My 1983 Jaguar XJS died on me the other night. Kirbert has been helping me with suggestions to try but so far no luck yet. I am not sure but I would like to throw some parts at it. Everything works as far as fuel and spark. The injectors are getting power to them but something is not pulsing them to fire. I know newer cars have trigger units on the crank, cam and other parts but not these cars.
I was hoping that some nice Jag owner would not mind me trying an extra ECU or throttle potentiometer on my car. I would like to narrow it down or get lucky and actually find the problem. There is a local Jaguar dealer in town but I just naturally figure they won’t have the capability to check either item.
I will pay for shipping both ways and offer to buy if it gets my car running. If not, at least I would know what to look for out there in the old inter web.
Hi Russ.
My no pulse to injectors first came from amp coax break down. The next time is was indeed the ECU. A modern revamped 6ECU did the trick. You can substitute the next model ECU (16ECU) into an 83. I have heard the injector power resistor can cause your problem because of where it is as it gathers gunk in the connector. Clean out the connector plug is about all you can do for that. Good luck.
BTW where are you located?
Hi Trev. I did check that coax and it has continuity from pin 18 where it plugs into the ECU all the way to the ign amp module. Unless that box itself is bad the coax is good.
I will try to locate a 16ECU then if it does plug into and work on y Lucas 6CU. Mine was revamped from AJ6 Engineering and I guess I should ask them if they can check mine but knowing the next model will fit and work could really help out.
The injector power resistor was mentioned as a possible problem and to clean it. I sprayed the plugs, male and female and used a small hand held toothbrush like wire brush to clean the pins and a small round file to clean the female ends. Still nothing.
What do you think about the throttle positioning sensor, or potentiometer being a problem? I appreciate the suggestions Trev.
I live in Bossier City Louisiana, USA. Not the place to ask difficult questions about older foreign cars, lol.
Don’t want to harp on the coax but continuity is one thing…the other two are… does the sheath earth and is there short between the sheath and the core? I have heard that’s short may only occur during starting and look good on testing. I was advised to replace the engine bay section of mine as a matter of course.
As for the TPS being involved, I will have to look up my notes and get back to you.
Trev
TheTPS Is one of the primary fuel mixture devices. From memory the TPS has little to do with the pulsing of the injectors. I believe I have once started my 82 with the TPS unplugged. If I wrong about that I’m sure that someone will jump in. Might be worth sending your ECU back to AJ6 for a check.
Trev
I have heard that the coax can get very brittle like most do in the engine compartment. The wire that comes out of the ign amp is a small white wire, well the other is blue and white but the one concerned is the smaller plain while wire. On my car that wire has a female/male connector. it connects to a covering that leads to a bundle just behind and above the oil pressure sending unit. From where it connects just after the ign amp to where it enters into that bundle, it is covered in a heat protected, loose fitting sheath. Somehow the coax is connected to a black wire that must be grounding for the coax shielding in that now lager white wire that came from the ign amp.
Following me? lol, I guess pictures could help. Anyway that black wire is grounded to the intake manifold right bank. I am not sure just where to get a wire that is the same as this coax wire, with the grounding black wire and how to splice a coaxial wire together! It would seem that it must be one continuous coax all the way to the ECU. Right, wrong? The coax wire is still supple and not brittle like some others that are not protected in that area.
If there is a contact between the sheath and the core it seems that there would be no way of detecting it. The center wire of that coax if checked, would show as a short if the wire itself was not broken from pin 18 of the plug to the end where it plugs into the smaller white wire of the ign amp, but if the sheath and the center core were touching it would still show as a short but now the signal from the inner wire would probably not get to the ECU but go to ground instead. I understand how this problem could cause the ign amp signal to be compromised. I don’t see how you can replace that coax without replacing the whole wire from engine bay to the ecu though.
Am I missing something?
Getting a headache, not really but I am interested in getting this right.
Russ
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
7
TheTPS Is one of the primary fuel mixture devices.
Well, no, not really. It only serves A) to tell the ECU when the throttle is at
idle and B) to provide an “accelerator pump” function, popping all 12
injectors when stomped.
That’s in OEM mode. If the ECU in question has been modified by AJ6
Engineering, it’s possible the TPS does more – including providing primary
mixture control for a brief period after throttle changes because the MAP
sensor is too slow for good throttle response.
From memory the TPS
has little to do with the pulsing of the injectors.
When the throttle is suddenly opened, it’s supposed to pulse all the injectors
to provide accelerator pump style enrichment. Hence, turning the ignition on
and twisting the throttle bellcrank while listening for injector clicking is a good
way to confirm that the circuitry from ECU to injectors is working properly.
I believe I have
once started my 82 with the TPS unplugged.
Yeah, should work. Won’t drive well, though.
Might be worth sending your ECU back
to AJ6 for a check.
We’re trying to eliminate all the cheaper and simpler possibilities before we
get to that.
– Kirbert
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
8
If there is a contact between the sheath and the core it seems that
there would be no way of detecting it.
Sure there is. Disconnect the coax from the ign amp. Go into the trunk and
disconnect the main connector from the ECU. Then test for continuity from
the core of the coax – the connector that connects to the ign amp – and
ground. If there is continuity, it’s shorted to the shield.
I highly recommend wiggling wires around when testing for continuity in that
coax. The nature of the failure modes mean that sometimes wiggling it can
make it go from good to bad and back again. If it flickers on and off, it’s bad;
it should be secure no matter the wiggling.
Kirbert, I was wondering how the system knows how to increase the amount of fuel the injector releases with timing pulses as you slowly increase the gas pedal during normal driving. It seems the throttle positioner or potentiometer would dictate that…
Russ
I will try the wire wiggling too. sounds like a good test for that wire.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
10
Kirbert, I was wondering how the system knows how to increase the
amount of fuel the injector releases with timing pulses as you slowly
increase the gas pedal during normal driving. It seems the throttle
positioner or potentiometer would dictate that…
Nope. This system bases fuel flow on RPM and manifold pressure, as
measured by a sensor built into the ECU itself.
Kirbert, do you know of anyone who has installed a 16ecu in place of the 6cu ecu? I am considering it. My 6cu was enhanced by AJ6 engineering but it seems that Roger Bywater is a very busy man and it might take him quite a while to get to it even if I send it to him.
What do you think or know about this?
I did try the coax wire wiggle test but it still shows continuity from the trunk to the ign amp. I still don’t see how I can replace just part of the coax though as it seems to be a complete wire from engine bay to the trunk. Splicing a coax wire that is bigger is an almost impossible task.
Let me know.
Thanks
Russ
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
12
Kirbert, do you know of anyone who has installed a 16ecu in place of
the 6cu ecu?
Yeah, I did! It is truly plug 'n play. Not so with the versions that came after
the 16CU, though.
I am considering it. My 6cu was enhanced by AJ6
engineering
The 6CU that’s been warmed over by Bywater will likely greatly outperform
the dead-stock 16CU. The improvements from 6CU to 16CU had to do with
refinement and processor upgrades but don’t really show up that much in the
driving experience, while the AJ6 enhancement does make a big
difference. Still, you might use a 16CU to confirm that your problem is in
your ECU.
I still don’t see how I can replace just
part of the coax though as it seems to be a complete wire from engine
bay to the trunk. Splicing a coax wire that is bigger is an almost
impossible task.
Nuthin’ to it. Just peel back the shields and splice the cores together. It’s
not even all that necessary to fiddle with the shields other than connecting
one to the other, perhaps with a piece of wire, but if you wanna do a
top-notch job you could wrap a bit of aluminum foil or tape around the splice.
Some have even reported splicing in some non-coax for that coupla feet and
it worked fine.
I guess my biggest problem with AJ6 and Bywater is that I have written to him, he did verify that the 6cu in my car was enhanced but that he seems to be overwhelmed with work. I texted him and asked how to send mine to him to have it checked out but he doesn’t reply. Are both AJ6 Engineering and he in England? I can’t tell if he is still affiliated with them or not. And what happens when he can no longer work on the older ECU’s?
He doesn’t seem the type to talk much unless maybe you have known him for a long time. I have a sense of urgency in that I am not sure how to work with him to get mine checked out. Any help with that?
I have satisfactorily checked the items suggested to try to narrow down why my car quit. If it costs some money to get my ecu checked out to rule that out I am willing to pay.
Yes, AJ6Engineering and Roger Bywater are the same entity. UK based. One man show. He might have an apprentice, but from my dealings with him, he is the best.
If you suspect the ECU, try ebay. There are usually a number of them. This will tell you if you have a problem in this Department or it is indeed the coax.
I lucked out and found one on ebay for about $150, which Roger later “enhanced”. I now have two working units from the super rare 36CU breed…
Good luck,
Steve
PS Shipping to UK and back, plus the cost of the repair won’t be cheap.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
15
If it costs some money to get my ecu checked out
to rule that out I am willing to pay.
Easiest and most straightforward test is to swap in a different ECU, either
6CU or 16CU, and see if the engine runs. Generally, there are two ways to
go about this:
Find someone else with an early/mid-80’s XJ-S and swap their ECU into
your car just for a minute to see if it fixes the issues. The problem is that
many owners worry that you’ve got a short somewhere that burnt up your
ECU and they don’t want you burning theirs up as well.
Buying an entire spare ECU. Obviously, a bit expensive just for a test.
We used to have someone here in the US that repaired these things, but I
dunno if they’re still in business. That’s probably why Bywater is so busy.
The 6CU is pretty old tech, and reportedly even the PCB is an early
configuration that doesn’t hold up as well as modern designs. There’s also
the issue with the capacitors on the board, that apparently they don’t hold up
well with age, and some have even suggested just going over the board and
replacing them all with new caps of the same ratings.
Have a look on eBay.co.uk at item number: 221723133437
I have no affiliation with the seller who is a commercial XJS breaker. The asking price of the 16CU ECU is approximately $150 - if the seller won’t ship to the USA you could always get yourself a courier quote from a company such as www.interparcel.com (again, I have no affiliation with the company).
Before I went to the trouble & expense of trying a different ECU, I would replace that coax cable with new to rule out the “known” cause of problems similar to yours - probably the least expensive course of action too.
Rgds.
A.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
18
…I
would replace that coax cable with new to rule out the “known” cause
of problems similar to yours…
I wouldn’t bother unless it is visibly brittle and cracking from exposure to
heat. Of course, I haven’t heard of any that weren’t. Once you peel that
looming off, the problem is usually obvious.
Let’s not forget that there are myriad other possibilities here, starting with a
bad EFI relay.
I believe I have 2 spares, if so, I could part with one. don’t want to lend it out but more than willing to test yours on my car, no charge but shipping both ways could take a week.
jrinam, I would like to try one of yours please. Let me know what you need to send it. It looks like yours will be the best option to either fix or eliminate the ECU. I am looking at a 16ecu now but I think I will wait to see if yours could do the trick.