Bass-Ackwards TPS readings

[StevenB]

A problem has developed with the motor stumbling off-idle and trying to die when releasing the throttle. In an effort to track down the potential problem, I checked the TPS to see if perhaps it needed adjustment.

'89 4.0
AW4 tranny
4WD

Input voltage checked at 4.91 volts - closed throttle
Output voltage checked at....... .75 volts - closed throttle and increased as throttle was opened.

On an automatic equipped transmission vehicle???

To me it sounds as if the thing is acting like a manual-transmission equipped TPS..on an automatic equipped vehicle.


Has anyone seen anything like this before? Any suggestions would be greatly appreciated. The Jeep runs, and runs fine except when just coming off idle or occasionally when I let off the throttle, it wants to die out.

[Matthew Currie]

For an automatic you test the transmission, rather than engine, section of the TPS, and the two read differently. Just in case you need to review, here's my handy cut-paste condensation of the service bulletin specs:

For an 87-90 4.0:

Test is done with throttle closed, connectors connected, ignition on, engine off. Insert probes through back of connectors to make readings. Use a high impedance meter. Adjust TPS by loosening mounting screws and turning in mount.

For manual connect + probe to terminal A, and - to B. Take a reading, which should be in vicinity of 5 volts but probably lower. Note the reading well. Now move the positive probe to terminal C. The reading should be 16 percent of the previous reading.

For Automatic, use the square, 4-terminal plug. Connect probe + to terminal A, and - to D. Note reading, which should be in vicinity of 5 volts. Now move probe + to terminal B. This reading should be 82 % of the first.

[StevenB]

Well, that would explain it, then, wouldn't it. :doh: ::dunce::

Thankees for the procedure synopsis as well.

[gian]

Guys,
Is there a need to follow a similar procedure on 91+ XJs?

[RWKHausSupply]

OK here's one for ya...

What if the Votage feed to the sensor is only 3.46V not running with ign on..

I am thinkn that with it running I bet I get closer to 5V.

I also dont get more then like 1.2V no matter where the adjustment is n a new TPS at idle position while ign on and not running.

[langer1]

Guys,
Is there a need to follow a similar procedure on 91+ XJs?

No 91+ no longer uses the duel TPS.

[Matthew Currie]

OK here's one for ya...

What if the Votage feed to the sensor is only 3.46V not running with ign on..

I am thinkn that with it running I bet I get closer to 5V.

I also dont get more then like 1.2V no matter where the adjustment is n a new TPS at idle position while ign on and not running.

That's a pretty low reading. I'd suspect a wiring or connector problem somewhere, and also check if it really does rise when running. As far as I know, that's supposed to be a regulated voltage, relatively independent of battery voltage, so something is wrong somewhere.

[RWKHausSupply]

That's a pretty low reading. I'd suspect a wiring or connector problem somewhere, and also check if it really does rise when running. As far as I know, that's supposed to be a regulated voltage, relatively independent of battery voltage, so something is wrong somewhere.

well I will check the differnce in running not running tomorrow. I say that though because the dash voltage meter shows like 11V while not running and 13.5ish while running. So if they get the feed from the same circuit then I would guess that the ecu would see from 11 to 12+ and then the semi generic ecu would then shoot up from the high 3's to in the 4's.

I will check tomorrow.

[8Mud]

OK here's one for ya...

What if the Votage feed to the sensor is only 3.46V not running with ign on..

I am thinkn that with it running I bet I get closer to 5V.

I also dont get more then like 1.2V no matter where the adjustment is n a new TPS at idle position while ign on and not running.

I had almost the same thing going on awhile back. It turned out to be a dirty connector at the TPS. Somebody had sprayed it with WD40. I got radically different readings depending on with end of the connector I tested at, the TPS end or the harness (ECU) end. The ground for the TPS (through the ECU) is at the dipstick holder and a known trouble spot. Is 3.46 volts tested at the TPS ground or a chassis ground? A quick test for your TPS ground is to do a volt test from the TPS ground to the chassis, this will tell you how much standing voltage you have due to dirty connectors or bad grounds. There is always a little resisitance in this circuit, internally in the ECU.

[RWKHausSupply]

I had almost the same thing going on awhile back. It turned out to be a dirty connector at the TPS. Somebody had sprayed it with WD40. I got radically different readings depending on with end of the connector I tested at, the TPS end or the harness (ECU) end. The ground for the TPS (through the ECU) is at the dipstick holder and a known trouble spot. Is 3.46 volts tested at the TPS ground or a chassis ground? A quick test for your TPS ground is to do a volt test from the TPS ground to the chassis, this will tell you how much standing voltage you have due to dirty connectors or bad grounds. There is always a little resisitance in this circuit, internally in the ECU.

Ok just checked it and DAYM all test at ECU side of TPS connector I get 3.46 testing like it says to. And then if I go to the battery ground rather then harness ground I get 4.9v. Whats the deal?? Does that mean that the ECU isnt getting full voltage either? I read where you said the ground is. But where exactly is that? And is that the ground for only the TPS circuit or for the ECU?

[old_man]

It means that you have a bad ground in the system. Clean you negative battery terminal, also where the cable attaches to the block, and then the braided connection from the head to the firewall.

[RWKHausSupply]

It means that you have a bad ground in the system. Clean you negative battery terminal, also where the cable attaches to the block, and then the braided connection from the head to the firewall.

Well I never "cleaned" the existing connections anywhere. I did add a new 1000CCA autozone best battery, about 12' of 8ga wire that made I think 6 different location grounds that all come right from the battery to all over the engine compartment and two places on the motor. Also redid the alt power cable up to 4ga and have a 140amp alt. As well as a few replaced + wires here and there under the hood where I thought they were very old and brittle looking. This is a 1990.

SO my real concern I think is where the ECU grounds or where the TPS gets its ground from? It runs/ran great. But I replaced the throttle body with a 61mm and had to swap over the TPS and IAC so I had to reset the tps adjustment and found this.

The last adjustment was done with a DRB by a great guy here on the board so I dont know if it was an issue then or not.

[lawsoncl]

Guys,
Is there a need to follow a similar procedure on 91+ XJs?

91+ isn't adjustable, so no.