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]

Re: Shifting at high RPM
 

Is that modulator getting vacuum?

Re: Shifting at high RPM
 

Yes

Re: ****ing at high RPM
 

I did what you suggested, I posted pictures but couldn’t upload videos.
The gear is in good condition,

Re: ****ing at high RPM
 

Did what you suggested, initially the valve seems moving slowly but with repeated movement of the weights it did move, I noticed some scratches on the surfaces, in the pictures, cleaned it with brake cleaner.
I’m thinking of buying new governor and vacuum modulator, however I’m not sure what brand to buy. Any recommendations? 🤔
Note I live 5000 miles from USA, so I must make sure I buy the correct part.
Thanks 🫡👋🏻

Re: Shifting at high RPM
 

If it moves, there is no reason to waste money on another one. Cleaning it out was good. If anything buy a weight kit and an extra gear for it if you anticipate issues in the future. There had to have been a gasket on there, if there wasn't there would nave been a huge leak while driving. I'd pickup a couple of gaskets also. As to the shifting?? I don't know, if the valve was sticking somewhat and is now loose. I'd put the governor back in and try driving it and see what happens. If it still won't shift, there may be a sticking valve in the valve body under the vacuum modulator. See if you can pull it out with a magnet. What condition is the oil in....nice and clean?

Re: Shifting at high RPM
 

Greetings
There was no leak at all prior to removing it, no gasket just silicon. Oil is new and clean.
I’ll put back, see if ok,
Regarding your suggestion of fishing the valve, how do I pull it out?

Re: Shifting at high RPM
 

One possible thing that might cause a shifting issue is if the o-ring on the filter tube is damaged, worn, or missing.
Amazon link that shows the o-ring as part of a kit: https://www.amazon.com/ACDelco-TF231...2935891&sr=8-3

I noticed that some kits don't show the o-ring. It is needed or you won't get good fluid pickup and will have symptoms similar to low fluid levels. Including bad shifting.

#7 on the diagram shows where it goes. Another thing some folks miss is removing the old one if it sticks in the opening when you pull out the tube #14.

I would finish your check of the modulator and related stuff before pulling the pan. Pulling the pan probably means replacing the pan gasket + 4~7 quarts of ATF. No point in doing that until you know the problem isn't in the modulator you already have off.

Re: Shifting at high RPM
 

the problem may stem from the silicone used to seal the gov cover. An automatic transmission is no place to use silicone. A small chunk of it will get in the governor and case it to stick.

Re: Shifting at high RPM
 

I totally agree. However, This truck never had any issues with trans since I owned (2015), it parked for 3 months due to paint work. Otherwise all ok.

Re: Shifting at high RPM
 

You are correct,
This is not the issue, otherwise shifting problems will happen after the oil change.

Re: Shifting at high RPM
 

This is a huge issue ^^^^. You may have already dis-lodged the culprit, but A small piece could have lodged itself anywhere in the valve body. Did you try to pull the valve out behind the vac modulator? It's about a 1/2'' in diameter and about 2-3'' long. Magnet will be useful

Re: Shifting at high RPM
 

[Quote]...... Another thing some folks miss is removing the old one if it sticks in the opening when you pull out the tube #14.

If the tube butts up against it ..yes. But you can actually double up the o rings if you have the deep pan to help retain the extension tube.

Re: Shifting at high RPM
 

Greetings to all,
Sorry for late response. I was sick (corona or flu) not sure.
Again, trans oil is new and clean, drove it less than 200km since last oil & filter change, all GM products. No silicone inside the governor housing. When replacing the old filter, I made sure the old o-ring is removed, the new o-ring was lubricated with trans oil, then it was fitted in the filter neck, then it was placed inside the housing. Truck drivers normally.

Update 12/23/2023 8:30pm my time.
I cleaned the governor tested the valve it works fine, made a gasket from gasket paper, put it back, added new oil to replace the spilled oil. Drove the truck, no change, it will not shift.
I’ll try to fish the valve body behind the vacuum modulator, and see.
Otherwise I’m out of ideas.

Re: Shifting at high RPM
 

He may have to pull the valve body Something is stuck somewhere, 400s just dont give problems as a rule

Re: Shifting at high RPM
 

I agree. I think he is going to see if the valve under the vac modulator is free first, that could be sticking in the case