91 M30 Shifting Issues

[91M30]

My 1991 M30 has a rather harsh 1-2 shift when cold, but does the 1-2 fairly smooth when warm. It is also always slow to do a 2-3 shift or to lock up into overdrive unless you back off on the throttle; i.e., if throttle is held steady to hold RPM at 3000, it will not shift unless you back off very slightly on the throttle.

I've checked the TPS functionality and adjustment per the manual and it appears to be within the resistance range and voltage output ranges specified as currently adjusted.

The car only has about 62,000 miles. I've done a transmission flush and filter change, but it didn't change the symptoms. Any ideas as to what could be the cause? Speed sensor?

[Q45tech]

The shift pressure is varied by the readings of a thermistor in transmission to correct for viscosity changes in ATF.

Lots of tests to gauge health in FSM.......with Consult.

[91M30]

The shift pressure is varied by the readings of a thermistor in transmission to correct for viscosity changes in ATF.

Thanks for the response, Q45tech. After reading your response, I took a second look at the symptoms chart in the manual.....and it does indicate that a faulty fluid temperature sensor could cause hard 1-2 shifts and and problems shifting from 3-4.

What is the procedure for replacing the thermistor (fluid temperature sensor)? And roughly how much should the part cost?

[M30 Steve]

Hey JWT,Good to see another M30 up and running. My 90 has almost the same trans symptom you describe. I think the hard shift 1st to 2nd when cold is typical of this trans. I bought mine at 40k miles and it has always ****ed hard when cold, when I get a couple miles on it it shifts perfectly.

I was playing with my TPS several months ago, after installing a head. In adjusting the TPS by trial and error I noticed it shifted late from 3rd to 4th (4000 rpm). I adjusted it again after a test drive and it ****ed perfectly at about 3000rpm. I used some paint on the TPS body so I was able to return to the origional setting if needed.

I thought you might try adjusting the TPS before going into the trans? Please note I am a back yard mechanic not a pro technician, but I have a few hours in working on the M30.

A note, when you move the TPS slightly you will notice a large difference when you test drive it. If nothing else you can reset the TPS back if you have marked the origional location.

Hope it helps, good luck.

[91M30]

Hey JWT,My 90 has almost the same trans symptom you describe. I think the hard shift 1st to 2nd when cold is typical of this trans. I bought mine at 40k miles and it has always ****ed hard when cold, when I get a couple miles on it it shifts perfectly.

Thanks for the feedback, Steve. Hmmm.....so that's normal?

QUOTE]Originally posted by M30 Steve "] I was playing with my TPS several months ago, after installing a head. In adjusting the TPS by trial and error I noticed it shifted late from 3rd to 4th (4000 rpm). I adjusted it again after a test drive and it ****ed perfectly at about 3000rpm. I used some paint on the TPS body so I was able to return to the origional setting if needed. ]

I thought you might try adjusting the TPS before going into the trans?

A note, when you move the TPS slightly you will notice a large difference when you test drive it. If nothing else you can reset the TPS back if you have marked the origional location. [/QUOTE] Good input, Steve.....sounds like a good thing to try. Do you remember which direction you rotated it; i.e., CW or CCW as you look at the part? Did you take voltage readings as you rotated it? The FSM says it should read about 0.5 volts closed throttle and about 4.0 volts at wide open throttle. As I remember, mine was reading a little lower than the FSM numbers, but I couldn't get it to adjust properly for both ends of the spectrum. Can anybody tell me which (closed throttle reading or WOT reading) is more important to match up with the FSM recommended setting?

I was doing some diagnostics per the FSM on the Fluid Temperature Thermistor yesterday and found:

With key off, and harness disconnected, the fluid temperature sensor thermistor resistance is about 1.8K ohms cold and about 0.5K ohms after driving the car about 15 minutes. FSM says it should be about 2.5K cold.

With key on, everything connected, the fluid temperature sensor thermistor input signal is about .75 volts cold and 0.25 volts after driving the car about 15 minutes. The FSM says it should be about 1.5 volts cold and about 0.5 volts hot.

Can anybody tell me if the readings I got are far enough off to cause problems?

[91M30]

I was playing with my TPS several months ago, after installing a head. In adjusting the TPS by trial and error I noticed it shifted late from 3rd to 4th (4000 rpm). I adjusted it again after a test drive and it ****ed perfectly at about 3000rpm. I used some paint on the TPS body so I was able to return to the origional setting if needed.

I thought you might try adjusting the TPS before going into the trans?

A note, when you move the TPS slightly you will notice a large difference when you test drive it. If nothing else you can reset the TPS back if you have marked the origional location.

Hope it helps, good luck.

Update: After reading his post, I tried what Steve suggested above. At first I rotated the TPS CCW from the factory setting until the voltage output agreed with factory settings at closed throttle (.4V-.5V)........and the 1-2 shift got much harsher and the 3-4 shift went to higher rpms, i.e., much worse.

Then I rotated the TPS CW slightly past the factory setting a little and the 1-2 shifts got smoother and the 3-4 shifts now happen at lower rpms.......so I rotated it a little further CW and it now shifts reasonably well. And the engine runs and performs as well as ever.

However, the present setting that works much better for shifting gives voltage outputs that are well out of the tolerance specified in the manual. Are there any negative effects to this out of tolerance setting (emissions, mileage, etc.) that are not obvious; i.e., what besides transmission shifting does the TPS setting affect?

Do the results indicated above indicate that I need a new TPS?

Anybody else out there have any more input?

Thanks.

[M30 Steve]

I wonder if a pro transmission shop can diagnose the problem? I recieved your post but must admit I do not have a working knowlege of the engine-trans controls. Sorry I am not more help. I can check voltage on my 90 TPS if you like.

Good luck, Steve

[91M30]

I wonder if a pro transmission shop can diagnose the problem? I recieved your post but must admit I do not have a working knowlege of the engine-trans controls. Sorry I am not more help. I can check voltage on my 90 TPS if you like.

Good luck, Steve

Thanks for the offer, Steve. Yes, it would be interesting to know the voltage output of your TPS. See my companion post re: Throttle Position Sensor Adjustment Effects