1998 GMC transmission problem?

chopped · April 03, 2009, 12:35:14 PM

I can't find anyone around here who can figure this out. From 55 to 65 mph the tach gains 2-300 rpm and goes back down. Over 65 it runs fine. A 98 GMC 4X4 auto. Trans rebuilt 8000 mi. ago. Trans shop says it's not the trans.Repair shop says maybe fuel pump (replaced 1 year ago) Said it had 55# at idle, sounds OK to me. Any thoughts?

Bruce Dorsi · April 03, 2009, 01:12:13 PM

Quote from: "chopped"I can't find anyone around here who can figure this out. From 55 to 65 mph the tach gains 2-300 rpm and goes back down. Over 65 it runs fine. A 98 GMC 4X4 auto. Trans rebuilt 8000 mi. ago. Trans shop says it's not the trans.Repair shop says maybe fuel pump (replaced 1 year ago) Said it had 55# at idle, sounds OK to me. Any thoughts?

Converter locking and unlocking under light load cruising? ? ?

It seems about that's how much lock-up affects the rpm's on my '98.

chopped · April 03, 2009, 02:34:53 PM

Quote from: "Bruce Dorsi"
Converter locking and unlocking under light load cruising? ? ?

It seems about that's how much lock-up affects the rpm's on my '98.

My thought too, that's why I went to the trans shop. He reads the codes and says it's not the trans. He could be wrong. Does the lock-up disconnect at one of the connections on the side of the trans? One has a couple wires the other six or so. If I disconnect it wouldn't that prove the lock-up theory? The dealer(import) where the wife works put a throttle senser on and gave up. Seems I can't even pay someone to fix it.

Crosley.In.AZ · April 03, 2009, 09:45:20 PM

Quote from: "chopped"
Quote from: "Bruce Dorsi"
Converter locking and unlocking under light load cruising? ? ?

It seems about that's how much lock-up affects the rpm's on my '98.
My thought too, that's why I went to the trans shop. He reads the codes and says it's not the trans. He could be wrong. Does the lock-up disconnect at one of the connections on the side of the trans? One has a couple wires the other six or so. If I disconnect it wouldn't that prove the lock-up theory? The dealer(import) where the wife works put a throttle senser on and gave up. Seems I can't even pay someone to fix it.

you have a 4L60E trans in that 1998 vehicle... you do not simply disconnect the lock up wires.

What you must be looking at is the MLPS on the shift linkage. MLPS = Manual Lever Position Sensor ... couple of plugs on it

Control wire harness is on the passenger side of trans case , round plug about 1.5 inches in diameter

wayne petty · April 03, 2009, 09:49:40 PM

the converter locking and unlocking will not set a code... unless it is broken it won't set a trouble code..

there are a bunch of various things that will cause the converter to cycle on and off at cruise... you may even need to have your computer reflashed with an updated program...

do you have many 96 and newer cars..

might pay to invest in a OBD2 scan tool... the price seems to have gone up on this one a few bucks..

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=98614

but i have one and it works really well... i have not connected to any gm transmissions with it... on tuesday a 98 safari mini van is coming over so i will test it on that... i know with snap on scan tools you can read the converter on and off signal..

there are a bunch of code readers... but you really need to look at live data... and graph some of the sensor voltage outputs to see whats going on..

just having an obd 2 scan tools makes sense... most shops charge 50 to 80 bucks to scan a code. (professional scan tools start at $2,500 and go upwards of $8K) dealers.... if there are any left... charge even more than that.. i know several mechanics that are parts changers... sets a code.. throw parts at it... till you fix it... it gets expensive for the customer or the manufacturer if it is still under the warrantee ...

disconnect the battery.,,, clean and wire brush both ends of the ground cable.. bad grounds are a huge problem... and most really good drivability techs start there... takes 10 to 30 minutes... depending on the car.... even if it looks clean... use that stainless steel tooth brush to clean the ends after removing the bolts that hold them in place...

if you can see where the ecm harness grounds to the motor clean that one also... the ecm only grounds through its own harness... and only to the motor... not to the body like most people would think...

would you think that a light bulb with a dirty connection would work properly.. how about plugging your home computer into a loose outlet.. its just not enough ...

why don't you PM me your year make model number.. plus the. 3rd, 5th , 8th and 10 digit of you vin number...

chopped · April 04, 2009, 07:20:38 PM

Think I have an out. A friend of a friend has a trans shop and is trading some carpenter (which I retired from)work for whatever the truck needs. He's doing the truck next week then I'll make it up in work on his home . Works for me, I'll let you know how it turns out. Thanks to all who helped but this probably was going to be past my comfort level.

wayne petty · April 04, 2009, 07:30:21 PM

it still could be a tiny problem.....

clean the battery cables... both ends if you can....

chopped · April 05, 2009, 08:30:47 AM

Quote from: "wayne petty"it still could be a tiny problem.....

clean the battery cables... both ends if you can....

I liked that he said I should try that cause I may not need him. Didn't help.I did see the four year battery was five years old. Replaced it last night.

wayne petty · April 05, 2009, 06:58:38 PM

just to let you know.... that the torque converter clutch solenoid is controlled by the ecm.... via a torque converter clutch solenoid... there may be a pressure switch on the valve body also... that prevents it from coming on unless it is in the right gear... it is also shut off by one of the brake light switches...

(brake light switches have 2 or 3 sets of wires to them... brake lights, cruise control and torque converter solenoid circuit.. ) the converter clutch solenoid power is supposed to be cut when the brake pedal is pushed slightly..

there maybe throttle position switches on the throttle pedal also... inside the truck...

so many things it could be... without looking at live data... its hard to tell..

i was searching for a data list of normal operating voltages usually found on gm cars that are working well...i have not found it yet....

Danimal · April 05, 2009, 09:02:32 PM

Could this solenoid making the lock up go in and out produce a 'shudder' condition where it feels like you are driving on spalled concrete? Downshifting kills it immediately. I'll have to try the brakes tomorrow. It doesn't happen all the time but right around 55-60 mph in a 2004 Trailblazer AWD 6 cyl.

Sorry to * the thread...

wayne petty · April 06, 2009, 01:45:31 AM

as for the shudder.,... there were several problems that could cause that..

this link is what i was looking for... it is what you should expect to see when scanning a gm car and reviewing the live data...

http://econtent.autozone.com:24991/znetrgs/repair_guide_content/en_us/images/0900c152/80/0a/7f/e5/large/0900c152800a7fe5.gif

one might want to print that image... and keep it around someware...

it will come in handy...

it does not show the shift info..... i am still working on that... the 98 astro is out in the back right now... i plan on playing with it monday... along with its first water pump at 218,000

chopped · April 06, 2009, 07:33:14 AM

Quote from: "Danimal"Could this solenoid making the lock up go in and out produce a 'shudder' condition where it feels like you are driving on spalled concrete? Downshifting kills it immediately. I'll have to try the brakes tomorrow. It doesn't happen all the time but right around 55-60 mph in a 2004 Trailblazer AWD 6 cyl.

Sorry to ##### the thread...

Come on in, my truck was doing the same thing. I was going to get the wheels balanced, then figured it was tied into the trans.

Crosley.In.AZ · April 06, 2009, 08:41:33 AM

the photo posted is not the TCC solenoid. Not the correct type, shape or color.

In a 4L60E that would be the 3-2 solenoid of 1997 and newer 4L60E.

The 1998 would have the lock up solenoid that plugs into the pump and a PWM solenoid that would be a dark gray color or black color plastic. The white solenoid is a different ohm rating that the PWM solenoid.

You may also have a worn valve body in the area of the PWM valve assembly , this is very common. This also sets a DTC and turns the check engine lite on. A code P1870 is very common if the TCC ( torque converter clutch) is slipping too much.

PWM = pulse width modulated

Danimal · April 06, 2009, 10:50:04 AM

No trouble codes on mine. Just goes into a nice resonance that shakes the whole car and feels like the wheels are coming off. Sometimes it stops on its own, other times I hit the brakes or downshift. I drove back from CT this weekend and it is now doing it at higher than 55/60.

At 222k I suppose it could be time for some work to it but I don't want to do that!

Danimal · April 07, 2009, 09:45:35 AM

Messed with it on the way home last night. Certainly acts like it stops as soon as you step on the brake.

Is this something that requires a tear down to change?