Th400 electronic issue

[Falkasorus]

I have an 87 c10 when I bought it the dash cluster wasn't working. After finding the plug to be shorted and installed a new fuse it all works.. my truck now shifts funny when driving through town and only periodically. I contacted the person I bought it from and they failed to mention that happened to them so they just pulled the fuse. Said they had it looked at and it wasn't the transmission???? Any help would be much appreciated.

 

[Frankenchevy]

Only thing electronic on a th400 is the kickdown switch. Only has an effect on wide open throttle.

Maybe you have a vacuum leak and the modulator isn’t getting a good signal.

 

[Falkasorus]

Idk if this helps. It's a TBI I appear to not have a kickdown switch and the only fuse I replaced before this started happening was the guage/idle fuse. My guess the reason it was pulled and a 3 cluster pack installed down below.

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[Frankenchevy]

I can see that you don’t have a kickdown switch present. Are you positive you have a th400? Anything is possible, but I’m not sure how common a th400 was in an 87 c10. Maybe you have a 700r4 and the TV cable is maladjusted?

 

[Falkasorus]

Yea I just replaced the fluid and filter its a th400. I haven't found the schematic yet. But does the tps sensor tell the detent solenoid to operate? It's been frustrating. I remember the day it started and that was the day I plugged in the gauge/idle fuse.

 

[Frankenchevy]

Oh gotcha. I know nothing about factory tbi. I wasn’t thinking that the tps of that unit would take the place of the 86-earlier kickdown near the gas pedal. I’m sure someone with tbi knowledge will be along to help.

Yes, whatever switching mechanism is supplying +12vdc, is supplying it to the case connector on the drivers side, then to the solenoid inside for th400 kickdown.

 

[Vbb199]

400 has the electronic kickdown at the gas pedal

I mean, im no expert here, but i think someone put a th400 in place of the 700r4 that was in there.

On the 700r4 the tv cable would control passing gear kickdown and such, on this 400 you have, you're gonna need to source the goodies for your gas pedal

 

[Vbb199]

Is there a 4 prong plug anywhere dangling near the transmission that isnt plugged into anything?

 

[Vbb199]

If someone maybe really did order a th400 in 1/2 ton, then i believe the tps does in fact control your passing gear switch.

The 1 ton truck that had a tbi 454 with a th400 i believe worked off the tps.

 

[Falkasorus]

I will crawl under it and check tonight. I'm also going to try and get it to do it and then pull the fuse and see if it goes away for sure. If it does then I'm even more confused hahaha. As far as I know and have read the single wire to the detent solenoid should be controlled off the pedal? It's just odd that it happeen to me when i installed the fuse and the other owner and they just put a mechanical cluster and ran with that. But I had no fuel gauge with the cluster dead.

 

[Falkasorus]

Is there a way to Vin search what transmission came in it factory?

 

[Frankenchevy]

Is there a way to Vin search what transmission came in it factory?

Your RPO sheet in the glove box may be telling.

 

[Matt69olds]

Unplug the kickdown connector from the transmission. If it shifts normal after that, you know that whatever controls the kickdown solenoid is the problem.

The TBI trucks used a kickdown solenoid relay. The relay was triggered by the PCM. The relay may be stuck, the PCM itself might be triggering the relay, or there might be a wiring issue causing the rely to stay energized.

 

[Falkasorus]

Thanks

 

[Falkasorus]

If I understand right I have the one on the exterior of my trans and the internal one. They are fired by the pcm what tells the pcm when to fire? TPS sensor haven't found I diagram yet.