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Transmission shifting back and forth at constant speed

jeff in co

jeff in co

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I've noticed over the past few days that under light pedal, cruising at constant speed, my transmission is shifting up and down every 10 seconds. If I get on the gas, it's fine but as soon as I just keep a "maintain speed pedal pressure", it shifts back and forth. It looks like the rpms are changing by about 300 rpms so I'm guessing it's the torque converter engaging/disengaging.

When I drop it down to "D", it still does the same thing.

I had the transmission serviced, drained, flushed, new filter and synthetic fluid added about 2 months ago.

Another thing that "might" be causing this is my speedo is jumping around a lot! I know that there is the eye behind the speedo for the computer/cruise so perhaps it's getting some strange readings and can't quite determine how fast the truck is going....hence the constant shifting. I just don't know though. :dunno:

I did replace the speedo, the speedo cable and lubed the cable.....but it still jumps around quite a bit. When it was cold this morning, it was not jumping much.....just when it gets warmed up.

Any thoughts? TV Cable perhaps??
 
What transmission do you have? I have the 3 speed auto th400 and kinda had the same problem. It ended up being a cheap fix. Vacuum trans modulator was bad.
 
Sorry, 700r4. 1989 Truck. When they serviced the tranny, they said it looks great and saw no issues.
 
Might want to adjust your TV cable, mine was doing that because with even light pedal, the tranny wanted to kick down into a lower gear.

Sent from my SAMSUNG-SGH-I727 using Tapatalk
 
i had that problem when my modulator went out.its like 15 bucks to replace.i would check the tv cable as well.both tell the trans when to shift.
 
Monitor the throttle position sensor, the position of which signals the computer to apply or interupt the ground that controls TCC solenoid operation. If it is steady , it could possibly be the speed signal problem you are having confusing the computer
 
In 1989 the ECU controls lockup, right? So you could have an erratic MAP or TPS signal causing this.
 
this happened to me in my old k5 I had. It was a TH350.

it had erratic shifting like you describe..

I couldnt figure it out. so I just drove it around. for a while
then it ended up binding up inside. something went wrong.

i was driving then just hear metal grinding. like gears .. it wouldnt come out of first after that.
 
I called the tranny shop and asked him about the bouncing speedo as possibly contributing to the transmission shifting around. He said it could be possible that the gear for the speedo within the transfer case was worn.

So, I disconnected the cable and put it on a drill. Everything from the cable forward to the speedometer was smooth pointing to the transfer case gear as being worn.

I think I'm going to change this out and see if that helps. How do I go about removing this and will t-case oil drain out if I remove this cap? I was thinking to first remove the bolt and then maybe use some pliers to turn the large cap (is this threaded into the case??)

thanks for any tips. As an fyi though, the tranny guy was concerned that if it didn't fix it, I am putting the tranny in jeopardy so I do need to fix this issue.

DSC_0402.jpg


DSC_0401.jpg
 
It's easier than you think. The whole gear housing just slips in there on an O-ring. Remove the retainer bracket and give the housing a little twist/pull and it will pop right out. Easy to do on the truck. A little fluid may come out, but it's no big deal. Top off when you're done.

At first I was thinking it was unlikely for the converter to lock/unlock based on speed. It only has to unlock when vehicle speed gets close to 0. Otherwise it's based mostly on throttle position, engine speed and vacuum (i.e. MAP). So if vehicle speed is going down that much, wouldn't it also be shifting? But then I remembered that the ECU doesn't actually control the shifting - only the TCC. Beyond that, it doesn't even know what gear you're in. Sure it's impossible to go 70mph in 1st gear, but the ECU doesn't know what axle ratio, transmission ratios or tire size you have or attempt to calculate - it only tries to choose the best times to lock and unlock. I believe the "unlock on shift" is internal to a 1989 transmission anyway.

This 10s period you describe also seems significant, like there is a circular cause and effect going on. Could there be a leak in EGR that reduces vacuum when it runs causing the converter to unlock?
 
Blue85 said:
It's easier than you think. The whole gear housing just slips in there on an O-ring. Remove the retainer bracket and give the housing a little twist/pull and it will pop right out. Easy to do on the truck. A little fluid may come out, but it's no big deal. Top off when you're done.

At first I was thinking it was unlikely for the converter to lock/unlock based on speed. It only has to unlock when vehicle speed gets close to 0. Otherwise it's based mostly on throttle position, engine speed and vacuum (i.e. MAP). So if vehicle speed is going down that much, wouldn't it also be shifting? But then I remembered that the ECU doesn't actually control the shifting - only the TCC. Beyond that, it doesn't even know what gear you're in. Sure it's impossible to go 70mph in 1st gear, but the ECU doesn't know what axle ratio, transmission ratios or tire size you have or attempt to calculate - it only tries to choose the best times to lock and unlock. I believe the "unlock on shift" is internal to a 1989 transmission anyway.

This 10s period you describe also seems significant, like there is a circular cause and effect going on. Could there be a leak in EGR that reduces vacuum when it runs causing the converter to unlock?
Click to expand...

Thanks for the tip. Sounds pretty easy to remove/install.

Glad to hear that the ECU only controls the torque converter as it feels like it's just the converter going in and out. Shifting from OD to D.....still same converter reaction.

I parked it today and will try and find a new gear here locally. If not, may need to order online somewhere.

I have recently replaced quite a few parts (last 6 months) EGR, IAC valve, TPS, etc. I also pulled the vacuum tube from the canister after sitting for several hours and as soon as I unplugged, I heard air escaping. So I don't think I have much of any vacuum leaks. All new tubing too. Also, if the needle was jumping around too much, the cruise wouldn't engage. Once the needle settled down at a different speed, the cruise would engage.
 
One other remote possibility. Most all the lockup systems drop out when you hit the brakes.
Works off the brake light switch.
If it were faulty or out of adjustment, it might cause the converter to go in and out of lock.
 
Try disconnecting the detent cable and take it for a spin. If that fixes the issue, just readjust the cable right behind the mount. Worst case scenario replace the cable if it's stretched.

Good luck
 
cameronsaddress said:
Try disconnecting the detent cable and take it for a spin. If that fixes the issue, just readjust the cable right behind the mount. Worst case scenario replace the cable if it's stretched.

Good luck
Click to expand...

I did disconnect the speedo cable last night and put it on a drill....results were no bouncing. I also drove around with the cable disconnected and I did not feel the converter going in and out. So hopefully, this is the problem.

I found this site for new gears:

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

A good write-up with gm part numbers:

http://www.73-87.com/7387garage/drivetrain/speedo.htm


Also, found some alternate sources as the transmissioncenter is falling out of stock on some of the gears:
http://atransmission.info/gm-transmissions-parts/700r4-parts/700r4-speedometer-kits-and-gears.html
http://www.performanceautomatic.com...ries/speedometer-gears/speedometer-gears.html
http://paceperformance.com/i-7134410-8640518-f-gear.html
 
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If you drove around with the speedo cable disconnected, then the ECU would always see 0 mph, so it would never lock up. I don't think this test is conclusive.

If your gears look good, you might also check out the VSS unit. There is a optical sensor inside the speedometer that counts the revolutions of the cable and sends it to the ECU as vehicle speed. This is also where the cruise control gets the speed from. If this unit is damaged or the wiring is bad it could cause these problems as well. If the cruise control is working and the TCC is still cycling then the problem is probably somewhere else.
 
He meant the speedo cable and it seems Jeff understood that. But it is a good idea for anybody else following this thread to point that the T/V cable must be installled on a 700R-4.
 
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