Fuel System Driver - FSD/PMD

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Note: The FSD/ PMD (same thing) is a small black box on the side of your injection pump. It is a signal amplifier that powers the fuel solenoid on your injection pump. It has 2 transistors mounted on it's backside that generate heat that has to be dissipated. In it's stock location it can over-heat and fail.

Stanadyne DS-4 Injection pump

New FSDThe story usually begins like this. You’re driving along and the engine just quits. You lose your power steering and the truck is very hard to steer. You pull over to the side of the road and you are able to restart. After a few days the stalls become more frequent and the truck will be hard to start. You realize that you can’t trust the truck and something has to be done. This is the classic symptom of a failing Fuel Solenoid Driver (FSD) also known as a Pump Mounted Driver (PMD). This is an electronic switch that controls the fuel flow on a Stanadyne DS-4 electronic injection pump. Electronic injection pumps are used on all Hummers with the GM 6.5 turbo diesel between 1996 and 2004. A failed FSD will not display an engine light. Pictured above the FSD is a small black box about the size of a deck of cards bolted to the side of your injection pump. It is an amplifier that receives signals from the engine computer to pulse the fuel solenoid which tells the injection pump to send fuel to the engine. It has 2 power transistors mounted on the back that generate heat that has to be dissipated. The injector pump is mounted deep in the center valley of the engine in a very hot environment. The FSD is exposed to heat that can cause it to fail over time.

The Stanadyne DS-4 pump has been used on GM trucks since 1993. Owners of older Chevy and GMC trucks and G-Vans have experienced these problems years before Hummer owners have. GM had so much trouble with early generation electronic injector pumps that they had to warranty the units for 100,000 miles. When AM General purchased these motors from GM they did not get a 100,000 mile warranty. In 1999 AM General bought the plant and took over production of the 6.5 engines later that year.

Injector pump burried in the engineHeat generated by the PMD is supposed to be carried off by the diesel fuel flowing through the pump.  While this is true, the damage is being caused right after the engine is shut down.  After you stop the engine the fuel of course stops flowing and the water pump stops circulating coolant which in turn causes the engine temperature to actually spike for a short time.  This is called heat soak.  This rise in temperature heats the PMD which is bolted to the injection pump sitting on top of the engine.  The continual heating and cooling cycles cause the PMD to fail over time.

Note that damaged wiring or a bad injector that's drawing too much current can take out a PMD. Also check the the fuel system shutoff solenoid on the injection pump. An electrical problem here will cause the engine to die.

Because the injection pump is deep in the valley of the engine the PMD mounting screws are almost impossible to get at unless you remove the manifold (which is a major job. I've spoken to a couple of mechanics that were able to get the PMD out by using a very short torx bit wrench but I can't see how it was done.

If your PMD needs replacing it seems unwise to put it back where it was, down in the heat. Why not save all the trouble and labor and simply mount the PMD on a heatsink somewhere away from the engine where it can stay cool.

Flash Off-Road has a specially designed heat sink and extension cable that let you remote mount your PMD in a cooler spot. The kit used in this article is from Flash Off-Road. This solves the problem once and for all.

PMD’s have a calibration resistor pack that describes the pump to the electronic control system. Because there is variability in the manufacturing of injector pumps the resistor is used to calibrate the engine computer to the pump. The resistor pack is a small circuit board with a resistor that is inserted deep into the PMD’s socket. New PMD’s don’t come with resistors.

The #5 calibration resistor is considered the "average", and any pump will work with a #5 resistor. Most owners replace the resistor with a #9 because it gives them a little more fuel and possibly a little increased performance. The resistor is only used when the control computer forgets what the value is, which occurs after the batteries are disconnected or a hard reset is performed with a tech tool. If you don't use a resistor you will get a p1218 error code when the computer tries to read the value. Your truck will still run.

Whenever I run into a failed PMD I always replace it with a remote mounted unit on a heatsink and just abandon the defective PMD on the injection pump.

As of 2009 there are 2 kinds of PMD's

Stanadyne in all their wisdom came out with what is supposed to be a better PMD. The problem is that the new PMD has a different socket style than the original one. This means that it won't plug into existing harnesses and extension cables and isn't compatible with the original calibration resistors. It's my opinion that they did this to attempt to thwart the aftermarket PMD replacements. Luckily, there are plenty of options for PMD's that have the original socket. In fact, the GM factory replacement PMD has the original style socket. If you do get one of these new PMD's Flash off-road has an extension cable that works with them.


FSD with Heatsink Compound Extension Cable

FSD and heatsink Remote mount PMD kits come with a heatsink, heatsink compound, terminal grease, mounting hardware and an extension cable. If you get a brand new PMD it will come with a heatsink pad / gasket. Use this instead of heatsink compound.

Silicon Grease

The terminal grease is a thick plumbers silicon grease you can pick up at a hardware store. It's used to coat the seal on the water tight silicon rubber connectors so the elements don't corrode your connections which will stop your truck from running.

First Decided Where You Are Going to Mount the PMD Heatsink

While I’ve seen the PMD’s mounted in many places a good place to mount the new heatsink on a Hummer that isn't used off road is near the battery compartment on the right side of the truck. It’s away from the engine, fairly clean and gets good air circulation. If you go off road and kick a lot of mud up into the wheel well you can clog the fins.

99 GMC 3500 6.5 diesel driver side between battery & fuse box.

If you go off road and kick a lot of mud up into the wheel well you can clog the fins.

This heatsink is mounted on the Battery hold down bracket. Just make sure the hood will close. This installation has plenty of room.

Electrical coverOn 1999 and newer trucks the plastic electrical cover is a very good out of the way place that stays clean and has very good air circulation.

Horn bracketIf you have a 98 or older a real nice place is on the horn bracket. You wanted to get rid of those lame stock horns anyway.

You have to get to the injection pump so depending on the year of the truck you will have to move the big black rubber air intake hose that sits on the top of the engine out of the way. This usually involves loosening a few hose clamps and pulling the hose free.

Top of the EngineReach down to the wiring harness going to the PMD and lift up on the tab and pull the plug out of the PMD. If you’re going to remove the old PMD or resistor now is the time.

Some mechanics say that they can remove the resistor from the old PMD while it is still mounted on the pump by using a small snap ring pliers with a dental mirror and flashlight to grab the two small holes in the resistor pack and pull it out.

I say "no way". I've looked at this and tried it a few times and never was able to do it. I had trouble getting the resistor out of the PMD with the PMD out of the truck. I usually just get a new resistor which will cost between 20.00 and 30.00 dollars.

How to Remove the old PMD

If your PMD isn't defective and or you want to salvage the resistor try this method to remove the PMD without having to take the the truck apart. I'm cheap and wanted to save the 30 bucks for the resistor or better yet the 230 bucks for a new PMD.

Saw the Corner Corner sawed off

If you're careful it isn't hard to break the old PMD off the side of the injection pump. You don't want to somehow break your injection pump or cut a bundle of wires in a harness.

First cover up the air intake with a rag to prevent the possibility of loose bits of old PMD from getting in the engine. What you do is very carefully place a hack saw blade diagonally across the top corner of the PMD facing the firewall . Position it so it's on the plastic right next to the metal base. Move any wiring harnesses away so they don't get cut. Saw about 1/4" deep into the corner so you go through the mounting bolt. Take a screwdriver and tap it in between the PMD and the pump. Work the screwdriver around while gently prying. The mounting screws all just snapped and the PMD fell off the pump. In fact as you can see in the picture the metal in the corner of the PMD cracked off probably because I first put the screwdriver in the sawcut and not between the PMD and the injection pump.

Saw the recepticle Push the resistor off the pins

If your old PMD is defective you want to get the resistor out of the socket in one piece. If you're reusing the PMD leave the resistor in place.

Put the PMD in a vise so the socket is facing up. Take a hacksaw and carefully cut the plastic socket in two places. Break back the tab of plastic and the resistor and pins will be exposed. Take a small screwdriver and push the resistor off the pins.

Installed FSD ResistorInsert the calibration resistor into the new PMD with the resister number facing up, Notice that there is a tab on the resistor pack that meshes with a cutout in the socket. If you put this in the wrong way you won't be able to remove it without destroying it so make darn well sure you get it right the first time.

One of my customers used a special tweezer with hooked ends to pull the resistor right out.

One of my readers sent me some information on how he gets the resistor out of the PMD. He fashions a spring clip out of a large safety pin. He cuts off the head and bends the ends out so they will engage the holes in the resistor.

Assemble the PMD to the Heatsink

Next you want to mount the PMD to the heatsink. New PMD's come with screws. Otherwise, you will need 4 - 1" 6-32 screws. If you are using new screws get some stainless steel hex head cap screws. The heads on these screws are small enough to drop down in the PMD counter sunk holes. If the screw heads are too large to drop down you will not be able to correctly tighten the PMD to the heatsink. When I couldn't find hex head cap screws I've bought Phillips head screws and had to grind off some of the diameter so they drop down.

If you have the heatsink pad which comes with new PMD's use it. Otherwise, coat the bottom of the PMD with heat sink compound (the same stuff you use on computer cpu's) and mount it tighting to 25 inch pounds. This is fairly tight. I like to use locktite blue on the heatsink mounting screws.

Smear terminal grease on the silicone plug gasket and insert it into the socket on the extension cable. Make sure the cable locks together. If it doesn't I've had to file down the flat surface of the extension cable so it engages the locking tab. Leave the ground wire attached to the injector pump. Route the cable over to the new location keeping it off hot surfaces. I like to clamp and wire tie it so it doesn't move.

In this case we are going to mount the cooler on the plastic face in the battery compartment. The Hummer's batteries are far enough away and are sealed units so there will not be a problem with battery acid or corrosion. If you are going to be doing lots of muddy off roading you do run the risk of getting the cooling fins full of mud in this location.

The 6' cable will allow you to mount the cooler almost anywhere you want. Others have mounted it near the center of the firewall, near the air inlet in the radiator stack or near the radiator overflow. .

Cutout for the FSDTrace the outline of the PMD on the black plastic shield next to the battery and use a saw or rotary tool to cut out a hole for the PMD to fit into. Put the PMD into the hole and mark the 4 heatsink mounting holes and drill them out. Mount the heatsink to the plastic shield and plug in the extension cable using terminal grease. If you go playing in the mud make sure that you wash off anything plugging up the fins of the heatsink.


Horn BracketWhen I mounted the heatsink on the horn bracket of my 98 Hummer, I ground off the 2 horn mounting tabs and smoothed the surface. I located and drilled 2 holes in the heatsink and bracket. I primed and painted the bare metal bracket to prevent rust.

We finally had a little rain here. The windshield wipers were going and as they moved across a spot on the windshield they seemed to be scraping over some frozen spots except it wasn't freezing out. I didn't think anything of it. A couple of days later the sun came out. I was at the gas station fueling up and cleaned the windows off. When I got back in the truck I saw a bunch of very small black spots on the windshield. I first though I had driven by someone who was spray painting so I figured I'd just take a razor blade to the windshield when I got home. Then it hit me. When I ground off the horn brackets I had sparks flying everywhere. The little black specs were from the grinder. Luckily the razor blade cleaned off the specs. The moral of the story is to protect the area you are grinding in, especially the windshield.

Grind off Tabs Completed Bracket

A Technical Discussion About the FSD/ PMD

by Peter Hipson 11/15/2005

This all started when I made the comment that the PMD (pump mounted driver) has 2 - 500 watt transistors and would seemingly have to dissipate 1000 watts of heat. Actually it cannot dissipate more heat than the power it draws from its power source, the PCM. The PCM is powered by two circuits, one battery circuit that feeds the PCM, and the truck's brake lights. This power is used solely to keep the PCM's memory alive, and a few key-off tasks and represents a minimal amount of power consumed, none of which is sent to the PMD.

The second power source keyed to the ignition feeds the PCM. This source has a 20 amp fuse. Fuses are expected to blow at the amperage they are rated at, so if you wanted 20 amps, you'd need a fuse larger than a 20 amp fuse. In the electrical engineering game we use a figure that is about 70 to 80% of the fuse ratings, so a 20 amp fuse would be expected to pass about 16 amps steady state. So we have the potential that the PCM (which includes the PMD driver module) drawing 16 amps. But...

The same power fuse also feeds the fuel pump! Now, though many don't realize it, the fuel pump is rather inefficient electrically! It typically draws 5 to 7 amps when running! That leaves us with 10 amps for the PCM and the PMD. That's perhaps 130 watts of power.

However we must take into consideration other parts of the PCM that draw and utilize power. The actual computer module draws and uses power, perhaps an amp or so. (Say 10 to 15 watts). The Injector pump's solenoids and timing stepper motor also draw power (which is eventually converted to heat...) and these actually do draw a significant amount of electricity, maybe (let's be generous, and say 5 amps, you'll see why in a few minutes).

The PMD/FSD driver is in series with the solenoids. Something like a big electrical switch. The current through the PMD cannot exceed the power drawn by the solenoid, in fact, if the signal was a constantly varying saw tooth, or a sine waveform (the worst to control) the average power dissipated by the PMD would be equal to half the power drawn by the solenoid. So, say 2.5 to 3.0 amps. OK, at 3 amps, we're talking about 36 watts of power that the PMD dissipates. That's a long ways from the power dissipation that we see quoted so frequently.

However, this amount is still significant at 36 watts in a package the size of a deck of cards would make the unit too hot to handle after a short while (say 10 to 15 minutes.) However, that is not the problem with the PMD!

Silicon devices are designed to work in one of three environments. The consumer grade devices are designed to be used in an ideal environment, not too hot, not too cold. Commercial devices are specified as 0 to 70 degrees C for their working temp. The other end of the scale is military specifications. These are typically -55 to +125 degrees C. That's really, really cold, and quite hot. In the middle are devices designed for rugged commercial applications, such as automotive, where the range is between these two extremes (usually -25 to +100 degrees C) which is what the PMD's devices should be rated at. I doubt they use military grade devices in this device!

The problem is that the injector pump is eventually going to get about as hot as the engine. The engine temperature at operation can be estimated to be 85 to 90 degrees C. That is 10 to 15 degrees less than the temperature of the devices! The PMD will create its heat increase (delta t) without regard for ambient temperature. That is if the PMD temperature is 20 degrees C, and as it operates it increases by 40 degrees C (to 60 degrees C) then if it starts at 85 degrees C and increases that 40 degrees, it will get to a temperature of 125 degrees C! Well above the design limits of the device.

The stock PMD/ FSD was designed to get cooled by fuel running through the injection pump. This arrangement works well until outside temperatures heat up. In locations like Arizona and Southern CA where the temps run over 100 degrees in the summer you see more PMD failures.

When we design heat sinks, we specify their efficiency as being a certain number of degrees of temperature rise per watt of power applied. Typical designs range from about 0.75 degrees per watt (very efficient, and probably very large too) to as much as 6 or 7 degrees per watt (less efficient.)

Above we said that the PMD may dissipate in a worse case about 40 watts. Mounted to a heat sink this would translate into a temperature rise of about 30 degrees for that really efficient heat sink, to 260 degrees for that really poor one. The heat sink size would have to be between these two values, and if we use an ambient temperature of 48 degrees C, and a maximum temperature of 100 degrees C, we have about 52 degrees C to play with. We see we need a heat sink that will give us a rise in temperature of not more than 1.2 degrees per watt.

The above is worse case design. Maybe smart design, too. But in the real world this calculation is very conservative. The PMD only dissipates a small fraction of the heat that the above calculation implies because it is basically digital (A pulse width modulated signal) and it only dissipates heat when it is changing state from on to off, and off to on. When fully on, the dissipation of heat becomes a factor of the voltage drop in the transistor .With a standard silicon transistor, that would be 0.7 volts, or with 5 amps about 3.5 watts. A FET is much more efficient, and would probably only dissipate maybe 0.5 watts when fully on. The fact (if it is a fact) that the transistors are rated at 500 watts, indicates they are FET's. But since they are in a TO-5 case (that diamond shaped package) tells me that someone simply multiplied the maximum voltage and the maximum current for the transistor and came up with a number. A TO5 case limits power dissipation to considerably less than 500 watts, as there is insufficient surface area contacting the heat sink to move that amount of power.

In summary, the PMD can only create about 40 watts of heat which is all the power it receives, and it can't create heat from nothing. However, 40 watts of heat can be significant, and does require a heat sink to keep it from being destroyed.

Some Technical Updates

One of my readers sent me this information.
The PMD transistors are bipolar (not MOSFET) transistors and used in a linear current-limiting mode at 12 amps. The solenoid resistance is very low so a prudent design approach is to assume that the PMD needs to dissipate all of the power. This is an oversimplification but I always design my automotive controllers for worst-case. In particular, my fuel injector drivers never blow up. At this power level the dissipation is about 130 watts, times the energize duty cycle which is a maximum of about 40% or just around 50 watts. The transistors themselves are rated to -65 to +200 degrees C (wider than automotive) so that they are ok to use in that temperature environment but not without proper heatsinking. HOWEVER the SOAR is being exceeded under higher voltage conditions. The transistor is capable of about 3 amps continuous (each) at 14 volts of drop, assuming effectively infinite heatsinking. Basically this is simply a design fault. The transisors simply violate the SOAR (SOAR is 'safe operating area)curve, a straightforward way to disaster.

Transistor Spec sheet

You are correct about the voltage drop difference between a MOSFET and BJT but in this case the BJT is used intentionally in its voltage-dropping mode. A MOSFET would offer no particular advantage with this circuit, HOWEVER if used in a switching (PWM) injector driver, that situation changes dramatically. In that case one MOSFET is switched on by the injector control signal. A second MOSFET 'chops' the current when it reaches 12A. In other words, it turns on until the current hits 12 amps, then it switches on and off rapidly so that the current averages 12 amps. The 'left on' MOSFET recirculates the current in the solenoid so that very little heat is generated. When the solenoid is to be closed, both MOSFETs are switched off, and the solenoid closes.

PMD Part Number Characteristics and Misc Info

Starting off - the Stanadyne DS4 injector pump (as an overall unit, including the PMD) has gone through over 20 revisions since it's introduction. Many of the improvements are internal, but 2 of the improvements are involving the PMD itself.

Introduced in 1994, the (pretty accurate) rumor mill out of Detroit has Stanadyne losing the argument with GM about the placement of the DS-4 injector pump, which we all know is in the middle of the "V" between the valve covers. This unfortunately places the injector pump electronics in the middle of a ton of heat.

The heat is balanced (in theory) by the fact that the PMD is being cooled by the diesel fuel itself running through the pump. In theory, this is true, in actuality - the damage is being caused while the engine is OFF, not while it's running.

Reports have shown that the pump itself will peak in temps around 140-150 degrees F. When the truck is off, the heat soak will push the temps to anywhere from 180 to 200 degrees, depending on ambient temps of the season and location.

The reason why the unit fails (from reports via Stanadyne dealers and Diesel Forums) is basically the transistors themselves will lose connection due to the epoxy compound (used to seal the unit together) weakening, eventually allowing the transistors to literally work their way loose. It's a bad design overall, compounded by continual heat/cool/heat/cool cycles. Add to the issue that you need a good and clean ground (the case itself provides some of the grounding!), and you really have a major issue if the mounting screws start to oxidize.

Getting technical, it seems (from what I can tell by looking at a destroyed PMD) that the unit is basically a huge set of transistors (one of them I believe is a Darlington) in which one triggers the other to fire an electrical pulse, and will vary the pulse based on the input from the PCM (engine computer).

I'm not crazy about the design myself. The dual layer PCB has a tendency to delaminate, and the board design (when looking at the reverse engineered designed diagram) should have been far more robust for a diesel workhorse engine.

Getting to the specific PMD question you're posing - what we are seeing here is classic failure of first and second generation PMD's. This is something that owners of 1994-1996 Chevy and GMC trucks and G-Vans have experienced years before Hummer owners have. Because AM General didn't put in the 6.5L TD until later on in the engine's life span, non-Hummer owners have experienced these failures way before Hummer owners will. Add the fact that many Hummer's aren't daily drivers (mine is!), and you extend the lack of issue visibility even further.

GM had such a problem with PMD's and early generation injector pumps that they had to warranty the units for 100,000 miles.

According to Stanadyne rebuilders there have been 3 variants of PMD's, with the latest one being either an improvement or a problem, depending on who you talk to. PMD's with a 30214 on it's case are the worst. This is the first generation and the most prone to fail. It seems that this electronic module overheats and goes bad. The engine will probably start when it's cold and have a hard time as it gets hot. If the engine doesn't start It was suggested that you pour some water on the left side of the injection pump to cool off the PMD.

PMD's with a 34264 are second generation, and are (ironically) preferred by some Stanadyne tech's because they will either work, or won't work, like "a light bulb" according to one tech. A Stanadyne tech in Ontario, Canada (note the location - definitely a low risk of heat soak in that area!) has gone over 100k with a second generation PMD. The third generation (34583) actually has gone through some mini-revisions of it's own. This unit was designed to allow you to "limp home" (yeah, right) by operating for a short while, then fail. The latest variants of the third generation are the most sought after versions. The latest versions sold by us are the most dependable.

All the new PMD's superceed these older varients and are greatly improved.

You have to remove the injection pump to remove and replace the PMD although I've heard from some mechanics that they have removed them by using a very short torx bit wrench (below). They may have ground one down to fit in the narrow space.

One owner was able to remove the PMD with a heavy duty "putty knife" (the kind where the blade goes through the entire handle and you can hit it with a hammer) I sheared off the thin torx screws attaching it to the pump and remounted the new one to a cooler and reused the resistor that was embedded in the socket.

Owners have told me that the best way to go is to get a unit that has a remote mounted heat sink off and away from the hot engine.

If you get a new PMD you will need a new calibration resistor unless you take the old one out of the defective pmd. The resistor describes the pump to the electronic control system. The resistor must match the pump, not the FSD/PMD. When I purchased the PMD, there was no resistor with it - I had to buy one separately. If you get a complete master cooler kit a new resistor will be included.

Any pump will run with any resistor. The resistor is a way that stanadyne can insure that all the pumps leaving the factory will put out the same amount of fuel. When GM says the engine has 200hp it will mean that each new engine will in fact be to spec. The #5 resistor is considered the "average". The resistor is only used when the control computer forgets what the value is, like after the batteries are disconnected. If you don't use a resistor you will get a p1218 error code when the computer tries to read the value.

You can get the computer to see the new resistor by initiating a TDC Learn with a tech 2 scan tool. The computer will check the resistor value on it's own every 50 startups.

The resistors are keyed for correct installation. Notice the curved top and the alignment tab in the socket. The resistor number faces up. You can see the number printed on them by looking at them. Use a dental mirror and a flashlight. To remove them, use small snap ring pliers. There are two small holes in it that you can hook the pins into to pull it out. I was able to do this without removing the PMD from the pump.

The PMD module contains a pair of high-power driver transistors that generate heat during normal operation. Knowing the PMD module drives the fuel solenoid approximately 7,200 times a minute at just 1800 rpm brings the operational requirements into clearer focus. These driver modules are worked very hard and generate significant levels of heat. Use more throttle, and the fuel solenoid is driven harder and for a longer duration. This means the PMD will generate even more heat under high load or high-speed conditions. Tests have shown that even a low fuel level in the tank or a non operating electric fuel-lift pump will cause the PMD to work harder and generate more heat Add a high ambient temperature (or a dry, thin fuel), such as that found in the Southwestern US during the summer, and you begin to see why these heat induced PMD failures occur more frequently in those areas of the country.

p0251 (Injection Pump Fuel Metering Control) is the generic OBDII code that gets reported when the Stanadyne Injector pump can't read the optical sensor properly. p0370 (timing reference) is generally referring to the PMD or the electronics behind the optical sensor. p0251 will throw a "Check Engine" light. p0370 will not. Getting a p0251 will generally give you something else paired with it.

Definitely have the injector pump/PMD diagnosed. Best case - you dump a bunch of Stanadyne Fuel System additive in for a few weeks, clear the code out with the OBDII tool, and you'll never get it again. Worst case - you replace the Injector Pump and/or the PMD.

My Hummer has only 51,000 miles on which I do not think is a lot for this kind of diesel engine, so I called Stanadyne the manufacturer of the injector pump . After I read them the codes they explained that the optical sensor in the pump had failed. They gave me the model number of the pump used in all diesel hummers DS5521.

PMD Calibration Resistor Values
Resistor No.
Fuel Change MM3/Stroke