North American Turbocoupe Organization



TPS Problem
turbird77 Offline
Member
#1
I'm having an erratic idle/random dying problem with my 88 TC, and have been suspecting the TPS for awhile now. Today, I had some spare time, so I probed the wires with a multimeter to check, and found out that the TPS isn't even getting any voltage to it whatsoever now. Unplugging the TPS turns on the CEL, and after around 10 miles of driving the ECM learns to ignore the lack of a TPS sensor signal, substitutes in default values in it's place, and the erratic idle then smooths out. Not looking to take the easy way out here though, and would like to actually fix the problem.

Obviously I need to check the entire length of wiring now. To anyone who has had this problem before, what are the things to look for here? Does the TPS harness itself contain any resistors or diodes that may have burnt out, or am I stuck just slicing open the entire harness and tracing the wires all the way back to the ECM port?
88 TC - Work in Progress.
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turbird77 Offline
Member
#2
Anyone know?
88 TC - Work in Progress.
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Jeff K Offline
Administrator
#3
Which wires to the TPS did you probe? First, check between orange VREF(+) and black SIG RETURN(-). Should have very close to 5.0 V KOEO and KOER. If that is OK, check black to green (TPS OUT). If you are backprobing the connector, are you 100% sure your probes are making contact with actual metal?
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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turbird77 Offline
Member
#4
Yes, I'm sure. Was checking the orange (+) and black (-) wires. Tried both backprobing the connector, and piercing through the wires insulation directly with pins. No voltage is getting to the TPS for some reason.
88 TC - Work in Progress.
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Jeff K Offline
Administrator
#5
What volts do you get between orange and black wires with the TPS disconnected (i.e., measure volts @ harness connector with TPS unplugged).
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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turbird77 Offline
Member
#6
I checked that originally as well, it's getting nothing to the harness connector. Went out tonight to check again and still nothing. I've also tried several other new and known working TPS sensors, and nothing changes. Is there a resistor or diode somewhere in the TPS's wiring that could be causing this? Maybe a corroded connection point at the ECM itself?
88 TC - Work in Progress.
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Jeff K Offline
Administrator
#7
No diode or resistor in the TPS circuit. If the 5V VREF was missing at all the sensors due to corroded / damaged VREF pin at PCM, you would have out of range codes for all the sensors. Check for VREF at the VAM connector - should be 5V between BK/W (-) and W/BK (+) wires. There is a big splice (I forget where it is) where the 5V VREF branches out to all the sensors. Possible splice damage or harness damage from the splice to TPS.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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turbird77 Offline
Member
#8
Ok, that is exactly what I wanted to know. Thank You. The only KOER code I am getting is for TPS out of range. So as you suggested, I went out and checked the VAM connector, and it is indeed getting the 5V it should, so that must mean there is a problem in the TPS wiring itself. I guess now the only thing left is to trace the TPS wiring back to that splice you are talking about. Hope it's not too deeply buried inside the harness wire bundle. Only this one bug left here to work outta my TC before it's perfect, and I'm determined to squash it.
88 TC - Work in Progress.
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Jeff K Offline
Administrator
#9
If its real difficult to find the harness damage or splice, you can take a VREF 5V signal from anywhere else under the hood and run a wire to the TPS connector. There is VREF at the ECT, ACT, KS, etc.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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turbird77 Offline
Member
#10
Finally squashed this last bug outta my TC last night. Turns out the Orange TPS harness wire was heavily corroded inside for some reason, and blocking the 5V signal allmost entirely. Had to cut it completely out, and run a new length of 12ga wire from the VREF splice point to the TPS harness connector. Now, the TPS is getting the voltage it should, and after I set the TPS sensor to 0.98V, she is now idling smooth as butter, and accelerating much smoother as well now. No more KOEO or KOER codes are coming up either. Finally I have a perfect 10 TC. Happiness. Smile
88 TC - Work in Progress.
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