TPS Volt Questions

I finally went out to purchase a mulitmeter and i bought a decent one from Kragen. The main reason for this was to check my TPS voltage readings. Lately the Jeep (87 6 cyl manual trans) has been idling somewhat inconsistantly. Upon startup it will rev anywhere from 2200-2900rpms and sometimes it sticks and will not come down, the only way i was able to get it down was to unplug the TPS from the throttle body. The normal idle once it comes down is anywhere from 500-1000rpms. Also when im driving and it doesnt matter if its been 5 minutes or an hour when i push the clutch in coming to a stop the tach will drop down to zero and sputter and then rev back up to about 1000 rpms. Occasionally after driving a while it will actually stall. So i took my readings and this is what i got:

TPS connected to throttle body:
Positive lead on "A" and the negative lead on "B"= 0.82v
Positive lead on "C" and the negative lead on "B"= 0.1v

TPS disconnected from throttle body:
Positive lead on "A" and the negative lead on "B"= 5.01v
Positive lead on "C" and the negative lead on "B"= 0.00v

according to the FSM my readings with the TPS connected to the throttle body are completely wrong. the first reading should be 5v and the second should be 0.8v. So am i correct in this? and would this indicate a major problem? new TPS? possible adjustment? am i doing this wrong?

Thanks in advance
Brett

Also, i tried adjusting it but loosening one of the screws on the TPS and when i rotate it the voltage doesnt change at all. So far im thinking it is a bad TPS, anything else to check before i get a new one?

87CherokeeChief said:

Also, i tried adjusting it but loosening one of the screws on the TPS and when i rotate it the voltage doesnt change at all. So far im thinking it is a bad TPS, anything else to check before i get a new one?

Click to expand...

TPS is simply a potentiometer. There's no reason to disconnect TPS from TB either.

You are looking for a reference voltage from the ECU of about 5Vdc +/- 10% or so. You must use the sensor ground in the harness or at the ECU to make all voltage measurements. Do not use engine/battery ground or you will experience errors.

After you have 5Vdc coming in on pin A (pin B is sensor ground), you need to see "signal" voltage that changes as the throttle is opened/closed from pin C. For manual transmission, 0.8 is just about right based on my FSM. I own an auto so I've never seen a manual tranny TPS but I know electronics and how this works.

TPS failure is usually crappy throttle response anyway. You can sometimes see it with an analog meter on pin C to pin B and then operate the throttle. You'll see 0.8-swing up near 5Vdc as the throttle opens. If it jumps in some part of the travel, the TPS is probably bad.

Your issues sound more like 02 failure and/or open-loop (default) operation to me. Connectors and wiring on this era XJ really suck too. Often, the sensor is fine but the wiring is the real problem. Replacing the sensor usually brings a faulty connector around for a while making you think the NEW sensor fixed the problem. Until it acts-up the next time.

XJXJ said:

TPS is simply a potentiometer. There's no reason to disconnect TPS from TB either.

You are looking for a reference voltage from the ECU of about 5Vdc +/- 10% or so. You must use the sensor ground in the harness or at the ECU to make all voltage measurements. Do not use engine/battery ground or you will experience errors.

After you have 5Vdc coming in on pin A (pin B is sensor ground), you need to see "signal" voltage that changes as the throttle is opened/closed from pin C. For manual transmission, 0.8 is just about right based on my FSM. I own an auto so I've never seen a manual tranny TPS but I know electronics and how this works.

TPS failure is usually crappy throttle response anyway. You can sometimes see it with an analog meter on pin C to pin B and then operate the throttle. You'll see 0.8-swing up near 5Vdc as the throttle opens. If it jumps in some part of the travel, the TPS is probably bad.

Your issues sound more like 02 failure and/or open-loop (default) operation to me. Connectors and wiring on this era XJ really suck too. Often, the sensor is fine but the wiring is the real problem. Replacing the sensor usually brings a faulty connector around for a while making you think the NEW sensor fixed the problem. Until it acts-up the next time.

Click to expand...

Im still confused as to why my readings are way off. I can check the 02 sensor with a voltmeter too correct? also what is open-loop operation, when a sensor fails it goes into a default mode since the loop is broken???

If I had the same symptoms, the first thing I´d look at, would be a sticky IAC.
Your TPS "A" contact, is imput voltage which will probably be arounf 4.8 volts, with the connector connected and the ignition on.
"B" contact is ground, use your ohm meter, with the connector disocnnected, test the engine side of the "B" black wire to chassis ground, there will be a very little resisitance, some but not much.
"C" is output voltage, it will typically be 6.5 volts with the throttle closed.

Check the wires on both sides of the connector, a straight pin through the wire works well. There can be a difference in voltage from one side of the connector to the other, sometimes substantial.

With the ignition on and the connector connected, test the "B" pole black wire ground (both sides of the connector), with a volt meter, this is a standing voltage test. This will tell you pretty quick what shape your ground circuit is in. There is going to be a small bit of standing voltage, a tenth of a volt or so. Any more than this tells you someplace there is significant resisitance. Like the ECU ground at the dipstick or the motor head to firewall ground or the battery to dipstick holder ground.
I carefully sliced the TPS wires lengthwise (3/16-1/4"), on both sides of the connector and ran the wires over the top of the rubber elbow for the TB intake. Makes testing a snap and hasn´t hurt anything yet (been that way for years, through mud and rain). If the open slice in the wire makes you nervous, a dab of silicon will seal it back up and can be removed with a finger nail next time.

The appropriate TPS test for a 4.0L ('88) starts with insuring approx 5v exist between pin A (+) and pin B (sensor ground--not engine or battery ground) with the TPS connector disconnected and the key on. If this is good, reconnect the TPS and back-probe pin C (pos lead) and pin B (neg) with long needles (to avoid damaging wire and insulation). With the key on and throttle plate closed, you should see approx .8v; if you do, the TPS is probably OK and properly adjusted. No implies the need to adjust the TPS to .8v. If you can't get .8v by adjusting, then the TPS is most probably bad (but only if in fact you saw 5v during the first check). A second test is to determine if the voltage changes smoothly, without jumps, from closed throttle (approx .8v) to wide open throttle (approx 4.8v). A digital VM is not good for this test. You need an analog meter connected to "C" and "B"; then with key on, engine off, slowly open the throttle while observing the voltage; you should see a smooth transition to the max voltage. If not, the TPS is bad.
All of that said, your problem sounds more like an incorrect Air Idle Control system. This would include a dirty throttle body--both the passages for the Idle Speed Stepper (IDS) motor and arround and below the throttle plate. The computer controls idle speed by extending or retracting the pintal in the IDS. Under normal circumstances, at engine shut-down, the ECM retracts the IDS pintal to prepare for the next start-up. when you first start your engine, the ECU emmediately commences to adjust the idle speed based on engine temp and TPS position. If you start a warm engine, you should get a "flare up" in idle, followed immediately by a drop in idle speed to match the conditions the ECM sees from its sensors. If the idle does not drop, it could be the engine is warm (correct action), the IDS is stuck open, the ECM is not sending the appropriate signals, or the throttle-plate is being held open by carbon build-up, or even possibly stuck throttle cable. The dealer has special test equipment to "step" the motor to determine its functionality. You can determine if the ECM is sending a signal to the IDS by back-probing the appropriate pairs (one set to extend and one set to retract the pintal) with a LED type test lamp; you should see blinking lights. If you do, you know the ECM is trying to control idle and you know the wires are not shorted--which could be lethal to the ECM; but you can't tell if the signal being sent is appropriate--only the dealer can do that.
Back to your tests of the TPS, I'd insure you were in fact on the correct pins. The results you posted were not consistent, indicating maybe you became frustrated and made a mistake.

I misplaced a decimal point in my typical voltage readings. Typically 4.8 volts in (or 4.65-4.80) and *.65* volts out (which is a performance setting and fairly typical for the double automatic tranny TPS) till the recommended .80 volts out. I´ve never seen an XJ ECU that put out the called for 5.0 volts imput, with the connector connected. The ECU will make some adjustments for lower imput voltages, nothing to get in a tizzy about. And works fine with a .7-.8 volt output reading.

Hey guys, My '88 cherokee is having problems running at start up. It will start up then idle high then come back down and die. I've replaced fuel pump(for additional reasons...), MAP sensor, and we saw sparks coming from the top of the boot on a plug wire so I replaced plugs, wires, cap and rotor.

I started testing the TPS tonight and found that on one connection coming out of the TPS I got (after adjusting) the normal .8 volts to 5 volts. However on the other connection it was pretty much reverse. it read about 3-4 volts then as i "accerlerated" it went down to below 1 volt. I know one of those goes to the engine and another goes to the transmission, just don't know which one and if the second readings are correct.

I'd like to check the AIC, ECM, and such listed above. I haven't researched this yet but I should be digging in this week. Any input would be nice, but I just wanted to touble check those second readings I got.

Those 'reverse' readings are normal for the TPS plug going to the TCU.

I would suspect the IAC (idle air control) with those symptoms. Before replacing it though, clean the throttle body and see if that has any effect. Pay special attention to the area where the throttle blade closes in the housing. Just a slight carbon buildup there, will affect the idle.

Ok, I pulled the IAC out, it was black. So I cleaned it, cleaned the head of it at least. Is there a way to check to see if it is functioning?

The only way I can get my Jeep to run is to start it up and press the gas, otherwise it dies.

I tested my TPS (trans will not shift on it's own) and got about 4.2v on the A side, and nothing on the C side. The C side will gradually increase from 0v to near 4.2v with throttle input. The A wire does not change with throttle position, 4.2v constant.

This is an '88 Renix with AT, any ideas?

87CherokeeChief said:

Also, i tried adjusting it but loosening one of the screws on the TPS and when i rotate it the voltage doesnt change at all. So far im thinking it is a bad TPS, anything else to check before i get a new one?

Click to expand...

The tach droping to 0 is a clue, The tach signal is also the spark trigger signal.

langer1 said:

The tach droping to 0 is a clue, The tach signal is also the spark trigger signal.

Click to expand...

where would that be...?