Hello Good People...

Just when things were getting good. I must have put 600 miles on ol Cherry in the last 3 weeks. Just made it back to NYC and parked her up. Running WELL when parked. No sign of this problem.

Did some work in the last days in the AC Compressor area and in doing so had to mess around with the two power amplifiers to reroute the wire looks over the AC low pressure hose... Then screwed them back down with some fresh thermal paste. That may be something to do with this, or maybe not but it should be mentioned. Haven't driven since this maybe like 4-5 days ago.

Got in the car today and she started right up as usual. Drove for a bit and she was just getting warmed up. Then, tach started bouncing all around (I could be at a light, idling, and the tach would be a 2+ and I know from the sound of the engine that it's below 1) and the engine started running weak, funny, sporadically. Wanting to stall maybe but never quite stalling. I drove her back home and parked her. She couldn't and shouldn't be driven. 1st time for that,,, the ol girl...

I have a spare (new) crank sensor. I will do some more reading to try to get a better idea of the connection between the tach, ignition wiring and CPS and all that - not sure what to do first... I'm a little afraid (and excited) about a need to rewire a bunch of stuff.

What's this sound like to you all?

PS... Changed the motor mounts on my XK8 yesterday. Went smooth as butter. If it ain't one thing it's another.

 

I know nothing about Marelli, but (as they used to say in NYC in 1975) 1 gets you 10 that you have messed up the amps or the wiring associated with same!

 

Ahhhhrg... I never ever disconnected anything. Just moved the amps and the harness of it around and under the low pressure AC line. Then, bolted/screwed them right back into place... Is there a connection between the symptoms I am experiencing and (tach strange idle) and the amps and what is it...? Yeah. Importantly, what is the connection between the two?

 

I am pretty sure that, as with the Lucas ignition, the amps give the tach signal. The amps certainly would explain the rough running.

 

Yeah, the tach reading is all off,,, as is the running. It seems to intermittently lose power, regain it, drop out again. In the mean time the tach is literally all over the place. It's been running and the tach will be up around 3.5 and the car is idling at a normal rpm,,, then the tach will drop to 100,,, no real change in true idle speed (by sound and actual eyeball on the engine). It's nutts. I hate to start cutting the tape off the harnesses but I guess I better gotta... Y'all pros here have been telling me it sumpthin I aughta do anyways, just because. Frustrated (ing)

 

Considering that my problem seems to involve malfunction of the tach,,, I guess I'll start by looking for something being amiss in the "A" side/part of the wiring system... Most likely a case of self inflicted wounds...

Undiscovered country here for me. I'm open to ideas



Does WSU = white-slate-blue?

 

The front CPS was replaced in the beginning of my journey. I have put mayyyybe 2000miles on the car. The rear CPS has never been changed... But, considering my problems seem to be ignition, involving the tachs function, might the rule out the rear CPS which is more involved with engine speed, while the front CPS more to do with ignition...

Also, the CTS is new strating with my work on the car. Same as front CPS. In that,,,, what effect would a failed CTS have on the function or malfunction of the tach? The tach is doing things that have NOTHING to do with the actual RPMs and the engine speed. It has a mind of its own. Does this kinda rule out a faulty CTS?

I am open to suggestions...!

 

Does anyone know where the diagnostic port can be found. In all my digging in this car I can't say I've ever seen one, other than the one for the O2 sensors in the boot (which I still am unsure how to measure/read)...

Its not a lot of wires from the Amps to the coils to the ECU in the RH foot well. Maybe I'll do the long chase and inspection of the associated wiring - which is something I've needed to do anyways. Maybe it's time to replace these wires...? Also, my shielded wire isn't looking so healthy, so maybe it's time to reRUN that wire too. I'm a terrible solder-er... This crazy car!

The location of this component is NOT included in the EDiag bit location illustration, of course. Don't know why they do that.

 

I have no experience with these newer systems but, if the problem only arose after you were rummaging around in there, I'd guess you've fractured a wire or connection. Can you pull one connection at a time and test contiuity end to end? i would not do this while connected to amps or sensors btw..

 

Yeah Man... Unfortunately I think that's what the future holds for me... I have always been afraid of a situation like this with this car... Could be endless and impossible...

 

It seems that much if not all of the ignition system is grounded at this one point (pic). But, unlike many of the other areas of the ignition (and other) systems, this ground point goes unlabeled and unidentified in the ed... It's (ign) probably spelled out somewhere I just don't know where... Anyone know the location of the main ground point for the Marelli ignition system? Firewall?

 

I'm not good at reading wiring diagrams but given enough time, patience, and beer I usually figure it out.

I have the same issue with my Tach and SpeedO. I even took the instrument cluster out of the car to try to figure out this issue.

So, where do I start?
Contact cleaner on the electrical connections to the cluster. Done
Replace Transducer? Planning to take it off the parts car and swap it over to mine this weekend.

Could it have something to do with the wires up around the cruise control bellows? It seems like that is what one of the posts above is suggesting. (My car is an '87 though and probably very different than the '96) I could replace them with the ones from the parts car. Thoughts? Suggestions? Thx

Hoping to re-install the instrument cluster this weekend.

 

Well,,, a tentative "SOLVED"....

So I went out this morning (after finishing with the motor mount install on the XK8) and started cutting off the cover for the wiring to the A and B bank amps. Last week, I (foolishly) really gave them some good twists and turns rerunning them to make room for a the low pressure hose that I needed to bend DOWN some due to the inlet port on the low side fouling the bonnet when closed. Anyways

The wires were soft, flexible and looked damn near new. I disconnected the amps, leaving them on the rad cover, and removed the covering as far back as that 90degree hook in the long L shaped fuel pipe that they run under just before the FPR on the LH side. They looked great.

I then turned my attention to the amps, removing them and studying the plugs. B on the left hand side looked good. Both amp and plug. The A on the RH not so much. The plug was a bit discolored on the two connectors on the LH side of the plug as would be when the plug and amp are in place on the rad cover. So, I went about giving them as good a cleaning as possible. Small plugs and small spade didn't make this easy. I plugged everything back up and started the car. She fired right up BUT the problem persisted. Idle rough and unsteady,,, tach needle not corresponding with engine speed. I turned her off

About two months ago I cam across and new genuine Marelli Amp on eBay and took it CHEAP. So, I went and plugged that in on the RH side replacing the A Bank amp. I learned that the A Bank amp, unlike the B Bank Amp, work thru the coil to produce the signal that runs the tach... So, my focus today (thanks to the ED) was only and mostly on the A Bank part of the ignition system. Plugged in the new amp and started the car, also built a new plug for the CTS during this, and started her up. She went to idling smooth as silk...

Failed amp? But why? The reason I say tentatively SOLVED is that I have a HAUNTING suspicion that a failure in MY rewiring something's todo with new coil plugs and messing with and rewrapping wires in the RH corner at the rear of the engine bay a couple of weeks ago caused the failure.

I don't think I'm out of the woods just yet...

Tomorrow (after installing a new door handle in the sweet running 928) I plan to start really really really giving another and BETTER go at cleaning up messes I myself have made in this wiring system... I think it's faulty wiring or shorting in some wiring that put some stress on that amp... I could be wrong but I'd rather have a better look.

Another foolish thing that may have contributed to the failure of that amp was that when I moved them last week to rerunn the wire under that low pressure hose, I didn't disconnect the amps from the connectors and knocked them around in the process. Who knows, them things could have been original. 30yrs old,,, and why would they take well to being rough housed. Stupid move.

Below is a picture of the "Diagnostic Socket" the ED talks about. Also, I am pretty sure that that common ground point for the ignition system in the ED is one of the two on the LH fire wall...

How does one TEST the quality of a ground point and wire? Measure for resistance?

Both of my CPS test at 742ohms exactly and the plugs are clean.

I am ALL ears for pointers and suggestions...

What years were the Marelli car in production anyways? Just a few years of the xjs run. I could just be me but there seems less info on Marelli than the other XJS years.

"Diagnostic Socket"... No clue what to do with this. Can I test for good ground here?


I was HAPPY at the wires condition... Clean, soft and flexible. Felt new-ish.

 

Well found.

Probably ag erelated, and the moving and twisting was the last straw.

Marelli was the XJS 5.3 ltr cars 1989ish on. The S3 V12 (5.3) stayed Lucas. The 6ltr, Sedan and XJS were all Marelli, up to the last of them in the X300, where Coil over plug and NO distributor was used.