1997 M1078 Smoking Allison Transmission Selector

[Ambeihl]

Got a chance to hook up all the batteries tonight for the first time. As soon as I got the grounds all hooked up the transmission selector started smoking. I was told that this had happened when they tried to jump it through the NATO port also. I will be pulling the unit out of the dash tomorrow and will post any relevant pictures. Anyone run into this before?

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[someoldmoose]

Just a guess. Probably why it needed jumped in the first place. Glad ya only had to let the smoke out and not needing an extinguisher, or more. Check every circuit before putting the cables back on. Glad ya saw it quickly. To answer as asked, not in that vehicle but in others, yes. Anything with volts and amps going through them can burn.

 

[Suprman]

Water leaks in around the lights above the windshield and drips onto the dash. They keypads are splash resistant but over time water gets in. I had one that would randomly put the truck in drive on its own so be careful.

 

[Ambeihl]

Well I opened it up this morning and I looks like it was a MOSFET that was letting all the smoke out and burnt the screen with it. I just still am unsure as to why it burnt up in the first place. This repair looks doable but it would take a ton of desoldering just to get to the MOSFET. Tell me what you think.

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[Suprman]

There are places that repair that. Not cheap though. I have good parts on hand. You just have to be careful. On the older stuff bad harness or other issue can cause controller damage. If you look at the 2 plugs and follow the harness back a bit there will be some wires that are cut and just left sitting. I had an issue once with them shorting I taped each one off and that fixed it.

 

[Ambeihl]

Ok that would explain why this shorted like this. The main set of wires coming from the transmission hangs really low and the sheathing was off of it. I looked for obvious breaks in the line but didn't see any. I'll looks more behind the dash now.


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[Suprman]

There are unused wires coming off the trans harness they are usually just cut and laying in the dash. You are going to need to ohm out each solenoid in the harness before you plug another controller in. Does it still smoke with the trans harness disconnected? This is the TX truck that had the harness unplugged at the trans? Someone was troubleshooting something.

 

[Ambeihl]

I have not hooked up the batteries since last night but I am going to do that shortly. Yes this is the TX truck. I am going to have to finish what they started.

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[Suprman]

The trans control goes thru the cab hump on the drivers side. You can take the grill off and disconnect there. May need pliars to carefully turn to unlock the mil connector. Taking the steering wheel off makes working in the dash so much easier.

 

[scabskunk]

That is a VFD and that burnt area is normal.

If i remember right.
When the display is made there is a small chunk of metal that when the display is under vacuum they heat that metal and it goes to vapor inside the display.

 

[Ambeihl]

Took the entire wiring loom out that starts at the cab and runs to the transmission. No visible signs of tears or any exposed wires. Also looked behind the instrument cluster and no luck either yet. There is a rouge ground wire that runs under the dash and is just under the first available screw. More troubleshooting to come.

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[coachgeo]

Please upload pics here instead of hosting at taptalk. It will make the record of this adventure so much more valuable to future readers who are searching for solutions to problems like yours.

Look also for other things shorting too your grounds sending power wrong directions?

 

[Suprman]

Could just be a bad controller. There are wtec3 pinouts online. You can ohm out the solenoid circuits.

 

[NDT]

Send your controller to Transmissioninstruments.com, he can fix it and tell you what solenoid that board controls, so you can look for causes effectively.

 

[Ambeihl]

Well after more dissection of the control board, a transistor had fried and needs to be replaced. After scrapping off the potting I found that the part is a TYCO 6N15G surface mount transistor. I am having 0 luck finding a replacement part at my local electronics supply house. Getting it repaired by someone else will cost more than half of what it costs to buy one. So once I find the part I am gonna break out the soldering iron and make the repair myself. I will keep you posted with the progress so that others may benefit from this repair. Also any leads on the transistor part would be helpful.

 

[Suprman]

6n15t4 may be cross ref. And they are available. Surface mount work is not easy with a soldering iron. There are special rework stations used for that type of work. Too much heat will damage the transistor, board and other components near it. Pay the repair guy to fix and test it. Cost you less in the long run.

 

[Ambeihl]

Just wanted to update this post with the resolution. I was able to get into the board and desolder the parts that i needed to only to figure out that this unit was not repairable. I am having to purchase another controller. I will be posting parts from the unit if anyone is interested in the boards for their own repairs

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