I have been having the high/erratic idle problem for some time and decided to try swapping the ecu. A local friend had one from an old XJ6 so I plugged it in. Result, smooth idle and smooth running. The only issue has been a P1199 (Fuel Level Input Current Low) when the fuel level got down about 1/4 tank. I reset it with my el cheapo Elm327 and Dash Command on my iPad.

I think I've seen some posts that said you can't use an XJ6 ecu in an XJS.
So far I've seen a significant improvement and no ill effects. I don't consider the P1199 a big deal as I can reset it with DashCommand or a full tank of gas.

 

In diagnosing a problem with my '95 XJS, I used an ECM from a '95 XJ6. It ran well but I too got a CEL and the code P1199 (low fuel). My XJS ECM has never commented on the fuel level signal/voltage and the electrical schematics do not show any relationship between the ECM and the fuel level sensor or anti-slosh module. Can anyone explain why this happens?

 

I posted this question 2 years ago and have still not found a resolution. The car (1996 4.0) has been running fine and passes North Carolina Inspection but I would really like to get rid of the P1199. Any ideas on how to hack the ECU to clear this up?

 

Fool the sensor?

I'm guessing it's because the fuel tank sizes are different, so the sensor shows different resistant levels.

Here's the expected resistance levels for the XJS (from the 500+ page workshop manual):



All you gotta do is find the expected levels for the X300, and somehow change the resistance so that they match up.

I couldn't find the X300 workshop manual online. I got my XJS manual from JagDocs, The Source for Jaguar Documentation! . This might work for you, only costs about $10 for the PDF download: X300 Workshop Manual [Jaguar X300 XJ6 XJ12 XJR]

Padre

 

Thanks - I'll give that a try.

 

Congrads on getting your high idle problem figured out. I have read in the past of others who also ran a XJ6 ECU in their XJS. Nice to have so many XJ6' to get certain parts off for the AJ16.

I wonder if you could just switch out the XJ6 fuel sensor to fool the ECU. Might not give 100% correct readings on the guage but if it shuts off the light would be worth it.

 

Found this on jagrepair.com:

Quite a difference in levels.

 

I think LuvmyXJS has the best idea: find an XJ6 fuel level sensor and just swap it out. Can't hurt to try, assuming they can be swapped.

Padre

 

I think just swapping sensors would mess up the gas gauge. I want to change the feed to the ECU without changing the circuit to the gauge.

 

Electronics is not my forté, but it seems to me it would be easier to modify the signal going to the gauge than going to the ECU. You'd want the ECU signal to be more accurate. But your resistance chart is very helpful.

Here are some discussions that may be helpful:

https://sites.google.com/site/shanno...our-fuel-gauge
how can I make a 90 ohm sender work with a 30 ohm fuel guage? | The H.A.M.B.

This interests me because I'm about to do a swap that may involve a lot of fiddling.

Padre

 

Here's what I've found so far. No problem figuring out the sender/gauge circuit. What I've been unable to find is anyplace it's tied into the ECU causing the P1199. If I can find that circuit I just have to add a resistor (300 ohms or so) so the ecu thinks it has plenty of fuel.

 

Perhaps I'm missing something, but wouldn't the *easiest* solution be to send the ECU you pulled from the car to a repair/reman service?

Given your original symptoms, it sounds like it might be simple solder joint type of repair.

 

"Easiest" - Maybe; if you can find someone to repair an ECU from an AJ16 and, assuming you do find one, you want to spend a minimum of $150 to have them look at it. The XJ6 ECU was free and so is my time. Plus, where's the fun in having someone else fix your stuff?!?

More than likely a simple reset would fix the problem but a PDU is required and the dealer wants $150 to do it.