Overdrive pops fuse #12

I recently rebuilt my overdrive and for a month or so everything was working great. Here's the thread about that process if you have any interest: http://forums.turbobricks.com/showthread.php?t=339523

Recently when I've gone to engage the overdrive it pops the #12 fuse. What's odd is that this only happens if I'm above a certain speed. If I'm just cruising along and engage the OD ~40 mph , no issues. But if I'm trying to merge on the highway then go to engage the OD ~60-65 mph, then it pops.

I inspected the wiring underneath and didn't see any issues. Also checked the fluid level and it was fine.

My guess is something is bad with the solenoid, but I'm stuck on where to go next.

I am having a hard time imagining how the speed would affect this, but maybe heat could be coincidental with it. By the time you get up to 60 things are hotter, then the fuse blows and you can't try it again at lower speed until you replace it. Just a wild guess...

You could start the diagnosis by disconnecting the solenoid and checking if the fuse blows when you push the button. If it does not, you have likely found the problem. If it does, I think I would next look at the wires around the shifter and tunnel to see if something has shorted to ground.

That's where I would start. Good luck.

Doesn't the wire go through under the shifter boot? I would take another close look right there.

Harry Tuttle said:

I am having a hard time imagining how the speed would affect this, but maybe heat could be coincidental with it...

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That's what I was trying to figure out as well. I was shifting at ~3500 rpm when the fuse went, maybe that shook the trans enough to jiggle a wire and cause a short?

I drove to work today and it worked just fine, but it's all city streets. On my way home if I get an open stretch of road I'm going to accelerate from 45 to 65 with the OD engaged just to see if anything happens.

VB242 said:

Doesn't the wire go through under the shifter boot? I would take another close look right there.

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True I didn't look at this area from the interior. But that's the switch wiring, wouldn't an intermittent short here just cause the OD to turn on/off rather than pop the fuse (just guessing I'm no electrical wizard)

oifish said:

That's what I was trying to figure out as well. I was shifting at ~3500 rpm when the fuse went, maybe that shook the trans enough to jiggle a wire and cause a short?

I drove to work today and it worked just fine, but it's all city streets. On my way home if I get an open stretch of road I'm going to accelerate from 45 to 65 with the OD engaged just to see if anything happens.



True I didn't look at this area from the interior. But that's the switch wiring, wouldn't an intermittent short here just cause the OD to turn on/off rather than pop the fuse (just guessing I'm no electrical wizard)

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A short on either wire to the body will pop the fuse.

Gotcha, I'll look around the shifter boot and see if anything stands out.

You may need to inspect the harness inside the shifter as it has power while the key is on.

What sucks is that last year, I replaced all of the wiring in the shifter and boot. So I feel like it should be good...

The wiring for the fourth gear switch and the solenoid go through the shifter boot to the trans. So the one for the solenoid can wear through the wire on the edge of the opening. If any of the fourth gear cutout shorted it would make the overdrive work in every gear which is bad with it's own set of problems. But it probably wouldn't blow the fuse.

dl242gt said:

The wiring for the fourth gear switch and the solenoid go through the shifter boot to the trans. So the one for the solenoid can wear through the wire on the edge of the opening. If any of the fourth gear cutout shorted it would make the overdrive work in every gear which is bad with it's own set of problems. But it probably wouldn't blow the fuse.

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I disagree having had this problem. Hot wire out to shifter will pop fuse at any time if shorted to body, hot wire back from switch to solenoid will short out and pop the fuse if switch is engaged. Well this has been my experience with a 1980 242DL I just noticed op has a later model 740 that has the relay setup, so my experience may not be relevant.

Wiring looks good under the shifter boot. I put some dielectric grease on the two solenoid connections and insulated the ground connection. I tested it on the highway up to 75mph and my commute takes me over some pretty ripped up roads (woot Go Indy ). No problems with all the bumping and shaking around. I'll just keep some extra 15A fuses in the car and see if anything changes.

EDIT: I didn't look hard enough. It popped 3 times on the way home from work (felt like a BA swapping out the fuse while driving). I found a small cut in the switch wire than was bumping against the reverse lockout mechanism. Taped it up and reinforced it with some gaffer tape. To test it I put it in 4th, engaged over drive then jiggled the shifter all over the place, no shorts!