TPS voltage readings while idling are erratic

88 XJS V12

When turning ignition on, I get a rock steady reading with my voltage meter of the TPS at 0.32-.34V. (measuring yellow/red wires)

But when idling, the voltage readings are all over the place on my digital meter, and my idle slightly stumbles (in other thread). Shouldn’t the reading also be rock steady when the engine is idling?

What would cause this?

Yes. That fluctuation might be a sign of a shot TPS, but it also might be a sign of a flaky connection somewhere.

Meter the voltage to the TPS while idling. Yellow and Green On the TPS side of the connector. If thats a steady 5v then it’s the TPS.

The YB ground and YK positive supply wires for the TPS also spur off to feed the CTS (YB) and ATS (YK) if that supply voltage is flakey then it would affect all three ECU inputs.

Voltage is steady at 4.95V with only ignition on.

With engine idling, intermittently voltage will jump around (flashing diff numbers between 5v and 16v) and then come back to 4.95V. Sometimes will go a good 10 secs before voltage jumps around.

I tested this with both yellow/green and green/ground.

Any correlation between idle quality and voltage fluctuation at the TPS ?
As Kirbert suggested it sounds like there a bad connection somewhere.

I’ve been chasing an idle stumble for months. But idle stumble does not change when voltage fluctuates.

Can I check other + connections to engine while idling? Perhaps main 12V supply is flakey?

What connection could change voltage supply only when car is running? Bad alternator diode?

Revving it does not change voltage. Flakiness with voltage seems to be more constant once engine bay temps go up.

Bad ground???

That’s why I’ve asked if there is a connection between the two, is there ?

Aha! When I disconnect TPS, but still check voltage on the green wire socket while engine idles, voltage is steady at 5V.

So TPS is shorted? Or plug? I’ve ordered a whole new & improved red pot kit, used, from simply performance for a decent price.

No, I don’t think it can short, it just looses contact. I would take the TPS apart to check it’s condition and see from there.
I added a small washer inside mine because it was doing exactly that.

So could the TPS be accidentally grounded? Still not sure what would influence voltage draw.

Is this part not applicable to a Lucas car?

I’ve no affiliation to the seller but it seems like a fair deal for a NOS part:

That is for the earlier V12 Lucas. I’ve already ordered the new red one (that is on the Marelli cars), but thanks for the link.

Ok, I’ve got a used red TPS in hand with all the adaptor pieces, except the spring pin C in the photo.

I assume I can just buy this in a hardware store? Is it a pin you hammer in? Or a cotter pin? Or a…

It’s what is commonly called a roll pin. When installed properly, it provides a connection free of slop.

Thanks, that’s what I hoped it was. Easy to find at hardware store.

Barely on topic, if at all.

Been watching a lot of You Tube. Some really smart guys fixing a lot f different things. some blabber a bit, but, oh well!! Makes it more interesting than a strict tutorial.

J.C. Smith. Deals in old trucks. Buys what are usually older Fleet vehicles, From pickup to semi’s… A lot of diesel, but some gas.

A Ford V10 with engine issues. Broke it down.

  1. A toothed wheel with a gap. TPS Ford style. the GAP tells the PCM, TDC!!!

  2. As an OHC engine, a very long chain. Tensioner and guide ala Jaguar!! I wonder?? Oil Pressure applies the pressure and takes up the slack. If they leak, the engine clatters at start up for a moment til pressure.

More on point :slight_smile:
A. Where did 14 v come from? Flaky alternator.
B. Does odd voltage matter, so long as the PCM aka ECU. gets the signal.

Or so sophisticated that variations in volts depict crank location in degrees?

C. Where in he harness does the TPS get it’s volts.


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