Testing fuel injector wiring 86 v12 convertible

Happy as a clam, finally progress, I went out to my garage ready to move on with my 86 XJS convertible V-12. I thought I would fire it up and revel in my accomplishment, let it warm up before draining the oil. AAArgh, no start, right back to where I was a week ago. Cranks over, fuel to and from rail, test for spark at B3 cylinder (easy access) had blue spark but didnt seem strong to me so I tested at the coil with screwdriver 1/4" from ground, no spark, same test at 1/8" no spark, I tested at less than 1/32" hardly any gap, finally a spark so Im thinking bad coil. I tested the coil according to my manual and it tested ok but on the lower end of the scale. I tested the old original coil and it tested the same as the new one in the car. I also tested the secondary coil (not connected but still there) just for the heck of it and it also tested the same as the other two. Can any one give me a better explanation or another idea of what Im seeing or should I say not seeing. I also have a new coolant sensor as gregmo suggested. It is acting like it did before the last time I started over. Could I have ruined the coil from one day to the next because of the spark gap then have it recover on its own to start, then not start the next day. this happened the last time too. I am stumped again. Any help again would be appreciated. Digging deeper tomorrow. Thanks Rod

One more quick question. I had the secondary coil hooked up while it was running as well as the main coil unintentionally oops. When I tested main coil alone I had gotten primary readings of 1.1-1.31 ohms with a secondary reading of 6000-7000,.with second coil connected primary reading was .7ohms. would the lower reading affect the output of the other coil or would they even each other out. Thanks again Rod

Coils is a bit of a gray area forme as well, @Frank_Andersen is more of an expert on this.
But I think that the two coils are connected in parallel, so the difference in the resistance readings you get would be correct if this is the case. Look at the wiring diagram to verify, and there are lots of posts on the archives about coils misbehaving.

The second coil is there because at the time there was no single coil capable of supplying 12 plugs with enough spark above 3.500rpm.
Later engines have a single coil.

An other common culprit for this kind of intermittent problems could be the Ignition Amplifier Module, an easy and inexpensive fix. If you don’t know the history of the one in your car it’s highly recommended that you change it anyway. Get only the GM module.

Good luck.

Frustrating, Put old coil back in with old secondary coil also installed just for the heck of it. Nice blue spark from coil to ground 1/4" away. Spark at plugs. No start again. Put old spark amplifier back in, again just for the heck of it. Still good spark. Installed in line gauge for fuel pressure to monitor fuel pressure 38lb pressure while cranking, when key is switched off holds at 30lb. Seems to be no problem there. Anyone with any ideas. Should I be thinking about another ECU yet? I`ll be checking fuel injectors tomorrow and wiring, again. Thanks for any insight. Rod

I had some odd it runs/it doesn’t run two years ago. After testing/fixing everything else, i guessed it must be ECU, and it was. But that could be an expensive test. I would double check everything, and also check continuity from sensors to ecu, before looking for another ecu.

For the heck of it, i just went ahead and bought the new replacement coil that allows you to remove the secondary coil up front. Much easier setup.

And just throwing this out there, but all this trying to start and only running 10 minutes, is your battery fully charged? This engine needs a fully charged battery to start. I would trickle charge mine a lot when i was going through diagnostics.

Could very well be the case. Had the same intermittent no start and it was the ECU.

Unfortunately the only way to check it is with a known good ECU.
As Greg said you should eliminate all other possibilities before.
Are your injectors clicking while cranking and no start?
Also check the Throttle position sensor. Details in the Book.

Today was fuel injector day. I tested the windings in all the injectors. All tested as they should. I rigged a connector with a light bulb to test the connections. When I cranked the engine with fuel pump disconnected all injectors were disconnected and each one lit the bulb as I cranked it indicating to me they operate as they should. I tried to test with a multimeter and Im not sure what my reading should be. The A bank is flucuating all over the place with my readings but each one was flucuating consistently if that makes any sense. The B bank was as steady as could be with the reading of 11.25 with the meter set at 20v. Still no start. I do keep the charger on low to keep the battery charged. Ill check distributor tomorrow again even though it tested fine earlier. All this redundant testing will show something sooner or later. I hope. Thanks to all. Any suggestions welcome.

If both banks were being tested in the same manner, one would expect the indications to be identical. If they were really that different, that probably points to a problem with the ECU.

Presuming you tested the voltage at the injector plugs, as Kirbert said, there is something wrong…

Worth checking resistor pack (between injectors and ecu on inside right fender). Corrosion on connector could be messing up voltage?

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.