ASD relay doesn't pull over - '96 XJ 4l Petrol - no fault codes

[adam_q]

Here's a list of the reading I took from the PCM multiplug, where there's a difference, "ignition on" is marked with * and cranking with **. Where my wiring colour differs from the '97 manual I have put the colour in (brackets).

PCM Connector
A1 -
A2 (BLU) 12V* 10.8V**
A3 -
A4 0V
A5 -
A6 - (BLK/YEL) Ground (as I recall)
A7 Not tested
A8 0.01V* 0.15V**
A9 -
A10 Not tested
A11 Not tested
A12 Not tested
A13 -
A14 -
A15 3.7V
A16 3.8V
A17 (Purple) 5.1V - this is the 5V sensor supply
A18 5.1V or 0V* Alternating 5.1V or 0V**
A19 Not tested
A20 Not tested
A21 -
A22 (Red) 12V
A23 0.87V * Varies with throttle position up to ~3V max
A24 4.20V
A25 4.24V
A26 -
A27 4.76V* falls to 4.6V ** (some vacuum then)
A28 -
A29 0
A30 -
A31 0V
A32 0V

"B" Connector not tested yet

C Connector mostly not tested except for

C3 12.1V* 10V** (this should be near 0V to pull the relay over)
C12 0V (should be 12V if relay is pulled over).

If any more measurements are necessary please let me know.

I won't type in all the lead meanings - these are in the service manual '97 pages 8W 80-70 and -74.

 

[adam_q]

Here's a list of the reading I took from the PCM multiplug, where there's a difference, "ignition on" is marked with * and cranking with **. Where my wiring colour differs from the '97 manual I have put the colour in (brackets). I have added the circuit function is in [square brackets].

I also checked the ground voltage drop from the battery, because I had read a few posts about bad earths causing problems, it seems quite low, the PCM ground is about 0.17V above the battery -ve post, and the engine is around 0.1V above battery post, both measured during cranking.

PCM Connector
A1 -
A2 (BLU) 12V* 10.8V** [FUSED IGNITION SWITCH OUTPUT (RUN/START)]
A3 -
A4 0V [SENSOR RETURN]
A5 -
A6 - (BLK/YEL) 0V(as I recall) [PARK NEUTRAL SWITCH]
A7 Not tested [IGNITION COILS NO. 1 NO. 6 DRIVER]
A8 0.01V* 0.15V** [CRANKSHAFT POSITION SENSOR SIGNAL]
A9 -
A10 Not tested [AIS MOTOR CLOSE SIGNAL]
A11 Not tested [AIS MOTOR CLOSE SIGNAL]
A12 Not tested
A13 -
A14 -
A15 3.7V [CHARGE AIR TEMP SENSOR]
A16 3.8V [ENGINE COOLANT TEMPERATURE SENSOR]
A17 (Purple) 5.1V - this is the 5V sensor supply
A18 5.1V or 0V* Alternating 5.1V or 0V** [CAMSHAFT POSITION SENSOR SIGNAL]
A19 Not tested [AIS MOTOR OPEN SIGNAL]
A20 Not tested [AIS MOTOR CLOSE SIGNAL]
A21 -
A22 (Red) 12V [FUSED B(+)]
A23 0.87V * Varies with throttle position up to ~3V max [THROTTLE POSITION SENSOR]
A24 4.20V [UPSTREAM HEATED OXYGEN SENSOR]
A25 4.24V [DOWNSTREAM HEATED OXYGEN SENSOR]
A26 -
A27 4.76V* falls to 4.6V ** [MANIFOLD ABSOLUTE PRESSURE SENSOR] (some vacuum then)
A28 -
A29 0
A30 -
A31 0V [GROUND]
A32 0V [GROUND]

"B" Connector not tested yet

C Connector mostly not tested except for

C3 12.1V* 10V** [AUTOMATIC SHUT DOWN RELAY CONTROL] (this should be near 0V to pull the relay over)
C12 0V [AUTOMATIC SHUT DOWN RELAY OUTPUT] (should be 12V if relay is pulled over).

If any more measurements are necessary or useful, please let me know.

 

[adam_q]

After doing a bit more searching on the web it seems that I have been labouring under a false impression slightly - the ASD relay only pulls over for 3 seconds when ignition is turned on, then it opens again. However, it should pull over again during cranking.

So this is the normal sequence of events:

1. Turn on ignition
2. ASD Relay pulls over [yes, this happens]
3. Wait 3 seconds
4. ASD relay opens [yes, this happens]
5. After a few seconds turn key to start
6. ASD relay should pull over again. [This doesn't happen on mine]

So the question is - what signal at step 6 tells the PCM to pull the relay over? Is there:

a) a signal for "starting" e.g. from the solenoid?
b) does it use the CPS to decide?
c) or the distributor sensor?

The manual doesn't really say what signals the PCM needs to decide.

 

[Runnin'OnEmpty]

Adam, the ECM needs to know the engine is turning over
in order to energize the ASD relay, and I'd suspect it gets
the signal from the CPS.

I base this on the fact that the distributor turns at only
one-half the crankshaft speed. Therefore the ECM would
get a quicker cranking signal from the CPS...???

Maybe the CPS plug-in is faulty, or the wiring is open from the
CPS to the ECM...???

 

[adam_q]

OK so today I attached an oscilloscope to Pin 8 (the CPS input) and there appeared to be no activity during cranking - no pulses coming in (at least, nothing down to less than 0.5V, and noise triggering the 'scope below that.) I'm therefore thinking that this is indeed the problem, and the CPS needs replacing.

I took the CPS off (a fiddly job, I kept getting cramp in my hands ;^( ) and reconnected, and tried testing it standalone by waving a screwdriver past it, but saw no activity on pin 8. I also tested the wiring back to the CPS harness plug and that is all OK.

I have noticed posts about OEM CPS replacements not being that good, and to use a Jeep original part, which sounds like reasonable advice. Any reason why I shouln't use (an original) one out af a scrap car or should I definitely get a new one (my scrapyard is much nearer than my dealers)? Are there any things to watch for during installation - e.,g. keep it away from steel parts?

Also does enyone know what pulses I should expect to see when it's working properly? What voltage level? The service manual just says to look at another manual to diagnose this part.

How does the CPS work? Is it a Hall effect sensor or a coil? is there an amplifier built in?

Lots of questions I know but it would be nice to get all this technical info into one thread for future reference.

 

[adam_q]

Update -

Phoned up Jeep for a new CPS - wait for it - £100. So that's $140 of your Earth dollars.

Luckily my (admittedly expensive) scrapyard had one for £36. Kooks like an original Jeep one, too.

Fitted today and it's runing again! Problem solved.

It seems that you get no fault codes for a failed CPS, and checking the voltages on the CPS lines they are indistinguishable from when the failed one was in there.

The only difference I could detect was the old CPS has an open circuit between two of the terminals. On the good one, with the meter set to "Diode", there is a definite diode or semiconductor-like response between 1-2 and between 2-3. Don't use an ohmmeter, use a DVM with a diode setting to check the 3-terminal CPS.

 

[RNMedic]

Adam I followed this thread and you were getting all the right answers, many of us keep a CPS as a spare in the Jeep and can change one in about 10 -15 minutes. Good luck.