Transmission Issue ??

Read · Jul 22, 2019

'85 C10 w/ 350 and 700R4 transmission. Truck is new to me and after driving for about 30 mins and it has warmed up the truck will begin to lunge forward on its own at cruising speed (40mph +) without any acceleration by me. Only does it after it has warmed up and once in 4th gear I believe. Short drives no issue. Any ideas or suggestions?

yevgenievich · Jul 22, 2019

Maybe it is trying to engage tcc lockup? Can check by unplugging the electrical connector on driver side of the transmission

1987 GMC Jimmy · Jul 23, 2019

Agreed. Something’s working because the only way they act up is stalling out at stops, chattering when the clutch starts to come apart, or simply not doing anything.

Preston · Jul 23, 2019

Any noise? What does the fluid look and smell like? Is it downshifting if you change the gears manually any difference? If you take your foot off the gas does it make a difference ? Or does it just happen while accelerating?

Read · Jul 23, 2019

Fluid looks fine. If I take foot off peddle or accelerate the issue stops. Only occurs when cruising or at a constant speed (ie. driving at 45mph). Also only after driving for awhile....doesn’t happen on short drives. Leads me to believe temperature is causing the issue

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Preston · Jul 23, 2019

As someone on this site told me not long ago start with the basics and cheapest issues and go from there, a process of elimination, is there a module on the firewall ( not sure what it’s called) has vacuum inlet and a few wires, to the left of center, and check your wiring going to the transmission, seems like something simple. Has the truck been setting for a while?

Read · Aug 2, 2019

I unplugged the lock up connector wire and so far appears that the issue has stopped. Suggestions on what I should/need to do or what exactly the issue is that needs to be fixed?

yevgenievich · Aug 2, 2019

Put a trans cooler and can keep the lock up disconnected. Otherwise have to check if it is the clutch the issue or the control circuit. Can reconnect connector and add a rocker switch to engage circuit that would force the wire to ground. If it does fine with the switch engaging it while cruising the the control circuit has a problem

1987 GMC Jimmy · Aug 2, 2019

Try two bottles of the Instant Shudder Fixx. I tried this when I had that issue, and it it improved. I did two or three more bottles, and it pretty much went away. That was over thirty thousand miles ago.

Read · Aug 4, 2019

Took it for a drive today, still with lock up unhooked, and it stalled while driving after about 30 mins. Started right back up and drove with no issue. Truck did not lunge this time while driving. Unsure if the stalling is related or a separate issue. Going to hook lock up wire again to see if lunging issue comes back so I can determine if it is truly a tranny issue. Odd thing is the lunging issue only happens after driving for a while and truck heats up. If it was a lock up problem wouldn't the issue be constant just not when heated up?

C10MixMaster · Aug 4, 2019

the lock up converter is controlled through a vacuum switch. if you engine is producing the amount of vacuum it takes to just trip the switch it could cause it to cycle and engage and disengage the lockup . mine will on just the rite incline.

Read · Aug 5, 2019

Would that mean the vacuum switch is bad or engine has a vacuum issue?

C10MixMaster · Aug 6, 2019

Read said:
Would that mean the vacuum switch is bad or engine has a vacuum issue?

not necessarily a bad switch or vacuum problem. Just the vacuum is running at +/- the cut out point of the switch. Their is a black and blue delay valve that may help.

Read · Aug 6, 2019

Thought the stalling issue may be related or do you think that is a separate issue?

C10MixMaster · Aug 6, 2019

Read said:
Thought the stalling issue may be related or do you think that is a separate issue?

i think stalling was a separate issue at this point.

start with your basics
verify there are no vacuum leaks
check to see if the switch holds vacuum
check to see it the delay valve is installed

it possible the current engine just doesn't produce sufficient vacuum for the lockup switch

if all checks out you may consider trying an aftermarket adjustable switch

Transmission Issue ??

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