Allison HT 741 Transmission Questions.

[big builder]

Hello all. I have an old firetruck we bought for engine parts. Detroit. 6v92. When we got it the seller indicated the transmission was working fine but that the batteries were reversed and it fried the computer. We took this into consideration. Now it would be nice to move this around under its own power. My question is . Is there any way the computer isnt fried and that this could be a very simple fix? I dont know anything at all about allision tranmissions. The transmission is an HT 741 coupled to a mechanical 6v92.

 

[Birken Vogt]

Yes. If there is a crowbar diode inside the computer it may have done its job and shorted out the reverse voltage causing a fuse to blow upstream. Now you have to go find that fuse. Replace it and it might work. Often the crowbar diode will fail shorted as a result of this abuse so it might still blow as many fuses as you throw in it. In that case you would have to open the TCM, find the crowbar diode, and cut it out. But it could also be that the TCM really is fried.

 

[big builder]

Hi Birken, Ok well I will go looking for some fuses I guess. I wont spend too much time on it becuase the trans is not that interesteing for me but moving it around would be great. Also someone maybe could use that tans as it only has a few thousand miles on it.

 

[56wrench]

never heard of a crowbar diode. is that a zener diode?

 

[56wrench]

never mind- i googled it. mrs google knows everything it seems

 

[Birken Vogt]

Google gives lots of examples of TRIACs and all sorts of nonsense but a crowbar diode that I am thinking of is simply a big silicon rectifier placed across the incoming power + and - whose purpose is to blow the main fuse if some dummy should hook it up backwards. It is not a specific type of diode, just an off the shelf diode that can carry a lot of current for a short time.