Duramax LB7 transmission Warm Up issue

Volume 1 Issue 2 - Duramax Issues and Answers

I have a 2001 GMC LB7 Excab 4x4. What I have happen sometimes – when I start it up and put it in drive with no warm up time – it will not move. After a couple minutes I can put it in drive and no problems. Also with no warm up time, when I stop, it feels like it goes into neutral. When I step on the pedal and rev the engine, the transmission goes clunk and then it starts moving. No codes.


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The first most obvious thing to check is the fluid level. With low fluid levels the pump cannot deliver enough oil to the clutches to apply them at low RPM’s or cold temperatures. If your fluid level is low, then the next obvious thing to check is: Where did the fluid go? It was common on the earlier Allisons to have a leak at the front pump seal as well as the bolts on the pump cover.

However, there was another common problem with the early Allisons that would cause this symptom, usually after sitting for a week or so. The cause was the torque converter slowly draining fluid back into the sump. There is a service kit available from GM to help with this condition, and it also provides the needed parts to fix up any leaks on the front of the transmission (part number 88996718). It includes a new torque converter relief spring and lube regulator spring.

Not to be overlooked is the possibility that the internal filter is not installed correctly or the filter seal is leaking. This has happened more than once after the transmission is serviced and the filter gets forgotten or the seal gets damaged. This would have a similar effect to low fluid levels. With an air leak on the suction side of the pump, it cannot deliver positive fluid flow to hold the clutches.

Joel Paynton

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Joel Paynton is an award-winning GM technician who specializes in Duramax fuel systems. He also does custom programming for any GM powertrain. Visit him on the web at www.payntonperformance.com.


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