HELP!!!! 91 GMC Tranny trouble

[andyvillen]

My 91 GMC 2500 Sierra with the 4L60E or 4L80E tranny is acting up. what it is doing is the Check engine light comes on and the Torque converter unlocks, then the light goes out and it locks back up. sometimes it does this off and on every few seconds... but mostly when its warm the engine light will stay on and it wont lock up.

Does any one know what this could be,??? and how to tell a 4L60 and 4L80 apart?

And By the way im new to the site.

Thanks.

[jogl]

I don't know the answer to your first Q. Have to leave that to the experts.

A while back somebody directed me to this site for ID'ing tranny's:

http://www.maliburacing.com/auto_tranny_id.htm

Good Day,

JP.

[andyvillen]

OK so its a 4L80E tranny? Why does the Check engine light come on when there are no codes???

[Subzilla]

Not intending to insult you but make sure it's up on trans fluid! My old Blazer TC was shifting in and out when it was quite low. The transmission leak had gotten worse and I didn't know just how bad. I realize my 700R4 doesn't have the computer stuff that yours does so I don't have the check engine light.

[john8662]

This is very likely a failing TPS (Throttle Position Sensor) on the injection pump, or the TPS is out of adjustment/voltage.

There is an article here in TDP that covers checking and adjusting the TPS, which works for BOTH the TH700/4L60E and the 4L80E transmission.

http://www.thedieselpage.com/members/features/tps.htm

and...

I'm moving this tread to the drivetrain forum...

[andyvillen]

For some reason i cant access that page? it asks me for a password.

[john8662]

The link is indeed secured, it requires TDP formal membership, which is not the same as forums.

Much of the same information is available on the forums, but in various help threads posted by other members.

Try performing a search using the search option in the blue bar above for 'TPS' or Throttle Position Sensor or Switch. You'll do best by performing an "advanced" search and only searching the 6.2, 6.5 or drivetrain forums for the information.

Hope this helps.

[andyvillen]

I did the ol Paper clip trick and looked for codes... can you only ground out the diagnostic test terminal to check for codes?

What does the Serial data one use.... and what does A.I.R and T.C.C stand for on the plug.

can i ground out the serial data and get a diffrent codes????

The problem im having is my Service engine soon light comes on.. but there is no codes stored in the computer?

[Robyn]

If the SES light is coming on for any length of time there should be codes stored.

As was mentioned the TPS could be bad, Or it could be another sensor thats bad and a temperature issue.

Get a cheap code reader and plug in to the ALDL connector to read the codes.

Just in case you are not familiar. To read the codes, jumper the far right two terminals of the ALDL connector top row.
Turn on the key to run only (engine not running) The SES light should come on and then start to flash. It will flash one set of numbers then pause then flash a second set. Example 1 flash followed by a pause then 2 flashes This would be a 12 (means system ok)


Again if the ses light is coming on there will be a code stored.

If its the TPS its not a biggy to change.
One plug and two bolts.

You can reference off the bolt marks on the old one to align the new one close enough to get you going.
The bolts go through two slots in the TPS and the washer heads will leave a round mark that you can measure off of and then locate the new TPS.

Let us know

Robyn

[andyvillen]

yes i have checked for codes by jumping the Diagnostic terminal to the ground on the ALDL.

All it does is Flash 12. no codes stored. i have a big service manual that some one photo copied. I am going to check the TPS tomorrow for resistance.

The problem only happens when its warm, when i first start the truck in the morning it wont come on tell about 5 mins in. then the light starts to go on and off.. and when it goes on, the torque unlocks.. then the light goes off and the torque locks back up... does that a hand full of times then the light just stays on for the rest of the day even if i turn off the truck and start it or even leave it for a few hours it will still be on.. only when i leave it on over night the light will go out.

the TPS is like 150 bucks, so i dont want to change it with out knowing that its the tps for sure.

[DmaxMaverick]

Sounds like a temp sender is losing communication (failed/intermittent sender or harness/connector), or a PCM issue. If the PCM senses a low operating temp of either the engine or tranny, it will keep the TCC unlocked. If 2 or more senders don't agree with premapped conditions (such as fuel burned compared to time, load and distance), the SES will come on. Depending on the condition(s), it may or may not set a DTC. The DTC may only be present during the actual fault, which you can't read w/o a Tech tool or real time scanner (Autotap, etc.). A serial data code scanner (Actron, etc.) can read codes on the fly, but not real time data. If the TPS faults, it should set a soft code (class B or C) that doesn't immediately evaporate. If it is the TPS, shift normality will also be effected, not just the TCC.

[andyvillen]

So weird thing happened last night, I was driving home on the highway and going about 80 kph and the service engine light went out and it went into torque lock up. It was good tell i got home and let off the fuel for a bit.. after it went back on i stoped and put it in neutral and reved it to about 1200 rpm and the light went out. and off again when i let off of the pedal. then after that for a bit it just stayed on.

[diesel65]

Check the engine speed sensor mounted on top of the oil pump drive, which also drives the vacuum pump.
Inspect the plastic cover and you may find a big crack.

GM had a service bulletin for a phantom check engine light, when you try to pull any codes you would only get code 12 which means everything is ok.

I don't remember the details of the service bulletin.

[andyvillen]

ok , i'll check that tomorrow

[Robyn]

I am thinking like Maverick on this one.

There is a temp sensor in the pan of the tranny (On the VB) that senses tranny fluid temp.
This sensor can and usually keeps the tranny from lockup until it reaches normal temp.
The coolant temp sensor located up on the water crossover will also cause the issue.

Try replacing the coolant temp sensor up top and the issue may go away.

You can (If you are quick) unscrew the old one and pop in a new one without draining the coolant.

The 700 tranny had a sensor on the VB. The 4L80, I am not sure about.

Anyway the sensor will cause the issue. If this is the only thing thats happening it very well may be the problem.

Without a tech tool to scan real time data from the rig while running its a guess.

Keep us posted

Robyn

[andyvillen]

So i changed the TPS and the check engine light is still on,

still no codes in the computer,


I have noticed that the tranny dosnt shift down to 1st gear when i come to a stop. then when i go to take off, i give it some gas and it shifts down.

Any one have any other ideas?

[JohnC]

Get a scanner on it and find out what's triggering the check engine light. Prior to '94 there weren't many engine conditions that could trigger the light. I think EGR may be one.

[andyvillen]

Vin # J No EGR stuff,

some times when i drive it, the check engine light will go out when im crusing down the highway. then when i let off the gas it will come back on, If i press on the gas just a little it will go back off. and so on.

[Robyn]

Sounds like an EGR issue and is probably related to the vacuum pump not working right.

Check to make sure you have Vacuum at the EGR solenoid (coming from the pump)

Otherwise the solenoids may be bad

[andyvillen]

where would i find that?