Common Issues With The 1000/M74 Transmission

Posted by on Tuesday, March 5th, 2013

The 1000/M74  is a very heavy duty automatic overdrive transmission built by Allison Transmission. It is primarily used behind the powerful Duramax diesel engine found in many later model GMC and Chevy trucks. It can also be found on GM chassis under custom motor homes, buses and other heavy duty vehicles. While this is a very heavy duty transmission, it is not without faults and problems. Let’s take a look at the 1000/M74 transmission.



Common Problems:

Electrical: The 1000/M74 transmission is loaded with electrical components. It incorporates multiple electrical solenoids and sensors, not to mention a fairly complex wire harness. All electrical transmissions suffer from similar problems. Solenoids often fail. They either develop an internal short, or end up with a break in the internal wiring that makes up the electrical magnet in the solenoid. Sensors can go bad for a number of reasons. Excessive heat or vibration can destroy them. Improper grounding can also cause them to malfunction. Excessive heat, driving conditions and age can destroy the wire harnesses.

Overheating: By far the biggest problem with this transmission is overheating. This is not the manufacturers fault. Overheating is usually caused by the vehicle owner. They tow loads way heavier than the maximum recommended limit. The also load their vehicle with more payload than is recommended. Doing either will put tremendous strain and stress on the transmission. Clutches and bands will sometime slip in server instances, creating excessive heat. Keep in mind that these transmissions are designed to run at a maximum temperature of 200 degrees. Every 20 degree about this temperature will cut the life span of the transmission by a factor of two.

Lack of maintenance: All vehicles come with recommended maintenance intervals. The problem is that most owners pay no attention to them. Filters get clogged over time and restrict the flow of transmission fluid. Transmission fluid breaks down over time and the result is a fluid that loses its hydraulic properties and ability to lubricate the transmission correctly.

Allison Transmission


Want to make your transmission last longer? Who doesn’t?  Here are a few recommended upgrades:

Fluid upgrade: Your transmission comes from the factory with an organically derived transmission fluid. It is specifically designed to meet strict requirements. Over time it will break down from the many heating and cooling cycles it endures on a daily basis. Therefore, I recommend you upgrade to a synthetic fluid. Synthetic fluids are designed not to break down like organic fluids. Just be sure to replace your organic fluid with a synthetic fluid that has the same properties as the original OEM fluid.

Deep pan: Install a deep pan. The extra fluid will help keep the fluid a little cooler. The fluid will also last a little longer before it needs to be replaced.

Allison transmission


Oil Cooler: Buy an aftermarket transmission cooler. A good rule is the bigger the better. Excessive heat is by far the number one killer of all automatic transmissions, and this transmission is no exception. Additionally, you can buy an aftermarket cooler that comes equipped with its own electrical fan. This will provide addition protect from overheating the transmission.

The 1000/M74 transmission is a great transmission. You should get years of trouble free service and reliability from it as long as you adhere to the information we provide.

A transmission will not last forever, regardless of how well you take care of it. If a decision has to be made to replace the transmission, I highly recommend you consider a re-manufactured transmission as your first choice. They differ from a rebuilt transmission in the fact that they are as good as a new transmission at a greatly reduced price and often include modifications and upgrades that make them much more durable. A re-manufactured transmission also comes with a much better warranty: three years compared to 90 days for a rebuilt transmission.

4 Responses to “Common Issues With The 1000/M74 Transmission”

  1. Don Thorson says:

    Spurious clunk from 6th to 5th gear.
    I have a 2007.5 2500HD (new body) GMC Duramax 6.6L LMM, Allison 1000. On occasion, maybe a couple times a yr, I will get a loud clunk from the back of my truck, usually when I’m slowing down from highway driving. This will happen when my speed is about 80 km/hr (50 mph) or a little less. One time, however, there were several clunks, until I took my foot completely off the gas pedal. It occurs when shifting down to 5th gear from 6th gear and the rpms low, and then it shifts causing these clunks. I’ve tried to replicate the scenario but can’t. It just does it at a certain speed and rpm that I can’t seem to replicate. However, it has repeated this scenario 3 times on the same road home when there was a bit of an incline where I slow down for an upcoming curve (just slowed down without putting on the brake and with the foot on the throttle slightly). There is 91,000 km (about 55,000 miles) on the truck. I had the trans oil changed at 50,000 km.
    I had the truck to GM Dealership where the transmission guy took a quick look and hooked up a gadget. There was no faults or failures displayed on the gadget. Do you suppose it’s something other than the transmission?
    Any suggestions would be deeply appreciated.

  2. Charles says:

    Try putting the transmission in neutral at around 25 mph. Coast and as it starts slowing, apply light braking. I bet as it gets to around 10-15mph, you hear the clunk again…and every time. I just got mine out of the shop Dealer) and they said this is normal…I am not buying it, so am going to a Allison mechanic that is not a dealer

  3. Clunk says:

    Sounds like either the transfer case, (they have a tendency to be ignored during regular maintenance), or the driveshaft, the splines tend to bind. Check the play on the output shaft coming out of the transfer case. If you can move the shaft up and down that is a problem.

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