1987 Nissan Pathfinder No TPS output voltage

Tiny
87FATHPINDER
  • MEMBER
  • 1987 NISSAN PATHFINDER
  • 200,000 MILES
I own a 1987(10/86) Pathfinder and I have a problem involving code 42 TPS. The input voltage is supposed to be variable from 0.5 to 4.0 volts and its supposed to change when the accelerator is depressed, but when I measure it I only measure 0.03 volts body grounded and 0.006v grounded to the black wire from the tps harness and nothing inbetween. I've tried using a different ecu and the same thing is occurring. I installed a new TPS and I've also checked continuity on all the wiring to the tps and everything is fine. I was measuring the voltage after installing and it all of a sudden dropped off and never returned. TPS was checked and is fine.
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Thursday, June 6th, 2013 AT 8:17 PM

9 Replies

Tiny
CARADIODOC
  • EXPERT
To start with, the feed wire should have 5.0 volts all the time the ignition switch is on. Check for that by back-probing through the rubber seal in the connector. If it's missing, disconnect the plug and check on the terminal directly.
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Thursday, June 6th, 2013 AT 9:41 PM
Tiny
87FATHPINDER
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I have done that, power supply is fine. I can't find any continuity issues either.
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Thursday, June 6th, 2013 AT 10:19 PM
Tiny
CARADIODOC
  • EXPERT
Dandy. That leaves four possibilities and you already tried a new sensor, so we're down to three. The most likely causes are a spread terminal for the 5.0 volt feed wire or the signal wire. If the 5.0 volt terminal isn't making contact with the sensor's terminal you'll find 0.0 volts on the signal wire. If the signal wire terminal isn't making contact you will either find 0.0 volts or 5.0 volts on it. I only have Chrysler's method memorized but about 95 percent of other car brands work the same way. With a break anywhere in that signal wire circuit, including inside the sensor itself, the voltage in the computer could "float" to some random value due to the circuitry inside. The computer could try to run on that value. To prevent that they use a "pull-up" resistor tied to the 5.0 volt supply. It is so high in value that it has no affect on anything, that is, until that break in the signal circuit occurs. Then the resistor places 5.0 volts on that circuit which is what triggers the fault code.

If the break is in the signal wire going to the sensor, you will find the normal 0.5 to 4.5 volts, (approximately) at the sensor terminal but 5.0 volts at the computer, (and on a scanner). If it's the terminal that isn't making contact you will find 5.0 volts when you back-probe that terminal.

Since you found 0.0 volts on the signal terminal, if the sensor is good and the 5.0 volt feed terminal is making contact, that leaves your meter probe as the problem. After working on tvs and vcrs for over 40 years, I can tell you that even experts get fooled all the time with 0.0 volt readings. To double-check yours, measure on the terminal side of the connector where you can see for sure you're making good contact.

It is also possible they're using a "pull-down" resistor. That will put 0.0 volts on the signal wire when there's a break in that circuit.
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Thursday, June 6th, 2013 AT 11:22 PM
Tiny
87FATHPINDER
  • MEMBER
I unplugged the harness from the tps and the ecu. I then checked between terminal e (white signal wire) and ground and there was continuity, meaning that there is a short to ground I believe. So if that's the case is there a likely spot to look for a short to ground?
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Friday, June 7th, 2013 AT 10:19 AM
Tiny
87FATHPINDER
  • MEMBER
So I was looking at the wiring diagram and it looks like the wire to pine 19 is always grounded through joint connector A. The air flow meter had the same thing on the diagram so I checked its harness the same way and it was grounded as well only I have no problems with that sensor. So, naturally, I'm as lost as ever.
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Friday, June 7th, 2013 AT 12:37 PM
Tiny
87FATHPINDER
  • MEMBER
The multimeter has been tested on the 5v line and works fine. I've tested everything I can think of to check it and it doesn't show any issues.
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Friday, June 7th, 2013 AT 12:41 PM
Tiny
CARADIODOC
  • EXPERT
I don't have a service manual to look at but I suspect you're misinterpreting a symbol on the wiring diagram. The signal wire obviously can't go to ground. I didn't think of that in my previous list of suggestions. Even the sensor's ground wire won't have continuity to ground when the plug is disconnected from the computer.

I would watch the meter while you tug and flex on the wiring harness. In particular, look for any sharp metal brackets the harness goes over, and anyplace the harness can slide back and forth when the engine rocks when shifting between drive and reverse. That usually occurs on the inner fender. I was involved with one that had multiple bare wires on the left fender near the Engine Computer, and I've heard of other people running into similar problems.
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Friday, June 7th, 2013 AT 7:53 PM
Tiny
87FATHPINDER
  • MEMBER
The signal wire was grounding out to its grounded shielding. I cut that back to a healthy point in the internal insulation. It is now working properly. Thank you for your help.
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Saturday, June 8th, 2013 AT 11:10 AM
Tiny
CARADIODOC
  • EXPERT
All right! One in a row. Happy to hear it's solved.
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Saturday, June 8th, 2013 AT 12:23 PM

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