Hummer Transmission Information

Transfer case and Transmission oil / Filter Change

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Changing Transmission Fluid


Note: There are three different filters for Hummers (which also act as the pickup for the pump). Older trucks have a different transmission and use one kind of filter For the TD models there are 2 different filters because the oil pans in the newer trucks are deeper. The filter is supposed to have 4 little "feet" (bumps, really) on the bottom to help prevent the filter from coming loose. If the wrong filter were installed, it is possible that it can come loose. This would cause the transmission to starve for oil.

Maybe your automatic transmission will last as long as you own your car. And it will if you take care of it. But if you don't, it, may fail in as little as 50,000 miles. To avoid a costly transmission repair, change the fluid and filter when needed.

Changing the automatic transmission fluid used to take time and had to be done by someone who was trained. The transmission oil pan had to be removed and the filter replaced.

For most technicians, it was "French­fried fingers" because there is generally no drain plug on the tranny's oil pan. The pan had to be removed. The fluid was usually quite hot, and it splashed out as the pan was pried loose. Once inside the transmission, the filter was accessible and could be replaced. Also, the band adjusters were accessible so they were often adjusted.

Transmission engineering and fluids continue to improve and many car makers do not publish a transmission fluid replacement interval. Others say the first service is due at 100,000 miles. Some say the transmission never requires a transfusion. But they may be doing their customers a disservice. It's better to maintain too much than too little.

Remove the dipstick. Place a drop on a clean, white paper towel. If the fluid looks pink and spreads out evenly after a minute, the fluid is still OK. If the fluid does not spread out or leaves a dark spot in the center, it should be changed right away. Ditto if the fluid smells scorched.

Evacuation systems suck out the old fluid, then refill the transmission with fresh fluid. They have advantages. The ATF can often be changed in less than 15 minutes. They can remove up to 95 percent of the fluid. They are safer for the technician. When you simply drain the fluid from the transmission you do not get most of the fluid which is in the torque converter.

If you want a change out machine, go to and look for a shop. You need to babysit them and make sure that the resevoir is completely clean before they hook it up. Otherwise they can pump dirty fluid back into your transmission. So babysit them and make sure they flush the machine and resevoir with a couple quarts of clean ATF *before* they hook it up.

Ford issued a technical service bulletin in July 1999 alerting dealerships to a new flush and fill machine available from their Rotunda Equipment Catalog. From an internal memo we obtained, Jim Blomgren of Ford's Transmission and Chassis Division stated: "The transmission sump filter will provide effective particle removal for at least 100,000 miles under normal operating conditions. Therefore, it is unnecessary to change the sump filter prior to a 100,000 interval when changing fluid with the fluid exchange machine." Richard Scott, manager of technical services at the Wynn Oil Co., told us that GM, Daimler Chrysler, Mitsubishi, Nissan and Isuzu have approved his company's TranServe II flush and fill machine. According to the Motor & Equipment Manufacturers Association, evacuation and fill methods are a clean, efficient and safe way to remove hot ATF. Many owners believe that this is a complete service and that removing the pan; inspecting for wear and replacing the filter are unnecessary.

About three fourths of the transmissions have felt filters; the remaining fourth mostly imports have mesh screens. Felt traps and holds particles that will not wash away during a flush and fill; mesh screens do wash. Though it is not common, the filter can get clogged. When this happens, fluid is restricted, pressure in the transmission drops. and the clutches slip or burn out. "Unless the filter is changed or cleaned, the transmission can be damaged," said Lenny Perry, co­owner of Lee & Franks Transmission Service on Kedzie Avenue in Chicago. "We like to drop the pan so we can inspect for damage."

Tom Saputo at Superior Transmission Inc. in Downers Grove agrees. "The filter is there for a reason. Like an engine oil filter, got to change it to get rid of contamination," said Saputo.

To complicate matters, the Filter Manufacturers Council reports that some transmission filters are 'non-serviceable'. But even if you don't change your filter, change your transmission fluid regularly. It is cheap, insurance against a premature breakdown or major overhaul. Change the filter, at least occasionally, unless it is inaccessible.

But if your car has high mileage and the fluid has never been, changed, think twice about having it done or risk the "morning after syndrome."

My Comments:

Reading between the lines, the transmission repair shops and the filter manufacturers want to replace the filter and everyone else doesn't. The transmission shops make a lot of money replacing filters and are upset about loosing this business to the oil change shops. It is often more harmful to open up a transmission and expose it to contamination in a dirty garage. Transmissions are assembled in clean rooms to avoid this problem.

I'm going to check out the fluid condition at every oil change or after any hard use, and do the flush and fill if the fluid isn't perfect or every 35,000 miles or so. I do recommend changing the filter after 50,000 miles. I changed the filter on my 95 after 25,000 miles and it was brand new. BTW, changing the filter is a real pain in the you know what.


My Shifter Lever is stuck in Park

The Hummer has an interlock that will not let you shift unless your foot is on the brake. How does it know your foot is on the brake? When you push the brake pedal the brake lights come on. When the brake lights come on they activate a relay under the dash that lets the shifter move. If your brake light fuse is burnt out your truck won't shift out of park. There is also a cable interlock that can get botched up.

Quick "Get Home" Fix

Pull the "speed control" fuse. This will disable the speed control sensor and your transmission will default to the default shift points. It will shift a little stiffly but it will get you where you need to go.

Used it many times on various trucks to get home when the tranny does crazy things. The fuse is five over and three up on the bottom fuse pannel. Only use this to get home as the odometer and speedometer will not work.

I want a Heavy Duty Aftermarket Transmission:

I decided to go with a built up transmission from ATS Diesel because of the warranty offered...I'm going with their tranny and torque converter which both have a 3yr 100K mile warranty. You cant beat that warranty... $2144 plus an 1200 core charge Transmission 5-star torque converter $1388 (8/2006)

The ATS is the transmission / torque converter I put in my truck, just had it done about 8,000 miles ago. I also added the B&M shift controller. It has performed flawlessly so far, and I was able to get the tranny and converter, and have it installed here for about the same price AMG wanted for the stinking transmission! I took a look at the thing out of the crate and the workmanship they do for the money is pretty hard to beat.

I lost Overdrive:

I had my trans rebuilt, on the way home after driving for about 30 miles I would lose overdrive. i would pull over, shut it off then regain overdrive for a short while. I replaced the tps (throttle position sensor). Overdrive works now, but after about 30 miles the check trans light flashes for one second and I hear the rpms go up for a second. if I continue to drive for about 60 miles the trans light and rpms do this about every minute or so.

Bent Flexplate (flywheel)

The most common cause of a bent flex plate (what an automatic transmission uses instead of a flywheel) is that the mechanic removed the bolts holding together the engine and transmission before unbolting the torque converter to the flexplate. This causes the entire weight of the transmission to be applied to the flex plate (and the transmission's input shaft, front pump, etc.) bending it. The correct method is to remove the transmission from the engine (regardless of whether in the vehicle or not) is to unbolt the torque converter, then unbolt the transmission bell housing from the engine. The engine and transmission must be supported during this operation. There is nothing holding the torque converter to the transmission except the flex plate. Once you unbolt the flexplate it can slide off the transmission causing damage to the torque converter or the mechanic.

If a mechanic first unbolted the bell housing (with poor support), slid off the transmission, then unbolted the torque converter. He would damage the flexplate, and probably the front pump assembly in the transmission.

I lost 3rd and 4th Gear

Dealer Service Bulletin 98-05-02 discusses 1996 and 1997 models loosing 4th gear. This occurs during high operating temperatures and/or high loads. There are no DTC's thrown. This is caused by leaking 1-2 and/or 2-3 shift solenoids.

My Transmission Won't Shift

First, physical position is not enough for the TPS (throttle position sensor). You need to align it, the procedure is in the (shop) manual, requires an adapter, a gauge block, and a volt meter. You can get by without the adapter by using jumper wires. If you are a machinist, you can make the gauge block. I have two, one that I made from Oak that was kiln dried It is finished to prevent it from absorbing moisture from the air. Aluminum is best, easy to work with (and doesn't absorb air.) The gauge block is .646" or 16.4 mm.

However, this is unlikely to be your problem (your shift points may be off with a miss-aligned TPS)

You need to get the MIL codes. Has your MIL light come on (many faults will not light the MIL light)? If yes, the problem is considered more serious than if it did not. Regardless to get the MIL codes (on 95 and earlier Hummers, with a 4L80E transmission, only) you can do this:

So called ALDL (assembly line diagnostics link) diagnostics connector which resided around your left knee is there for a service tool called Scanner. With Scanner you can communicate with your transmission control computer. Control computer measures and maintains over 40 parameters and stores fault codes if something goes wrong. Scanner is an expensive service shop tool.


You can read fault codes without any service tool if you short pins 5 and 6 of that connector. When pins 5-6 shorted check trans lamp gives the code with flash sequence. First comes 12 and then possible fault code. Codes are shown in sequence. E.g..12 is flash, pause, flash, flash, long pause and same again. Be careful playing with connector.

Connector pin order is:

from left upper corner: 1=np, 2=?, 3= np, 4=chassis ground, 5=signal ground, 6=enable PCM diagnostics, 7=np, 8= np

from left lower corner: 9=serial data, 10=np, 11=np, 12=np, 13=np, 14=np, 15=np, 16=unswitched battery 12V np = no pin at the connector

Erratic Shifts

My transmission shifts but a little hard in 1st to 2nd gear in HL or H but fails to shift in LL. It will only shift in LL if I take my foot off of the throttle (it will red line the RPM prior to shifting), and when it shifts it is real hard.

It was the Transfer Case Range Switch. It took around 15 minutes to fix and the problem is gone.

Can't get the gearshift out of Second gear position

It's likely that there's something wrong with either the shift rod, shift rod lever/bracket, and the arm that controls it. All are mechanical linkages that could get out of alignment.

Could you have been pulling on a heavy load (like a big shrub, etc.) from the front of your Hummer about the time this all happened?If that were the case, the rear (tubular) section of the front prop shaft assembly likely bowed/twisted and came around and slapped the shift linkage. Don't think this can happen??? most certainly can!!!

Also FYI; believe AMG released a new design shift linkage kit a few years back to help preclude binding, etc. Your dealer should be able to search out that info for you.

New Transmission Installed - Still Have Problems

I experience hard and delayed shifts with the new tranny I first checked the dtc. I had a code 22. I checked the tps (throttle position sensor) and it was out of calibration. I adjusted it according to the manual. (1.617v set with a 0.646 gauge ). No more hard shifts but the tranny was in limp mode. I just had 2nd & reverse. I rechecked my TPS adjustment, I had 0.72v at idle. It seems to be correct but I put in a new tps. I Adjusted it according to the shop manuel and the tranny still is in limp mode.
The only position where I have the all speeds is when the TPS is set counterclockwise, 0.1 V at idle. The problem is that shifts are delayed and are very hard. 2nd at 30mph 3rd at 45mph 4th at 55mph and OD at 60 mph with very bad down shifts. I was thinking about changing the TCM and Trans harness.

Best thing to do is drive with a scan tool attached, and watch what is happening. You can then see both the codes being set, and the TPS values. Make sure your 5V and a good ground at the TPS.

I see this complaint all the time, and the pressure control solenoid is the part we keep in stock.

New Transmission - Still Have Problems

I had a new transmission installed at 36,000 miles 1 year ago by Aamco in my 96 TD. Lately putting it into drive from park has been delayed, and doing the same on a hill is delayed and sometimes requires revving the engine. The tranny is still under warranty but is about to go out and i want to make sure that it is fixed right. The first time around I had the vampire so the cooling loop and tranny were both replaced. The guys at Aamco couldn't get the problem to reoccur for them but did say that there was air in the fluid. They are going to take the pan off and look. Aamco also said it could be the filter that is loose causing this to happen, they also said that the tranny fluid was slightly low but not enough to affect anything.

They took the pan off and discovered that the filter was crushed. it was creating a blender effect hence the bubbles in the fluid. they are replacing the filter

Rebuilt and Heavy Duty Parts:

I've looked into this myself but I decided just to buy a built up transmission. But from what I discovered is to look for all the upgraded 4l80-E HD transmission parts. i.e., planetaries, upgraded solenoids and valves. Stick with Red Eagle clutch packs and Sonnax parts. Here's a website that shows most of the upgraded 4L80 parts. For shifting performance, TransGo is a good brand since it geared for towing and HD use. During the process of trying to decide if I wanted to rebuild or custom, I did buy some parts that I no longer need..Trango Shift kit and a plethora of valve body parts.