Issue I'm addressing: High Idle
Possible Issue discovered in diagnosis: Faulty voltage going TO the TPS***.

Notable repairs that may relate:
- Previous owner replaced two oxygens (sounds like the post CAT but I haven't verified)
- Some previous owner replaced alternator with unknown. Also removed some warning lights in the dash. (How nice)
- Likely has had an ECU replacement as there is a spare in the trunk. (Isn't that just dandy!)
- I replaced noname alternator with a name brand Denso reman that tests positive (noname tested ok, but was an unknown quantity. - I don't mind having a spare)
- Replaced missing alternator indicator lights that are required for the charging system to work. System charging 13.4 volts @ 1500 RPM (the high idle) and lights are coming on and going out as they should.

Testing
- Tested TPS resistance and checks out.
- *** Tested TPS voltage and I am getting "overload" on the circuit I believe should be giving me 5 volts. *** Other two are showing .0x volts (effectively static) when I am testing them. (I am using a volt meter and a PowerProbe III to test with.

So, I am looking for suggestions on further diagnosing the TPS. I'm hunting through the system to see what other attempts at eletronical-magic have been made by the amateur electrical wizards that played with it in the past (although the aftermarket stereo seems to be a fairly clean install - but I still intend to replace with something that looks like it belongs.)

Recap: I purchased Jasmine, a 1996 XJS 4.0 a few months back. Runs well but idles high and wasn't charging the battery.

 

Sounds like you need to do the throttle adaptation. Unlike the previous generation with an adjustable TPS, on these AJ16 cars, ( and the X300 sedans, they are the same) when the throttle is closed, there is a particular voltage from the TPS at the point and the ECU is adapted to know that voltage is closed throttle. It will vary from car to car, so the ECU's are customized to each car in this way.

Once you have done the adaptation, then the ECU recognizes that it is at closed throttle, and then it will activate the idle air controller, it won't do that if it thinks the throttle is open. You'll either need to go to a dealer to have this done, assuming they have someone who is willing to do it, or do it yourself. To do it youself need a copy of SDD/IDS and a Mongoose cable. I forget which forum it was in, but there is a very lengthy thread about the Mongoose cables, genuine vs Chinese knockoff. Also I believe that V131 of SDD is the latest that doesn't requite an internet connection, but I'm not sure on that. The thread on SDD has all the details.

 

That is indeed the best way….but there is another.

You can take off the throttle body, enlargen the holes on the TPS, put it back on tightening the screws that it can just barely be adjusted by hand, so tight, but not locked in.

Then reinstall the throttle body, and with the car running, preferable warm, adjust the TPS until
the car is idling at about 720rpm. Then tighten the screws, locking the TPS in place.

You’ll end up with your arm close to a hot engine, and you’ll be adjusting and tightening that TPS essentially in the blind, because of how it’s installed….but it can work.

The AJ16 should come out of the factory with the TPS seeing 0.6v at idle. Over time things get older, sloppier, looser, and the TPS may learn that the new idle is at 0.63v. If the car’s TPS has been reset in its lifetime, that becomes the new baseline, instead of the factory 0.6v. So anytime the TPS is replaced, the engine needs to be told what that idle baseline is. It also seems to happen when other parts are replaced, for me the IACV, but apparently not for everyone.

Anyways, that’s the other way to do it. Good luck and please let us know what happens.