Can anyone tell me if I adjust the pot to .036 and then I start car runs great for about 10 minutes then the reading goes to .01 does it mean that the pot is bad? When I disconnect the red wire the wiper it will run fine. Just dont want to throw parts at it but this seems to be the problem. Did a sweep test on it it did seem a little jumpy. Thanks for any help

 

I still have an OLD analogue meter so I can watch the 'sweep'. Digital meters are NOT the best to use for small faults in the potentiometer.
Some digital meters have a bar graph along the bottom of the reading that might more useful?

bob

 

Motorcarman...you didn't answer his question.
Assume the meter is correct and the Pot goes to .01 DCV (which is probably "noise" in the system, and no voltage is present.)
Does that indicate it's not making a connection and thus no voltage

 

So what happens (as far as the engine is concerned) after the 10 minute period you mention has passed?

Or phrasing it another way, what undesirable behaviour are you trying to solve?

Thanks,

John
1987 XJ-S V12 (with a recent TPS swap)

 

The car will not accelerate. When I attempt to it sounds like its not getting fuel bogs down. When I disconnect wiper (red) wire it will throttle up. really confusing to me.

Thanks

 

You should get somewhere between -0.3 to +0.3 volts (measure across the red and yellow leads).when the throttle is in the idle position. If not, adjust it until it is. When you rotate the throttle pot, it should max out about 5 volts when you reach the mechanical stop. This should be a smooth transition. If it "jumps" alot, has spots where the voltage doesn't change or otherwise acts strange, it's likely time to replace it.

Thanks,

John

 

Check the 5v reference on the pot, which is the green and yellow wires this MUST be done with the pot in the circuit. The problem might not be the pot it might be the reference voltage from the ECU.

The TPS in the XJS as far as I can tell is there for closed throttle fuel cut and acceleration enrichment, the load is derived by the MAP.

 

I only get 2.06 on those wires I also notice if I move wiring harness on car around I get - readings. Maybe its in the wiring? Thanks

 

Yep engine bay wiring harness problems are very common on old XJS's

 

Fixed the problem with jag. It was the signal wire, had a bad spot in wire.
Thanks