TPS wiring questions

bigalpha · Mar 18, 2010

88 MJ, 6 cyl., auto

I tested the Jeep-side of the square connector. Pin D (black wire) should be the ground but when I hit it with the multimeter is says 0.0.

When I hit Pin B (blue wire) with the multimeter, it says 38.xx.

This means that the blue wire is somehow grounding out. Should I pull the ECU plug and test on that end of things to see if the wiring is grounding out funny before/after the ECU?

joe_peters · Mar 18, 2010

Yep, square four-wire connector, connector removed from the TPS, key ON, D is the ground, A should be reference voltage (approximately 5.0 volts).

Reattach the connector.

Key ON, check voltage using B (+) and D (-). Should be 83 percent of reference voltage.

So, you should adjust the TPS for the output voltage, voltage at B, to be 3.154 volts.

I would, however, fix the low reference voltage issue first--renew all your cables and grounds with particular attention to the dipstick tube grounds. Take them down to bare metal, treat with an anti-corrosion product (hardware/paint store), and make them TIGHT.

bigalpha · Mar 19, 2010

I'm more concerned that D is not showing resistance to battery while A is showing 39 ohms. At "idle", it revs at something close to getting ready to take off into space.

joe_peters · Mar 19, 2010

"D" is ground to source--ZERO ohms is good.

You shouldn't be trying to measure ohms on a powered circuit that is live--am I reading you wrong?

Describe the problem you are trying to deal with--of which attaining orbit is part.

bigalpha · Mar 19, 2010

joe_peters said:
"D" is ground to source--ZERO ohms is good.

You shouldn't be trying to measure ohms on a powered circuit that is live--am I reading you wrong?

Describe the problem you are trying to deal with--of which attaining orbit is part.

Surely the circuit in a 20+ year old vehicle is not going to be completely resistance-free. I didn't have the circuit powered on; the truck was off at the time; unless the circuit is always going to be hot even with ignition OFF.

Basically, the truck idles astronomically high. In the process of troubleshooting, I was messing with the TPS and then found the resistance problem that I describe here.

joe_peters · Mar 19, 2010

Ok--I misread your post, though you were checking ohms on "A" while the engine was running.

Hell, looking back over this I'm wrong all the way down.

What is your reference voltage--TPS square connector disconnected, measured between "D" ground and "A" power?

With the TPS connected, key ON, measuring between "D" ground and "B" output you said you are getting what voltage?

bigalpha · Mar 19, 2010

I didn't write down the voltages that I got when I was fooling around with it. I'll have to do it again tomorrow and write down the numbers I get. The ratio is supposed to be .8, right?

joe_peters · Mar 19, 2010

bigalpha said:
I didn't write down the voltages that I got when I was fooling around with it. I'll have to do it again tomorrow and write down the numbers I get. The ratio is supposed to be .8, right?

.83, but .03 wouldn't be off much.

Super high idle, usually massive intake leak.

bigalpha · Mar 19, 2010

This isn't even a super high idle - it sounds like it's redlining.

I tested according to the following:
Key ON, disco the square connector. Pin D to pin A = 4.7V
Key OFF, Pin D1 on TCU to TPS pin A = .2-.3 ohms
Key OFF, Pin D3 on TCU to TPS pin D = .3-.3 ohms
Key OFF, Pin D2 on TCU to TPS pin B = .3 ohms

The next step says that circuit is NOT open, to replace the TPS.

joe_peters · Mar 20, 2010

bigalpha said:
This isn't even a super high idle - it sounds like it's redlining.

I tested according to the following:
Key ON, disco the square connector. Pin D to pin A = 4.7V
Key OFF, Pin D1 on TCU to TPS pin A = .2-.3 ohms
Key OFF, Pin D3 on TCU to TPS pin D = .3-.3 ohms
Key OFF, Pin D2 on TCU to TPS pin B = .3 ohms

The next step says that circuit is NOT open, to replace the TPS.

What did you get Key ON, connected and back probe "D" and "B"?

bigalpha · Mar 20, 2010

joe_peters said:
What did you get Key ON, connected and back probe "D" and "B"?

Dang, I didn't measure that. I will go out in a few minutes and measure it.

bigalpha · Mar 20, 2010

So much for a few minutes.

TPS:
A to D = 4.64
B to D = 3.68

The ratio of this is .79 which is 4/10 off of correct reading.

Weird thing is that when I started it today, it didn't rev nearly as high as it did yesterday; though it's still higher than normal.

Do you think that the resistance readings between the TPS and the TCU are of concern?

joe_peters · Mar 20, 2010

bigalpha said:
So much for a few minutes.

TPS:
A to D = 4.64
B to D = 3.68

The ratio of this is .79 which is 4/10 off of correct reading.

Weird thing is that when I started it today, it didn't rev nearly as high as it did yesterday; though it's still higher than normal.

Do you think that the resistance readings between the TPS and the TCU are of concern?

For the idle it would be the ECU-TPS that would effect the idle. I don't think the ECU-TCU communicate.

Do this--pull the intake tube from the TB, check the tightness of the TB mounting screws. Also, there is a 5/32 (I think) allen that you adjust to keep the steel throttle place from fully closing and damaging the alloy throttle body. Back that screw off until it no longer effects the throttle plate, then screw it back in until you can just see the initial movement of the throttle plate and stop. Put the intake tube back on, and then adjust the TPS for the correct 83 % output voltage--see where that gets you.

bigalpha · Mar 20, 2010

joe_peters said:
For the idle it would be the ECU-TPS that would effect the idle. I don't think the ECU-TCU communicate.

Do this--pull the intake tube from the TB, check the tightness of the TB mounting screws. Also, there is a 5/32 (I think) allen that you adjust to keep the steel throttle place from fully closing and damaging the alloy throttle body. Back that screw off until it no longer effects the throttle plate, then screw it back in until you can just see the initial movement of the throttle plate and stop. Put the intake tube back on, and then adjust the TPS for the correct 83 % output voltage--see where that gets you.

The throttle body is on very tight as is the intake tube. There is no hissing during engine operation, nor did anything happen when I sprayed some carb cleaner around the TB.

I will try adjusting the allen screw and recalibrating the TPS and seeing what effect that has on things.

Is there any other data that you want?

joe_peters · Mar 20, 2010

No, just post up your results on the adjustments. Do you have a tach?

bigalpha · Mar 20, 2010

joe_peters said:
No, just post up your results on the adjustments. Do you have a tach?

No, I don't.

joe_peters · Mar 20, 2010

Ok. Let me know if it seems any better when you are done.

Did you check the vacuum lines where they attach under the MAP?

bigalpha · Mar 20, 2010

Roger. I should be able to fool with it some tomorrow. I'd go do it now but the woman is asleep and a jeep revving at 12000 RPM would wake her up.

I did check the lines a few times; lines are tight at the MAP and at the TB.

bigalpha · Mar 20, 2010

So I adjusted the screw that moves the throttle plate like you said. I then set the TPS at .83 V ratio between A-D and B-D.

I start the truck and it starts at the "normal" idle then get much higher over a period of one or two seconds. I let it run for a few minutes but it didn't idle down any. I restarted it a few times and 1 time out of 3 it immediately started idling crazy high.

winterbeater · Mar 20, 2010

What about testing the IAC? Have you pulled it, cleaned it and checked to see if it moves when you plug and unplug it while not installed?

TPS wiring questions

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