Testing fuel injector wiring 86 v12 convertible

Thanks for the insight. Im leaning toward the ECU myself. Im not rich but a couple hundred dollars or so wi]] relieve the frustration factor considerably. My schematic shows that the only thing between the injectors and the ECU is a power resistor. I will see if that is something serviceable or replaceable, it shows it as a seperate unit. I can’t find it in the manual, yet. I took my readings from what I could decipher from the trouble shooting section of the manual but they give readings using an epitester. All my readings were taken at the same time and with the same settings on the multi meter. The test light pulsing on every injector when I cranked it throws me off a little still, but my conclusion is that both banks should have tested at least close to each other which they definitely did not, not even close, that was the only test with such a out of the ball park readings.
If it turns out as the ECU being the culprit does anyone have any suggestions as to where to buy or what model number I should use. Is sending it out to be repaired an option and to who is reliable? As always thanks to everyone for pointing me in the right direction. Rod

You could try the cheapie fix, open the case, remelt all the solder joints, and replace any capacitors that are obviously bad (the tops get domed). Or just replace all the capacitors on principle. Capacitors are cheap, but it’ll be fiddly trying to buy all the ones you’ll need. Several members have reported getting their ECU’s working with such fixes, and it wouldn’t surprise me too much if most were returned to operating condition by such methods, but obviously no guarantees.

The 86 has the 6cu.
I upgraded mine with a 16cu from a later model, it’s a better ECU and it’s a direct swap.
Found mine on e-bay, I was lucky.
David Boger is a good source if he has one.

What model is the one you have?
Do you have Cats and O² sensors?

It’ quite possible to be fixed it by re-soldering all the joints and changing some capacitors, unfortunately it didn’t work for me.
The repair man is Roger Bywater, but temporarily closed due to ill health… sure hope he is well.

Came across this video regarding replacing capacitors if its any help.

86 was the change from 6CU to 16CU. If you have a 6CU, they are known for faults, and you can swap up to a 16CU. If you have a 16CU, they are more reliable, but can go bad (Mine did!)

So the ECU…I ended up sending mine to AJ6 Engineering, Roger Bywater, who is the expert. He found my fault and fixed it. Cost and shipping was about $300 I think? Problem is, the turn-around was 3 months, and that was over 2 years ago. I hear the turn around got up to 5 months, and now I heard he is semi-retired due to health concerns?

You can find used ECUs on Ebay for about $400, but you can not be sure what you’re getting. A roll of the dice. May be better to source a used ECU from a reputable forum member or source.

Check the archives, I remember hearing about someone here who found a place that fixed ECUs besides AJ6 Engineering. About $500+ if memory serves? At least you have a guarentee.

If you have the 6CU, it may be worth asking AJ6 Engineering if they have any rebuilt 16CU’s for sale. But they usually don’t, because everybody with 6CU’s need to replace with a 16CU.

Good luck!

The only no. I find on the ECU is LRZ 124 remanufactured by Lucas automotive. Does this sound familiar to anyone. Nothing on the unit suggests a 6cu,16cu or anything close. Thanks for any info.

There must be a DAC sticker


DAC1926 6CU 1980 pre-HE OPUS 9.0:1 O2 Sensors
DAC1936 6CU 1980 pre-HE OPUS 10.0:1 NO
DAC2596 6CU 1980 HE CEI 12.5:1 NO
DAC2597 6CU 1980 HE CEI 11.5:1 O2 Sensors
DAC3062 6CU 1981 HE CEI 12.5:1 NO
DAC3586 6CU HE CEI O2 Sensors

DAC4118 16CU 1985 HE CEI 11.5:1 O2 Sensors
DAC4119 16CU 1985 HE CEI 11.5:1 NO
DAC4478 16CU 1985 HE CEI 12.5:1 NO
DAC4585 16CU 1986 HE CEI 11.5:1 O2 Sensors
DAC4586 16CU 1986 HE CEI 12.5:1 NO
DAC6335 16CU 1989 HE CEI 11.5:1 O2 Sensors
DAC6336 16CU 1989 HE CEI 12.5:1 NO
DAC6337 16CU 1989 HE Marelli 11.5:1 O2 Sensors
DAC6338 16CU 1989 HE Marelli 11.5:1 NO

My 86 XJ V12 (Federal - O² sensors) had the DAC3586 and I fitted the DAC6335

DAC4118 is what Google shows for LRZ124- which according to the above chart is an ‘85 HE 16CU ECU.

Thanks everyone, never thought to Google it. That no. Is what came off of the sticker on the ECU.

I have a spare 6CU that I have yet to test; if you get into a bind, I could plug it into my car and see if it works. If it does, I could send it to you to try- you pay shipping.

Back tag is a Jaguar sticker DAC 4118/RB. Sorry I omitted it.

RB at the end of the DAC number means rebuilt. The DAC number is Jaguar’s part number. The LRZ number is a Lucas part number.

Thanks Mr. King for the offer. I may take you up on it. I am hoping to go to a local junkyard of all places to check on a couple of XJS parts that I found out about. I live in the sticks so if this pans out I`ll be surprised, but I have my fingers crossed. Thanks Rod

I picked up a 6cu ECU today to see if that would get me started, no go. It didnt make any difference. The only difference I noticed between mine and this 6 cu was that the fuel pump didnt run for the few seconds when I turned the key on. Pump ran while cranking though. I put my old ECU back in, pulled the A and B end injectors and both were shooting fuel when I cranked it. I checked for spark at B3 and have a nice spark. So I have fuel to and through injectors in what looked like a good pattern, good spark at B3. I left one plug exposed so I can be sure of steady spark when I crank it and an in-line fuel pressure gauge to monitor the pressure, 38lbs then drop and hold at 30lbs. When I try to start it, it sounds like it fires occasionally, very occasionally. I made sure that the battery is fully charged and even put more fuel in the tank just to be sure those arent the problem. My gauges inside the car are all haywire but the gauges were all corroded because of moisture. I cleaned up what I could but they were in pretty bad shape so I dont have a lot of faith in using them as a diagnostic tool. I didnt think they would have anything to do with my starting problem because they were that way when I did have it running. I installed an external mechanical oil gauge to be sure I had oil pressure. Just after I had the car running the last time and then shut it down I could smell a very faint electrical smell. I couldn't pinpoint where it was coming from but the wires from the throttle potentiometer were melted before I got the car so Im going to check that out tomorrow to see if that was where the smell was coming from When I first got the car the spark shield in the distributor was melted and misshapen. I had to eliminate it because it was so bad. Also the A bank oxygen sensor was disconnected and loose so I assumed it had been looked at by the previous owner. I Installed two new ones. He said it ran when he parked it and I had it running well the last time I had it running. I guess it could still be the ECU but I am going to troubleshoot more tomorrow. Im missing something, I just dont know what it is yetbut Ill find it. Kind of long tonight but maybe someone will see and recognize what it is Im missing. Everyone has been a great help so far and
I do appreciate it. We`ll get it yet. Thanks. Rod On a good note I did get my convertible top lift cylinders installed. Thanks again everyone.

Means that the ECU is bad, the fuel pump circuit is fried, and maybe the rest of it as well.
So we still don’t know if it’s the ECU’s fault.

Bad sign, as they are connected directly to the ECU.
There is clearly something amiss there, either the ECU shorted inside and fried the cables or vice versa?
You should check the TPS and the output voltage from the ECU (details and how to on the Book) and also that the cables to the ECU are intact, no short and good continuity all the way to the ECU plug.
I would do that first because if the TPS or the cables are shorted it could very well destroy a brand new ECU.

I don’t know how important this is, but they must have put it there for a reason…

Keep on lookin’, you will find it in the end.

I took the ECU apart the other day and the only thing I noticed not right to me was a few spots that looked repaired with a couple of not good looking solder joints. I checked for looseness and didnt find any. I will put a soldering iron to them if it seems to be a last resort. Nothing to lose I guess but Im not a professional solderer so I`ll do my best without doing more damage. Thanks Rod.

You can do no harm.
What kind of parts do you think were replaced?

Back at today. Checked throttle potentiometer and all tested good. I pulled the ECU again just to satisfy my curiosity and I used my micro torch to resolder the suspected bad connections. No dice, it didnt help me get started. No better no worse. I did manage to check the circuit though and I traced it to terminals 13&14 and 31&32. These circuits connect to the resistor for the A bank. Im thinking this is where my flucuating readings are coming from. Im waiting on a local guy to come through with another ECU for me. If this doesnt pan out I will be checking around. Meanwhile I will recheck the resistor pack . I cleaned the terminals but now I will take it apart and see what there is to see. Thanks for the input. If I make any headway I will post a note. Thanks Rod I see my text is still screwy when I post something, it doesnt show on my original. Im using my wifes laptop which is same model as mine and it still does its own thing. I dont know how to fix this so I hope anyone reading this realizes I tried to fix this. If I didnt know better I would say this laptop was made by Jaguar. Thanks again Rod

I picked up another ECU today and will see if I can get this thing running again. Waiting until tomorrow so I dont ruin my evening if it doesnt start. It is out of an older car with higher mileage so there is no guarentee it will work any better than the last one. It is a 6cu and not a 16 like my car had but I will see if it will get me started. I will update what and when I find out. Thanks Rod

Well this ECU 6cu didnt start me up either. I didnt have a lot of faith in either of these two 6cus, being from trashed cars, but I could not, not try them. This one as soon as I plugged it in, I got a red light on the left side of the steering column, just behind the telescopic section. This light is switched, if it is up the car will not crank over, down it at least it cranks. It should have cranked because the switch was down, but it didnt. I put old original 16cu back in and it cranked. I put the 6cu back in that I just tried it now cranked but no start. So Im back to my original 16cu. I just ordered one from ebay and when I get it in and installed Im hoping to report back with good news, although it is also a used one but a 16cu. By the way this is the switch and light that confounded me for about 3 weeks when I first got the car. It had a bad solder job on the switch in the steering column keeping me from cranking it over. Light would blink constantly. Thanks