Weird TPS readings and high idle???

Hello all!
I´m a newbie from Finland and I´m havin some trouble with my 90´ Cherokee ltd with 4.0l engine and automatic tranny.

My TPS seems to be OK but when I try to adjust it to so that output voltage would be 82% from input voltage, car starts to idle high at startup. It revs at 3000 and stays there.

My input voltage is about 4,69V and if I want the car to start properly I must set the output voltage high over 4V.

IAC is just renewed and EGR is disconnected.

PLEASE HELP!

i cant recall how to set tps without the scan tool from the dealer, but the factory settings are 12-14%. if what i think is correct yours is set to 18%. but it does sound like it is failing. there was a recall back in 1990 for that problem. i dont know if you have access to a dealer to find out if it has been done before or not. it may qualify for free replacement.but it could have been done by now.

So you think my TPS if failing? Well I´ll try to adjust it today after work so that output voltage would be 12-14% lower from the input voltage and see if it works.

I found that 18% from this forum and I thought it was correct. So with input voltage beeing 4,69 V I should have output voltage between 4,03 to 4,13 V am I right now?

Some instruction give 83% as a target. My mechanic uses 12% down. I would use the latter.

Did you clean the throttle body first?

DAMN! I didn´t do the cleaning... Will do it today after work ( it´s now 9:30am in Finland)

The TPS input voltage should be 5.0v. The output voltage should be ~82% of that i.e. ~4.1v when the throttle is WIDE OPEN, not when the throttle is fully closed with the engine idling.

Dr. Dyno said:

The TPS input voltage should be 5.0v. The output voltage should be ~82% of that i.e. ~4.1v when the throttle is WIDE OPEN, not when the throttle is fully closed with the engine idling.

Click to expand...

ok now im confused?? my tps at idle is 4.2 and drops to about 1.5 v or so as the throttle opens to full . Have I misread every article about setting a renix tps???

My TPS gives 4.69V input and 4.2V when throttle is closed. I measured it from the square connector when engine wasn`t running and ignition ON.

Gonna jump in here. There are two TPS´s on the 4.0 Engine with automatic tranamission. One gives throttle position information to the TCU (tranny) and one to the ECU/ECM (motor). The posts seem to have gotten them confused a bit. One on top of the other, piggyback, Mr. and Mrs. (or whatever).
The two TPS´s operate the opposite of each other (mirror). A typical value for the Tranny TPS (outer half) might be 4.64V in and 83%=3.85V out (per 88 FSM). A typical input voltage for the ECU TPS might be 4.86V in and .75V (15%) out (the opposite of the trans TPS).
As the throttle is opened the TCU TPS voltage falls and the TPS ECU voltage rises.
If the ECU half of the TPS, varies too far from specifications, it can cause a idle problems and on mine, a very high RPM (around 1800) for a second or two, at start up. The reason for the high RPM (a second or two) at start up, may be the reason for the TPS recall, the interlock saftey recall and seems to be worse the farther the ECU TPS is set from ideal values. My motor may not be typical,as I suspect a small intake vacume leak.
Anyway, the values for the TCU TPS affect shift points. I didn´t record exact numbers. Did notice a 1 volt difference in output voltage (1/2 volt either way from the 83%), seemed to be the acceptable envelope.
Lastly having two different, imput voltages just about makes an ideal adjustment/ratio for both TPS´s at the same time near impossible.
My solution was to split the differnce, shifts a little late (not necessarily bad with larger tires) and idles a little high (not necessarily bad as I idle alot with the radios on).
I tried two different TCU´s with the same output voltages.
A solution to the problem, may be to have the ECU, power both the inputs for both TPS`s. Not gonna try it, what I have is good enough.

Good brain bender for Dr. Dyno.

Could be the same thing, HEKULIINI is dealing with, the TPS, could be OK.

82% TCU TPS/ 16% ECU TPS as recommended by All-Data
83% TCU TPS/ 17% ECU TPS as recommended by the 89 FI FSM
12-14% as recommended by a 17 year Jeep tech.

Ratios may not work out exactly, due to different imput voltages for the different halves of the TPS.

A questionable O2 sensor (or vacume leak) can also cause high idle and affect driveabilty (can fool you) settings for the TPS´s.

Thank you mr mud you have explained it well , i never have recorded the ecu tps output while opening the throttle (which reduces the output voltage on the tranny tps side . ) I guess now i will see if my ecu tps output goes up. last time i looked it sat at about 0.8 v at idle? ill check it tonight.

8Mud,

Thanks for the valuable explaination! I have seen the 12-14% number on this forum from other people as well. I am wondering if the FSM numbers are the theoretical ideal with all new sensors etc., with the 12-14% number reflecting a setting that is better suited for "experienced" systems. Could be that 12-14% yields personal performance preference.

What variance in shift points did you see with what voltage changes/percentages?

My TPS is 4.64 in and 4.2 out or 10 %or so. I set my new tps at 83 %( 3.85v) didnt solve my problems , changed more sensors no help . One day I said the hell with it and jacked it to the above ( 4.64 in and 4.2 out ) now the xj is jack the bear , smooth idle beter all round performance . Just my .02 ......

I tried the 4.2 setting, motor idled really high. Though I did have some side issues, my IAC was frozen shut. The dealer had done a service on the throttle body (for the previous owner) and had only cleaned the parts of the throttle body that could be seen (the very top and outside) and adjusted the idle by bending the throttle stop tab (didn´t even bother with the don´t mess with me screw), when I cleaned the IAC things went kind of crazy. Also had a flacky O2, I replaced with a used spare.
I´ll probably eventually try the 4.2 volt setting, after an intake gasket change and a new O2 sensor. Might be chasing my tail.
Didn´t record the shift points, but tried the TCU TPS at various settings (before the O2 sensor change). Mine shifts at around 2100-2200 range (4.64 in 3.85 out), mild throttle, now. Had my shifts lower than 1800 and above 2200, at various settings. Same test drive, a couple mild hills, 50 MPH secondary road.

A little tip, running the TPS, cables over the top of the rubber hose, from the air box to the throttle body makes test a snap.

Hope my experiences helps, Hekuliini some, my XJ is still a work in progress.

Lowering the ECU TPS voltage (as suggested by Jneary 12-14%) helps lessen the start up surge (momentary high idle at start up).

its a 5 volt reference circuit but it runs lower. back in the chrysler training i went to back in the early 90's they recommended 12 14% to help reduce the high idle flare up. the computer system has the ability to "learn " your driving habits over a short period of time to give you proper performance depending on how you drive.

there is also a way to set base idle. remove the aluminum plug on the side of the throttle body and back that torx screw almost flush with the edge of the throttle body. then if it doesnt idle at about 950 rpm's then you need a small allen wrench and turn the idle stop screw until you achieve that setting. then set your tps to 12-14%. but make sure the throttle body has been thoroughly cleaned and the solvent has been run out of the system. also take a small dab of rtv sealant and aply it to the rorx screw on the side so it cannot back out by itself over time. one more thing. the ground wires by the dipstick should be moved to the inside of the dipstick tubu so they contact the engine block, that was included in the recall originally. hope this works.