Does anyone have a link to the schematic for an '88 VDP ? I'm trying to locate the incoming wire that receives signal from the crank position sensor to the ECM. It might help if you just happen to have a diagram of the ECM pin layout too. Thanks in advance!

 

hello
perhaps may help

best regards

 

frenchfairplaydriver:

That's a very interesting schematic! What is the source document? I'd like to add it to my library.

95Leaper:

You can download the entire 1988 Electrical Guide at the link below, courtesy of our forum member Gus and his website, jagrepair.com. Note that some of the XJ40 documents are in the XJ6 & XJ12 section or the X300 section, so it's worth exploring everything that is there.

1988 Jaguar XJ40 Electrical Guide

Cheers,

Don

 

Thanks Don B.
You saved me from posting it.

bob

 

Does anyone know what the voltage reading ( AC or DC Voltage ) or Ohm reading coming from the crankshaft sensor to the ECM should be reading to operate properly?

 

According to the AJ6 Engine Management System Diagnostic Guide, which you can download at the link below courtesy of Gus and jagrepair.com, the nominal DC resistance of the CPS coil is approximately 1.35 kilohms.

AJ6 Engine Management System/OBDI Diagnostic Guide

In operation, the AC voltage is around 2.5 volts, but a lot of meters can't accurately measure AC voltage that low. If your meter can measure frequency, it may show the frequency rising and falling in direct proportion to engine rpm. To read the actual signal from the CPS while the engine is operating, you would need a scope to display the waveform and voltage amplitude.

The easiest way to determine whether the ECM thinks it is receiving a plausible signal from the CPS is to observe the tachometer while cranking the engine. If the ECM is happy with the CPS, the tach will register about 200 rpm while cranking. If the tach shows 0 rpm, the CPS is suspect.

Cheers,

Don

 

I'm getting the proper ohm reading coming out of the crank sensor,now to locate a meter that can precisely read AC voltage.I'll have to check if that reading is making it all the way up to the computer. My meter is showing .6 volts AC but not sure how accurate it is. Also,I'm not getting anything from the tach while cranking. I've tested two crank sensors and they appear to have almost the same exact readings. One was used and one was new.

 

You can test for continuity between the CPS electrical connector and the appropriate pins of the ECM connector, and also check the DC resistance at the ECM connector when the CPS is connected to the harness - it should be just a little higher than approximately 1.35 kilohms.

I'm currently helping a friend with his X300, and the CPS (CKPS in X300 nomenclature) measures 1.3 kilohm but the tach shows 0 rpm while cranking. The wiring between the CKPS and ECM appears to be fine, so I have a new CKPS on order.

The fact that a new CPS didn't work for you is mystifying. So you have crank but no spark?

Cheers,

Don

 

Yes,it cranks fine,there's just no spark and nothing is registering on the tach when I crank the engine. The engine ran ok before it got a little wet (hood was open when sprinkler came on and I wasn't near the car to notice it right away) and then stalled and wouldn't restart. I thought there might be the slightest possibility that the inertia switch might have been causing the problem but it's fine. I'm getting power to both sides of the ignition coil and also to the amplifier. Assuming both crank sensors ( new and used) aren't bad,then my conclusion comes down to either the wiring between the sensor plug and the computer or the computer itself. Does the tach get it's signal from the crank sensor itself or does it come from the coil,like older cars used to? Could this be the reason why I'm not showing 200 rpms on my tach while cranking?

 

Hi 95Leaper,

I hope you don't mind that I revised the title of this thread so we don't have to keep going back to your first post to recall the model and year of the car you're troubleshooting.

The tachometer receives its engine speed signal from the ignition amplifier; the ignition amplifier is driven by the ECM; the ECM drives the ignition amplifier according to the timing strategy which is primarily based on the position of the crankshaft and engine speed; the ECM knows the crankshaft position and engine speed based on the signal from the CPS....

If the CPS is not working properly or there is a problem with its wiring, the entire chain breaks down.

P.S. The answers to these kinds of questions can be found by studying the Engine Management System Diagnostic Guide and the Electrical Guide. Both are required reading for a Jaguar owner/mechanic.

Cheers,

Don

 

So,would there be a possibility that the ignition amplifier may be bad and not sending a signal to the tach since it could have possibly gotten wet? And aside from checking for voltage coming to the amplifier is there any way other than replacing the amplifier to check for a bad unit?

 

I don't know, but it is possible the answer is somewhere in the Engine Management System Diagnostic Guide, probably in the section on Ignition Timing Control....

 

Am I mistaken or did you modify the wiring or connector on the CPS harness when you first started down this path? Are you positive you got the polarity right?

Maybe it was somebody else? - if so, please ignore!

 

I've tried two crank sensors and both are giving the correct ohm reading of 1.35ohms,one is a new sensor and one is used.

 

Hi 95Leaper,

You have so many threads going that I can't recall - have you measured the battery voltage sag while cranking the engine? If it falls much below 11V the ECM will not trigger the ignition. But the fact that you have 0 rpm on the tach would certainly seem to indicate a problem with the CPS signal to the ECM, or possibly the ignition amplifier. How does the wiring look at the amplifier and coil? Any signs of deteriorated or overheated insulation?

Cheers,

Don

 

Battery voltage is good while cranking and just to be sure it doesn't drop from cranking I've been using jumper cables to keep the battery at a constant level. The wiring around the coil and amplifier are fine and I'm reading close to battery voltage going to the coil and there is voltage going to the amplifier too. Do you think the amplifier may have gotten wet and been shorted out and would that cause nothing to show on the tach while cranking?

 

If I may chime in, having had this very problem in the past after cleaning up the engine bay on my 92; I got water underneath the amplifier which was causing a small short to ground in the amplifier wiring and subsequently the coil. Same symptoms, no tach no spark. It didn't dry out on its own and took me a week to figure it out... But in my case, I replaced the amp and dried the connections and it started working like a champ.

Don't know if you've left it long enough to dry out on its own, but definitely worth checking. The coil and amp connections are right on the frame so any little path will create a major headache... Best of luck in your investigation!

 

I have suspected the ignition amplifier since replacing the crank sensor didn't work. I still need to make sure my crank sensor signal is making it's way up to the computer though. Things don't stay wet very long here in South Florida but I'm still going to investigate the amplifier and make sure it's completely dried out just in case any moisture has somehow seeped into the unit or the connector but it seemed dry when I checked it last week. Thanks for the info!