Hmmm...My car was running with that ECU. Don't give up yet, it could be something simple!
I'm thinking your harness feeding the voltage to the ECU is messed up? That's what happened with mine and once I replaced the wires it ran great!

 

ECU is getting 12 volts.

 

How did you check them? end to end or end to ground. If you are looking for shorts to earth in a wire it needs to be disconnected at both ends.

You can use resistors directly across the ECU pins to simulate what the sensor is doing on the other end.

Welcome to the world of XJS ownership, these cars can be frustrating, don't loose hope just work through issues ONE at a time until you have it runing sweetly.

 

i checked wires end to end.

i dont understand what you mean by resistors. theres nothing to resist. theres no output voltage.

 

Check the wires to ground you may have a short to earth, this will cause the voltage to drop and current to increase can cause components to fry (ECU) and/or wires to burn.

If you replaced the ECU and still have the same issue then either you have fried the new ECU or the problem is in the wiring or sensors on the end.

Removing the wiring at the ECU and using a resistor to simulate the sensor can be helpful in faultfinding.

 

what value resistor? 400k ohms?

 

It depends on the value of the senor for example if your TPS is 1-500Z and you want to check it at 1/2 throttle you would use a 250Z resistor.

Just measure the sensor (or use the sensor spec) you want to test then select an appropriate resistor. Use 1/2watt resistors if possible.

 

See next post, this went in twice for some reason.

 

The same wire from the ECU feeds the 5v to the CTS as to the air temp sensor on the B bank air intake and to the TPS. The three feeds branch off the same wire to the engine bay from the ECU in the boot (trunk). First check if the ATS has 5v. It probably has as the two feeds come from the same bit of loom.

Assuming the ATS has 5v (or even if not) it might be worth your while carefully undoing the loom from the CTS and following it it back as far as you can and finding the join that gives the feed to the ATS and the TPS. At this point you should still have 5v (the CTS is getting it after all). Then, if the TPS (or ATS) is still not getting the 5v, run a new feed wire to it from this point.

One final point, if the CTS is an old one, renew it. The car will not run without it, and it will let you down at some point.

As far as I know, if you have 5v at the CTS, then you do not have a feed problem in the ECU. You have a wiring problem in the engine bay.

Once this is sorted, then the continuity of the return signal from the TPS to the ECU is the next thing to check, if needed.

Good luck, do not despair! Such things make jaguar life worthwhile...

 

"i checked pin 20 and 21 on the ECU, which is the 5 volt output for the IAT and its not reading any voltage at all. i checked pins 19 & 20 and theyre not reading voltage either. same goes for 19 & 7 and 20 & 7."

How did you check the pins? I cannot quite see how this can be done except with the multiplug out of the ECU in which case it will be dead, obviously. I think (but could easily be completely wrong!) that you would have to open up the loom after the ECU multiplug and get a point into the individual wires to test the pins properly, ignition on, too.

Apologies if this has got what you did completely wrong.

 

A little bit of electronics theory might help here.

The TPS is basically a potentiometer which acts like a variable voltage divider.

So lets say we have 10volts and 10Z (ohms) one side of the pot (we will call this A) has 10v the other side ground (we will call this B) and the wiper is variable.

Ohms law E=IxR so I=E/R

So total current flowing is 10/10 = 1amp

If we put the put the wiper at the mid point 5Z we know that the voltage will be a ratio between A and B as it is 50% the voltage will be half or 5volts on the wiper.

The important part here is if you have a short to ground lets say the resistance is 0.1Z (about right for a piece of wire) the current in the circuit is 10/0.1 so the current is 100amps POOF there goes the ECU. The other thing that happens is the voltage can not sustain the current and it avalanches and drops.

An easy explanation is Voltage is like the pressure in a garden hose before you pull the trigger and current is the water flow. With a nozzle on the hose the water goes a long way the pressure is still high (VOLTAGE) with the nozzle removed the water runs out the pressure drops (voltage drops) but more water can flow from the hose (more current).

Hope this explains what happens when a voltage is shorted to earth and why the voltage drops to almost ZERO.