1987 GL-10 Wagon Throttle Position Sensor Issues

[OCDan]

I have a 1987 GL-10 which I haven’t been using and decided to gift it to a friend in need of an inexpensive vehicle. I’m helping him resolve some issues with it.

 

It’s getting a 31 code. The TPS is the 4-wire type. Without going into detail, the original TPS was irreparably damaged during removal.

Based on information posted here https://www.ultimatesubaru.org/forum/topic/153231-1987-subaru-gl-throttle-position-sensor-help/?do=findComment&comment=1283981

I located and purchased this:

https://www.partsgeek.com/dpq5225-isuzu-i-mark-throttle-position-sensor.html

with part description “Throttle Position Sensor - 1989 Isuzu i Mark - Walker 1272-06053178”

This is what it looks like:

And internally:

Box label:

 

Prior to cutting any wires, I checked resistance as described in the EA82 FSM:

And measurements were within specs. So, I took a leap of faith and cut the wires on this new TPS and on the engine side connectors and soldered in a new connector on each side.

 

I installed the TPS and double-checked the resistance on the TPS pins and everything was still within specs.

So, I went ahead and connected the TPS to the engine side connector, reinstalled the components and belts which had been removed to gain access to the TPS screws. The engine started on the first attempt and was running smoothly. After it had been running for a few minutes, the RPMs suddenly jumped from around 1000 to around 2000. At some point, the CEL came on. I looked at the ECM light to see that it was blinking 31 again. I pulled the connector to check the resistance on the TPS. There was no longer any continuity at all on any of the pins, regardless of the throttle position.

At this point, I don’t know if the part just failed due to some defect, or if it was somehow damaged by some condition when the engine was running. Regardless, it can’t be returned to PartsGeek, due to my having cut off the original connector.

I’m fairly certain that I matched wires correctly between the 2 sides. The engine side harness wires have the following functions:

I measured the voltage on the red and it is 5 volts.

Red goes to red, black to black, green on the engine side to white on the TPS side and blue/green to blue, for the idle switch.

I’m hoping someone might have some ideas on how I can troubleshoot this.

Can you think of any scenario which could have caused the new TPS to fail?

Without knowing, I’m reluctant to plunk down another $210. If I do go that route, I’ll temporarily use its existing connector with alligator clips on its tabs. That way, if it doesn’t work, I could return it.

I would appreciate any advice!

[azdave]

I don't see what you could have done wrong if all your readings were as you say. It a simple potentiometer setup with an on/off switch on the side to let the ECU positively know when the throttle is in the full idle position. If you currently have no continuity between pins B&D, unlike when you tested it new, that would indicate that the carbon trace on the board had burned in half or was separated somehow. If you now connect the Ohm meter between pins B &C and then turn the shaft from stop to stop, do you ever measure anything but an open connection as the wiper sweeps around the carbon trace? 

The 5 volt signal should not be able to supply enough current to damage the sensor. Even if that supply wire measured 12 volts, it would barely generate 0.003 amps (0.04 watts) which should easily be handled by the carbon wiper.

I don't know what kind of time schedule you are under but I can offer to bench test the TPS and see if I can determine what failed. Not much to lose except for the time to mail it back and forth and the postage cost. I myself would not risk another new TPS until I had a better idea what failed. 

 

 

 

[OCDan]

5 hours ago, azdave said:

If you now connect the Ohm meter between pins B &C and then turn the shaft from stop to stop, do you ever measure anything but an open connection as the wiper sweeps around the carbon trace? 

Thanks for your reply azdave!

No, the connection stays open through the whole sweep. Thanks for the offer to bench test it. What would that involve? Just wondering if, with guidance, it's something I could do. The FSM Fuel Pump Circuit diagram contains this section on the TPS. Presumably, the new TPS circuit is the same...

I do have a good multimeter.

Thanks again!

Edited 1 hour ago by OCDan
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