Author Topic: Throttle Position Sensor Bad?  (Read 288 times)

Offline Jon87V20

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Throttle Position Sensor Bad?
« on: July 04, 2019, 04:17:07 PM »
Hi all,

Last year, my '87 Suburban V20 was having issues where on the highway it would randomly seem to throttle itself up hard for about half a second then start running rough. The Service Engine Light would come on when this happened. I was getting a code 42. I changed the ignition module and it seemed to clear this problem up but a few weeks later it happened again.

Shortly after this, the engine developed rod-knock so it's been parked for about a year.

I just bought a re-manufactured long-block for it and want to make sure I have any other issues sorted out before I put it all back together.

I was just testing my throttle position sensor following the document here on the forum (http://forum.73-87chevytrucks.com/smforum/index.php?topic=29996.msg251278#msg251278)) using the chart "Test Sheet TP 6".

Here's the readings I got:
A to B:
At rest = 1.75k
Rotated = 5.40k

C to B:
At rest = 5.61k
Rotated = 1.94k

Now these readings are quite a bit off from what the chart says they should be at.

I noticed my sensor doesn't look the same as the one in the chart, so I was going off the assumption that the left-most pin is "A", the center pin is "B", and the right-most pin would be "C".

Also, I had my selector on the multimeter set to "20k".

Knowing all this, is it safe to say the throttle position sensor is bad?

I also took and analog meter and watched the needle as I rotated the throttle all the way through it's range. The needle moved very smoothly and wasn't jumpy at all. Just something to note.

I don't necessarily think this is the cause or solution to the code 42 issue, just wanted to add that info in there in case that would be helpful.

Thanks!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Online VileZambonie

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Re: Throttle Position Sensor Bad?
« Reply #1 on: July 04, 2019, 08:44:51 PM »
You can read a potentiometer for fun using the ohm scale but everything should be tested live whenever possible (dynamic values). So if you are checking a TPS, you should do so with a fully charged battery while you backprobe the device using the min max feature on your meter.

As far as DTC 42, don't diagnose it until you have an engine running that isn't producing a rod knock.
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