Shifting at high RPM
 

Greetings
My 1986 GMC Sierra K2500, 5.7L, with auto transmission, developed 2 issues.🥱🥱🥱🥱🥱

1. It will not shift from 1st to 2ed as normal, it shifts at high RPM/higher speed.
I check the vacuum hose from the manifold port (behind the carb) to the vacuum modulator valve, suction is good. Not sure about the modulator valve if working or not, no one around knows how to diagnose the issue. Parts have to be ordered from US it takes 15-25 days.🤒

Tran has no click down cable to the carb, it has an electric wire (left side of Tran-driver side) to a switch attached near/at the accelerator peddle.
Not sure how to fix this high speed shifting.🤔

2. The choke light is on all the time, oil pressure is very good. Not sure why. 🧐🫡
:gmc2:

If someone can help :waah:

Re: ****ing at high RPM
 

From your description, sounds like you have a TH400.

Choke light on: Make sure the choke isn't stuck. Stuck choke can cause a way too rich condition which plays with vacuum and thus impact shift points.

Trans: Verify that the trans fluid level is correct. Low level can cause shifting issues.
One quick test - take a clean hose, pull off existing hose from modulator, attach test hose to modulator, suck on hose. If it keeps vacuum, modulator might be good. If it doesn't, modulator has leak and needs replacing.

Re: ****ing at high RPM
 

I’m not sure if it’s TH400 or R700. When starting the truck it will automatically run as choke is on, then when I press the pedal rpm drops.
Oil level is good, oil clean, I’ll do this test.
However what’s this switch/solenoid that is located at the gas padel with 2 wires (red goes to fuse box 12v, the other goes to side of the trans)?

Re: ****ing at high RPM
 

One of these.

Re: ****ing at high RPM
 

That is why I think it is a TH400. That is typical of a kickdown switch for a TH400. It detects near Wide Open Throttle and signals that to the TH400.

Re: ****ing at high RPM
 

That correct TH400.

Re: ****ing at high RPM
 

Modulators are adjustable. Poke a small flat bladed screwdriver in the end of the nipple and contact the pin inside. Turn the screw out 2 turns, hook the hose up and test drive.
If it gets better try another turn.

Re: ****ing at high RPM
 

Is that clockwise or counterclockwise?
Will that tell me if modulator is working correctly?

Re: ****ing at high RPM
 

I did the quick test, and it holds vacuum.

Re: ****ing at high RPM
 

It was difficult to find the pin inside and was hard to turn, but eventually I managed to turn it counterclockwise maybe 2.5 turns. Ill conduct a test drive and see.

Re: ****ing at high RPM
 

My truck has electrical choke with direct 12v, but still the choke light is on.

Re: ****ing at high RPM
 

What carb do you have?
Is it the stock quadrajet?

Re: ****ing at high RPM
 

That’s correct 👍🏻

Re: ****ing at high RPM
 

Your direct 12v, is that factory wiring or a seperate new 12v supply wire?

Re: ****ing at high RPM
 

Back of the transmission right hand side is a steel cover plate with 4 bolts 1/2'' socket or wrench. Remove those 4 bolts. There will be a little bit of oil that comes out ---maybe a a cup at most. Pull the governor out --it should remove easily with a small twist--and look at the plastic gear. You can tell if it is in good condition or not. If it looks OK, you can move on to something else. If not...... Sometimes the rear bushing in the case will walk out and catch the nylon gear and strip it out. If the gear is stripped out, you'll need a new gear of course and pull the transmission out and gut it so you can locktite (green) and lightly stake a new bushing in. Then you can reassemble or rebuild at the same time. The governor controls shifting in the transmission and the vacuum modulator will fine tune it.

Re: ****ing at high RPM
 

Nothing still doesn’t not shift,

Re: ****ing at high RPM
 

I’ll try, I’m not a mechanic, but I’ll try 🫡

Re: ****ing at high RPM
 

It’s a factory wiring
It’s a a single red wire that goes to the top left side labeled as ignition (IGN)

Re: ****ing at high RPM
 

The wire on the side has nothing to do with the transmission up shifting. That wire powers a small solenoid for kick down and only has power when really into the throttle. You don't have to be a mechanic to remove 4 short 5/16'' bolts and look at the governor. At the most you may damage the gasket when removing the cover. The gasket could be either a rubberized cork or a thickish paper.

EDIT: Another thing: when you get the governor out and the plastic gear looks OK, hold the governor by the gear with one hand vertically and flip the weights with the other and watch the small valve on the inside and see if it moves smoothly and does not stick. This can cause erratic shifting also.

Actually post up a pic of the governor when you get it out

Re: ****ing at high RPM
 

Much appreciated 👋🏻
Aha I see,
I’ll try to do it and post pics when removed

Re: ****ing at high RPM
 

X2 on the governor the 400s are good transmissions

Re: ****ing at high RPM
 

I managed to remove the governor, the bolts weren’t tight, no gasket, oil came down a lot more than a cup, clean & clear.
Did what you said “ hold the governor by the gear with one hand vertically and flip the weights with the other and watch the small valve on the inside and see if it moves smoothly and does not stick” initially I didn’t see it move then it worked fine, the gear teeth’s are good, some wear on the cylinder below the gear and the lower one.
Pictures attached

Re: ****ing at high RPM
 

I’m trying to upload but I get a msg saying I missing a token informed admin, I did.

Re: ****ing at high RPM
 

[QUOTE=Captain J;9267100]I managed to remove the governor, the bolts weren’t tight, no gasket, oil came down a lot more than a cup, clean & clear.
Did what you said “ hold the governor by the gear with one hand vertically and flip the weights with the other and watch the small valve on the inside and see if it moves smoothly and does not stick” initially I didn’t see it move then it worked fine, the gear teeth’s are good, some wear on the cylinder below the gear and the lower one.
I tried to upload vids but couldn’t, pics attached

Re: ****ing at high RPM
 

[quote=Captain J;9267106]