Non start. No revs at cranking


My 1990 Daimler XJ40, 4.0L refuses to start. When cranking there are no revs on the rev counter. CPS I thought. swapped it out and no difference. Checked wires to the CPU and they are fine. Tried another CPU, and no change. Fuel pump works and battery is fully charged and the starter motor cranks strongly but no spark. The zero revs on the rev counter when cranking has me stumped. Does anyone know of anything else I can try?

Thanks in advance.

John, what about the distributor cap and rotor? When was the last time you changed the fuel filter?

1 Like

John, is the replacement CPS known for sure to be good? If so, you might consider this: on your 1990 model the revs reflected on the tachometer are a result of:

Crankshaft position sensor ===> Engine control module ===> Ignition module a/k/a ignition amplifier ===> Tachometer in instrument pack

Here are excerpts from the wiring diagrams:

You might have a look at the wiring to and from the ignition module, especially the white/black wire that goes from the module to the coil and on to the instrument pack. Also, I suspect that if the ignition module fails the engine speed (revs) signal to the coil and hence instrument pack would fail.

Thanks for replying, Joe.
I replaced the distributor arm and rotor cap, coil and ignition amplifier. Fuel filter changed at the last service, and the tank is half full, and I can hear the pump, but fuel is not much use without a spark :frowning:
No revs showing on the rev counter when cranking usually means bad CPS, but a new one has not made any difference… I even tried swapping around all the relays that match under the dash.
I’m about ready to recycle the car…

Thanks Mike.
I’ll check out all those wires this evening, after work. It’s 10am Monday here now.

John, here are the output data for the two wires (pins 1 and 4 of the black 25-pin connector) from the ECU to the ignition module, in case you want to verify:

Thanks for the info, Mike.

I could be wrong but I think the coolant temp sensor can cause a 'no spark ’ issue if it has failed, I THINK it works like that to protect the engine in an overheating situation.

1 Like

Casso, I have not heard this before. The process of researching this has reminded me that sometimes the more I learn the less I understand. Here are two pages from Jaguar’s AJ6 engine management and diagnostic guide:

This does not indicate what will happen if resistance or voltage goes to zero (temp > 212°F/100°C). What is stated is that the engine will run, in ‘limp home mode’, if the coolant temp sensor signal is absent. In that case the check engine light (a/k/a malfunction indicator light or MIL) will illuminate and a diagnostic test code (DTC) will be displayed by the VCM. What I do not understand is quite how this sensor works. The guide states one pin of the sensor is connected to ground via the engine control module whilst the other pin is supplied with 5 volts, and the ECM monitors the voltage across the two pins, with said voltage decreasing (along with the resistance) as the sensor temperature increases. Now I am (obviously) not an electrical engineer, but I would have guessed the voltage would be greater when the resistance is lower, not vice-versa. I guess more research and learning is required on my part. In any case, I suppose theoretically at the extreme upper end of the temperature range the sensor resistance could be zero - as if the two pins were connected with a jumper wire. Anyone willing to risk testing this (i.e. willing to risk frying your ECM)? Although I suppose if simulating extreme high temp this way DID fry the ECM the engjne would indeed shut down? So many questions, so little time…

i think you can simply measure the resistance at the sensor. If it is low that 190 ohms at normal temperature the sensor is malfunction. Seldomly my xj40 gives temperature sensor error, but i am not shure which is the correct spare to replace it. Can someone suggest the correct P/N for a 1992 4.0 Daimler?

Even a dead short shouldn’t damage the Ecu, it should have built in current limitation to protect against this.

The sensor is NTC, so the resistance reduces as it warms up, therefore the 5v applied to pin-2 is restricted more when the sensor is cold and less when the sensor is hot.
The Ecu monitors the voltage on pin-2 as it being dragged to Gnd, which lowers as the engine warms.

I can’t see a temperature sensor stopping any Ecu from allowing the ignition system to spark. the risk of breakdown is unnecessarily too great.
As stated, it causes limp home mode and an error code.
If a temperature sensor failure was going to stop the engine, it would cut the fuel, not the spark.

I can’t see the complete wiring diagram, but the crank sensor is a two wire Hall effect sensor, so easily monitored. L159-17, L159-24.
An oscilloscope would be best for monitoring this, but a Dmm should suffice at low revs.
The sensor generates a small voltage, which rises with the engine speed, so you should be able to see a voltage output when cranking.
It’s the same type of sensor as an ASB sensor. (two wire)

Firstly check all power and Gnd feeds to the Ecu, Ignition module and coil.

If you manually and rapidly trigger the pulse from the Ecu to the ignition module you should be able to produce a spark and tacho output.
I need to look again at the Ecu wiring, but from memory, it appears there is a 6v feed which shunts to Gnd.
Use a low wattage/LED test lamp to observe this signal, which should changing for each ignition trigger.
The ignition module is just an amplifier to handle the high current of the coil.

You can test the coil and wiring by rapidly shunting the -ve line to Gnd…say by dragging a connected wire over the engine.

FWIW I once fitted a defective temp sensor and whilst the car did start, it barely ran, stalling and coughing for a few hundred yards when I pulled over and refitted the old one.

Didn’t get a CEL btw.

As far as the no spark, what about that red thing in the footwell that pops up stops the car from starting? Has it tripped?

Thanks for correcting that Mike, it was something I was told many years ago when I owned my first Sovereign and had a ‘no start’ problem with it. It was my fuel pump that had failed at the time but when I was trying to find the problem I was told to check the CTS and that had stuck in my mind ever since.

Thanks for all of your replies.
Nothing to report so far as I’ve had “one of those” weeks at work and not been able to look at the old bus at all.
We now have visitors arriving on Friday for the weekend so my “round tuit” has gone completely pear shaped. :upside_down_face:

John, I feel your pain! Report when you get ‘round tuit’ and if you don’t have resolution we’ll keep banging away at it. With all the collective brain power on Jag-lovers we can surely figure it out. Sorry to hear you’re having “one of those weeks” at work. Makes me even gladder (is that even a word?) I’m now retired. Nine months in and loving it!