GM Diesel Hummer Glow Plug Operation, Testing & Troubleshooting

Hard Starting

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Glow Plug The engine glow plugs are used to heat air entering the combustion chamber. Heating the air results in improved cold temperature starting and engine warm-up. A total of eight glow plugs are used; one for each cylinder. Each glow plug is threaded into a cylinder head port located just below the injector nozzle. The tip of each plug extends into a pre-chamber which concentrates the fuel/air mixture prior to combustion.

The glow plugs are operated by a controller/relay mounted on the rocker cover or intake manifold. The relay is in circuit with the ignition switch and is energized only when the switch is in the Start and Run positions. Turning the ignition switch to Start position allows current to flow through the switch to the controller. Once energized the controller allows current to flow through the relay and to the individual glow plugs.

If you start it up and it blows black smoke and then quits it's probably some bad plugs or the glow plug controller.

Glow plugs do go bad.  I had a 96 Hummer wagon with the 6.5 turbo diesel with 22,000 miles and the glow plugs went bad.  The engine just got harder and harder to start and wouldn't start once the temperature got below 50.  I didn't even realize what was happening until I traded in the truck for a 98 and the 98 started on the first crank.  I really don't know what caused the plugs to fail in this case.  Plug failure is usually caused by excessive heat and high voltage.  The heat can be caused by a defective glow plug controller keeping the current turned on too long causing the plug to burn up.  Overly advanced injection timing, leaky injectors, or  running gasoline in your engine will cause the combustion temperature in the pre-chamber to get hot enough to damage glow plug tips. Over voltage can be caused by voltage regulator problems in the alternator. Some aftermarket high output alternators can produce excessive voltage also.

ControllerSome say to replace all the glow plugs if you find one bad one. I would replace all the plugs if you find a bad one and you have 50,000 or more miles on the engine. If a glow plug is not defective, it isn't necessary to replace the glow plug! The are not wear items like spark plugs, and can last the life of the engine if they are not abused by a defective glow plug controller. Routine replacing of glow plugs is not necessary. Any dealer who recommends routine replacing of glow plugs, without any indication of failure is not working in the owner's best interests I'll leave that decision up to you. They only cost around 10 to 12 bucks each.

Tests have shown that the AC-60G is by far the best plug for either the 6.2 or 6.5 diesel engine. These plugs cost around $12.00 each.  This plug has a PTC (positive temperature coefficient) dual heating element which is thermally self limiting. This means that as the temperature of the glow plug tip goes beyond about 1400 degrees, the self regulating feature kicks in and reduces current flow. Since the temperature is regulated, these plugs never get hot enough to cause tip distortion.


When you turn the key over (for the glow plug controller) you should notice a voltage drop on the voltmeter which is caused by the high current draw of the glow plugs.  If there is no voltage drop perform the following tests.

1. Check all glow plug connections which should be tight and corrosion free.

2. Verify that engine temperature is at or below 95°F (35°C).

3. Turn multi meter to amps or current flow setting.

4. Connect test leads between feed wire to each bank of glow plugs and to output terminal of glow plug controller/ relay.

5. Turn ignition switch to Run position to cycle glow plugs. Current reading should be 50-55 amps at each bank when cycling.

NOTE: It may be necessary to cycle ignition switch to OFF position between checks to reset controller internal timer.

6. Check current at each glow plug. Disconnect glow plug wire. Then connect test leads in series between feed wire from relay and glow plug. Current should be 13-14 amps.

7. If current to each bank of glow plugs is less than 50-55 amps, or current at glow plug is less than 13-14 amps, further diagnosis is required. If current is zero at both locations, check fuses and circuit for shorts, grounds, opens. Fusible links at all 10 locations.


This procedure is written for the 6.5 NA engines. This procedure should not be used for military HUMVEE models because they are 24 volt systems and their glow plug circuit and controller is radically different from the civilian models. Military HUMVEE owners should refer instead to the proper TO for testing procedures.

Tools required:

1. Pliers, long nose.
2. Small screwdriver
3. Multi-meter (VOM) either analog or digital

Tools helpful:

1. Jumper wire with a connector (spade, female) to match the Hummer glow plug.
2. Jumper wire, with alligator clip for ground point


Glow plug test:

1. Perform this test with key off!
2. Set the multi-meter to ohms (conductivity) lowest scale (X1 is best)
3. Connect one meter lead to a good ground point (use the jumper wire if necessary)
4. Momentarily test the meter connections by grounding to another point the second test lead. The meter should read zero ohms.

5. Remove the lead from the glow plug.
6. Connect the non-grounded multi-meter lead to the glow plug. Use the #1 jumper cable as required.
7. The meter should read low resistance, 0.8 to 1.4 ohms is the nominal resistance
8. Any glow plug that reads high resistance should be replaced. When I replaced 8 of my plugs one was 28 ohms and another was 120k ohms. The current glow plug specification is AC-60G.

9. Perform steps 5 through 8 on the remaining seven glow plugs.


Glow Plug Power Distribution test:

1. Perform this test with the ignition key off. Damage to the meter may result if the key is accidentally cycled
2. Set the multi-meter to ohms (conductivity) lowest scale (X1 is best)
3. Connect one meter lead to a good ground point (use the jumper wire if necessary)
4. Momentarily test the meter connections by grounding to another point the second test lead. The meter should read zero ohms.

5. Remove the lead from the first glow plug. Do NOT remove the remaining seven glow plug leads at this time.
6. Connect the non-grounded multi-meter lead to the glow plug cable connector.
7. The meter should read low resistance, almost zero ohms is the nominal resistance
8. Any glow plug cable that reads high resistance probably has a bad fusible link, and should be replaced.
9. Reconnect the glow plug

10. Perform steps 5 through 9 on the remaining seven glow plugs.

11. Remove all glow plug connectors at the glow plugs.
12. Connect one lead of the multi-meter to a glow plug wire (that passed the above test) on the left (driver's side) of the engine.
13. Measure conductivity to any glow plug on the right (passenger's side) of the engine. This value should be zero ohms.
14. If there is an open (no conductivity, high resistance) then one or both of the main glow plug controller output fusible links has failed. Replace the glow plug harness, or replace the fusible link as necessary.


Glow Plug Controller

Note: There's a new GM part number for the glow plug controller for pre-2000 trucks.  The new GM/AC Delco part number is 12496472.  Many GM dealers do not show it as a "glow plug controller", but as a switch or relay.

Well, I tested out my glow plug controller, and the voltage at the glow plugs themselves is 9.8 when the controller is active.

Because of the low readings, I purchased another 3 wire glow plug controller from a local Chevy dealer (70 bucks), installed it, and the voltage at the plugs is now 11.1.

The glow plug controller is located on top of the engine on the passenger side on the firewall. It is a dark colored box, about the shape of a large deck of cards. It has two large 1/4" studs on the top. One of the large studs is the power feed. This is a fusable link from the starter motor. The other stud is the connection to the glow plugs. I believe that the forward-most stud is the one for the glow plugs.

There are three wires (pink, yellow and black ) at the top of the Glow Plug Controller on my 97.5 TD truck. Some trucks have 4. See AMG Manual page FO-8 Engine Harness Jumper (Turbo) for details. Almost always the black wires are ground. Almost always the pink is 'ignition feed'. and Yellow is control, goes to pin C6 on the ECM located under the doghouse..

The glow plug controller functions basically like a relay, but with some computer control. The computer reacts to temperature and other input, and tries not to burn out the glow plugs.

First diagnostic: check for 12V at the power feed terminal. It should be present, even with the ignition off. If there is no power, jumper from the battery to this stud. Be sure to use a large gauge wire (~ #10).

If there is power, the controller is malfunctioning. Using a heavy jumper wire (~ #10), apply power to the glow plug stud for a few seconds. Try to simulate what the glow plug controller would do. Don't just connect it up and leave it. After a few seconds, disconnect the wire and start the truck.

Glow Plug Controller test:

  1. Set your multi-meter to volts, any scale that will you to observe about 12 volts, DC.
    2. Attach the multi-meter ground lead to a good engine ground.
    3. Remove the #1 glow plug lead.
    4. Attach the multi-meter to the glow plug lead.
    5. Turn on the ignition.
    6. Observe that there is about 12 volts at the glow plug whenever the wait light is illuminated, and that the glow plug controller makes a clicking sound as it cycles. (note: steps 3 through 6 may be repeated for each of the remaining glow plugs in place of the harness test described above.)
    7. If 12 volts is not obtained in the above test, did the glow plug wait light illuminate?
    8. If yes, the problem is probably a wiring fault on the glow plug controller's output (to the glow plugs) harness.
    9. If no, the wait light did not light, and the engine is cold, either the glow plug controller is defective or is not receiving the correct signals to turn on.

Note: Removal of failed glow plugs is easiest if there is a glow plug removal tool available.

Controller Problems:

  1. The glow plugs don't work at all. This is the most common failure. Usually difficult starts when ambient temperature is below about 70F (21 C) though this varies from engine to engine.
    2. The glow plugs are energized all the time. This generally destroys the glow plugs. This failure mode is uncommon.
    Glow plug system failures:
    1. Most common: the indicator light bulb in the dash is either bad or not making good contact.
    2. Glow plug controller cycles but doesn't make the glow plugs go on. (caused by burned glow plug contactor contacts.)
    3. Older Hummers sometimes (due to defective glow plug controllers) have bad glow plugs. AC-9s fail easily. AC-11s are much more resistant to failure, but can fail. AC-62s rarely fail.
    4. There are a bunch of fusible links in the glow plug circuit. If a glow plug has failed, you must check the fusible link to ensure it is OK.
    5. The glow plug controller serves as the main power connector for the Hummer's electrical system! It is connected directly to the battery, thorough a fusible link. All other power then taps off from there.
    Strange problems can occur if the terminal nut is loose.
    If ambient temperature is over about 22 C, or 72 F, your truck should start regardless of whether the glow plugs are working or not. At least all the trucks I know of will start.
    If you want to replace the glow plug controller, do so. It is neither expensive, nor terribly difficult to replace. Disconnect the batteries (very important!), remove the dog house,  and ten minutes later you are set.


GLOW PLUG and Controller Removal and INSTALLATION

ALWAYS use plenty of penetrating oil A FEW days in a row BEFORE trying the get the glow plugs out. I always install my glow plugs with antisieze grease and they never are difficult to remove. Most of the time the newer style of non swelling plugs will come right out.

On my 98 the passenger side is the difficult side because there's lots of stuff in the way. I undo the coolant reservoir and the large air intake pipe and push them to the side. The plugs need a deep 3/8" socket. I had a lot of problems getting the socket at the right angle because there is a metal heat shield directly under the glow plugs getting in the way of the wrench. In fact when I was working on the front plug I dropped it down the heat shield and couldn't find it for over an hour. It ended up wedged in the motor mount. That took me another hour to fish out. You can get all the plugs in from the top. You do not have to take the doghouse and rear engine cover off.

Old Plugs and ControllerSometimes the tips of older plugs get damaged (swelled up) from the heat and can't be easily removed from the cylinder head.  The only thing you can do  is use whatever force is necessary to extract the glow plug.  You don't want pieces of glow plug to get into the combustion chamber. It will ruin your engine.

Remove the injector, stuff the pre combustion chamber outlet port with some kind of cloth that you can pull out in one piece with a pair of pliers. Break the tip off the glow plug and allow it to fall into the cloth. Fish out the broken piece (a magnet works), vacuum up any small pieces with a piece of tubing attached to a vacuum cleaner hose and then pull out the cloth once you are sure there are no pieces on it.

The extraction tools (see below) will allow you to get a plug out that has frozen threads buts it won't prevent the swollen piece from dropping bits and pieces as it gets pulled out. If you break one off you have a problem. Remove the injector and run the piston to full TDC before trying to remove it. If there is enough sticking out so that you can grab it with some vise grips you could try heating it with a very small torch tip. You will find that if you can get a pinpoint heat right on the plug for a short time then follow it up with some penetrating oil like PB Blaster it will come out. That assumes that there is enough to grab though. If not then you will have to use the extractor method.

Removing and replacing the controller is really easy and doesn't require removal of the engine cover. Disconnect the battery at the positive post. With the air intake pipe out of the way and the CDR valve removed simply disconnect the wiring harness plug by pulling up on the catch and lifting the plug out of the controller. Remove the nut holding the ring connector with the wires going to the glowplugs. Remove the 2 nuts holding the controller to the manifold. Pull the controller out exposing the terminal with the battery cable. Undo the nut and you are home free. Clean all the connections and install the new one.

Glow plug removal tool

How to use Glow Plug Extractors (I think they were from Bosch)

  1. Remove the sensor wire from the glow plug sensor connector, and unscrew the glow plug until all threads are exposed.
  2. Spray lightly with approved lubricant.
  3. Select the appropriate split nut from your service set (10mm or 12mm). Open the split nut and install it around threads nearest the top of the glow plug. Be certain the split nut closes completely.
  4. Install one of the set's furnished spacers between the split nut and the cylinder head. For some applications, use of spacers may not be necessary.
  5. Attach a suitable open-end wrench (1/16" or 18mm) to the split nut. Attach a suitable wrench (10mm or 12mm box-End or deep socket) to the top of the glow plug.
  6. Hold the split nut wrench in place (do not turn), and begin unscrewing the glow plug until it is fully extracted. Be sure to use the spacer needed to maintain a solid base during the extraction procedure. This will reduce side load on the heating element of the glow plug, preventing it from snapping off inside of the head.

Same thing happened to me last year. Only my broken plug was passenger side second from the back. No access at all. The key is to soak the plugs with PB Blaster or similar penetrating spray prior to removal

Your broken plug location may save you, the access is very good from the inside. If enough thread is outside of the block, spray and let soak for several hours then try to grab with small vise grips and try to turn out. The material is fairly soft so be careful.

If that does not work you can try to drill out the center and use an extractor to back it out. Since the center core is a stronger material than the plug casing it will be difficult to have the drill stay in the center. If it goes off to the side you will start to drill into the threads. If that happens it's time for a new head. It's only $3,000. I kept my old block with broken plug as a souvenir, or possible boat anchor.

Install glow plugs in each cylinder head Tighten plugs to 8-12 lb-ft.  You should use anti-sieze compound on the threads.


HMMWV Starting Problems

Thanks for all the advise on my glow plug controller. After installing new glow plugs and a PCB/ control box this winter (the new KDS) my truck started perfect all winter. Now in warm weather the wait light was just flashing and truck wouldn't start. Peter James of Sydney suggested disconnecting the batteries to re set the control box, I tried that tonight and every thing worked fine. Apparently the control box remembers the last few starts and defaults the prior starts. I had started a few times in warm weather without cycling the glow plugs