Hello,
I have an 85. I bought it 12-14 years ago. Pretty early on at the 75,000 mile marker, the trans blew. I only had 1st and reverse. It was rebuilt with a new torque converter and shift kit. The trans has worked perfect all these years. It now has 150,000 miles on it. Usually, with a light throttle setting ... it shifts through the gears and at 38mph, it shifts into OD. I guess this was a fuel economy innovation and it worked well.

It started hunting for OD and recently it has gotten worse. At 38mph, it will shift into OD, then down to 3rd, then up ... then down ... then up ... then down ... while accelerating up to 50 or so. It should be rock solid in OD at 50 but even at 55/60 it hunts.

What controls the OD??? Is it (fingers crossed) vacuum??? Wouldn't that be nice. It can't be the computer - computer control was introduced in ??? 1991 when the 700R4 was upgraded to the 4L60E - I believe.

I have the haynes and the chapter on automatics is about a paragraph long and says ... this is very complicated ... take it to chevy service department or something like that.

I'd appreciate if anyone had some info to pass on.

Thanks,
Tim

 

Overdrive is 4th gear. The ecm has no control over it, vacuum is also not part of the equasion. Nothing on a 700r4(4l60) is vacuum controlled.

4th gear happens when the 3rd gear clutch pack is applied, and the band clamps the reverse input drum. Sadly, your book is correct. The trans is the most complicated piece in the car and diagnosing , troubleshooting and repairing is almost unheard of for the average guy.

 

Pete,
Thanks for the info. So, the problem is in the box! That's the bottom line? I was hoping that wouldn't be the case.

You know, I always thought it was a 3spd with OD. But, you're saying its a 4 spd with 3rd having a 1:1 and 4th being 0.7:1. And, the difference being (what???) the way 4th is engaged?
Thanks again,
Tim

 

When 4th is engaged, the band clamps the rev input drum, which in turn manipulates a planetary gear set to achieve the .70 ratio.
Shooting from the hip, you may have a 4th servo problem, a weak 3-4 clutch pack, or a glazed or damaged band. You are correct that it is likely in the box though

 

Are you sure it's going in and out of overdrive, or in and out of converter lockup?

 

That's a good question. Don't know!

Let me tell you a couple things thats it's doing and maybe you can tell me. But, this is a driver and I drive it - fast at times but its just driven on te street. I drive in traffic but I don't recall that this car has ever gotten real hot. It's never overheated.

If I have it in (OD), it will start in 1st and shift normally up to 4th (at 38mph) and then it seems like it is floating and not in either 3rd or 4th ... kinda like on a downhill where you back off the throttle and it disengages and coasts. As I continue to accelerate, it doesn't stick in either 3rd or 4th. It kinda goes back and forth. If I shift into 3rd, it will stick and it will continue to gain speed ... maybe up to 55 where I shift into OD and it kinda is in. But, it still seems like it is floating between gears. If at that speed 55/60 in OD, I enter a slight downgrade ... and step on the gas (while on the downgrade) it rev's - it don't go! - it is disengaged. It rev's as in neutral. If I get to the bottom of the grade and then get onto the upgrade and give it gas, it will engage and pull (drive/go). Now that I think about it, last year, if on a similar downgrade, there were a few times that as I stepped on the throttle, the car shuttered as if it didn't know whether to be in 3rd/4th and then engaged 4th. All this seems to be happening in/around 4th gear - so I assumed it was the OD.

If I drive around in 3rd, it seems to work ok (... other than high rpm and lower gas mileage).

Action Plan:
Prior to your email, I was going to pull the pan, replace the filter and top it off. Maybe add some kind of Marvel Mystery Oil or Auto Trans something or another additive depending on the condition of the filter.

I appreciate any suggestion.

Thanks much

 

It does sound like the tranny is wanting to go out on you. It is worth following up on the torque converter lock that was suggested earlier. On my '94, it had similar symptoms and it was the converter that was not locking consistently. Long story short is the brake pedal switch was out of adjustment which would effect the torque converter lockup as it unlocks when the brakes are pressed. How I narrowed it down was to use my foot while driving to briefly pull the brake pedal upwards and it would immediately lock, when I released it, it would unlock again.

Hope this helps,
Ernie

 

Tim I have a 85 that I bought in 1992, still love the car even though I bought a new 96 LT-4 and drive them both. Anyway I hated the 85's automatic shifting. In city driving when put in OD positon or even in 3rd positon the torque converter would lock up under 45 miles per hour. I have come off the interstate not using the brakes, never touching the brakes in OD and using very light touch on the gas pedal the car would pull from 700 rpms in OD gear and torque converter locked up, really something. In city driving I would drive in 2nd for stop and go driving so it would not shift to 3rd and have the toque converter lock up below 40. Even in 2nd gear the toque converter would lock up. Anyway I fixed my annoyment with a simple on/off electrical switch. Like a previous poster has mentioned, tapping the brakes will unlock the torque converter. I used wiring from the electrical switch I mentioned and wired to the wire coming from the transmission to the stoplight switch. Tucked the small electrical switch in the carpeted area of the dash bottom to the right of my right leg. With this switch off, the torque converter will not lock up. I leave it off until I am on a highway or interstate, around town it is always off. I have read that there was a TSB for another chip available from GM years ago for this problem that would prevent the torque converter from locking up so early. Don't remember which wire I disconnected from the stoplight switch, but was an easy change/fix for me. My 85's original transmission still functions great and now has about 102,000 on it.

 

Hello Ernie,
I tried to lift the pedal and there was no change. When I was trying to lift the pedal, I remembered that I had a dually that I had put a gear vendor overdrive in. It had a 4spd od trans, and the gv gave it a second od. So, it would cruise at a real low rpm. It had a 454 with plenty of power and 4:10's. With both od's engaged, the rpm dropped to something like 1300 rpm. It would roll down the highway and just rumble like a train. Anyway, the gmc has terrible front brakes. I think they were adapted from a different vehicle. I had to put money into the brakes every year (a real pos). My truck had been running pretty good, and then it started with the upshift/downshift thing. It had to be going 70 to stay in double od. I eventually found out that one of the front calipers was dragging and that was the problem. Anyway, fyi - thanks for the suggestion.
-Tim

 

ZZ, Thanks for the note. You've taked me to a place I've never been - the land of torque converters and I've very confused. I thought that you wanted the torque converter locked up and that it was a good thing. I thought it unlocked at idle so you wouldn't be putting power into the trans (power converts to heat - bad thing). Can you clarify? What should I be trying to look for to see if the torque converter is locked/unlocked?

Re: the switch - I'm not getting it. What do you want to do? Do you want to insert a switch into the brake wire path and leave it open around town ...no signal) - then the converter will not lock.

If you close the switch and are on the highway, the torque converter locks. ???

Did I get that right?

Now, why don't you want it locked around town? Is it because the trans upshifts to the highest gear (for better mileage and lower performance).

 

Tim I use my installed switch so that I can manually control when the torque converter will be allowed to lock up. Normally for city driving with speed limits 30-45 I leave the installed switch off. If I want it on, just turn it on and I see the rpms drop when it locks up. My 85 is more responsive in traffic without the torque converter lock up. The switch does not connect to the brake light electrical wiring. The brake light switch under the dash has a separate activation for the brake lights and a separate wire to another switch that will unlock the torque converter lock up when the brake pedal is applied. The brake pedal operates both switches. You don't want to connect to the brake light wiring. All that you are really doing when you install the switch is really bypassing the original factory installed switch that would unlock the torque converter when the brake pedal was applied and giving you quick manual control of a new switch that can permit torque converter lock up when you want it. For maximum economy you will want the torque converter lock up on highway driving. To drive without the torque converter locking up you can just disconnet the wiring to the switch and see how you like it and then install the hand controlled switch.

 

ZZ, thanks a lot. The only thing I ever noticed was shift points and how slow/fast/crisp it was! I'll try that today and see what it does.

Question: What is the mechanism that controls torque converter lockup? Is it centrifical force, speed, engine vacuum, hydraulic pressure, ... or in other words, how does the signal control lockup. Is this stuff discussed in the chevy service manual?