Ok I see (sort of). The 47V Zener shown here is connected across the power downstream of the injectors but UPSTREAM of the transistors (N across N). There’s one big problem I have with this image from AJ6 BUT it is possible that the Zener is connected UPSTREAM at the pins on the OTHER side of the circuit board (not pictured)… and I haven’t examined this. BUT I see no way it can be connected upstream of the 5.6/6Ohm ballast resistors as there’s no wire for this.
I wish Roger was more of a regular here to answer how this diagram interfaces with the circuit board. I don’t see it in this configuration. In this config it looks like peak and hold pins would have to be fused? (I don’t think they are).
Don’t let this confuse you while I’m scratching my head over it…
The thing that gets me is the illus shows the Zener connected forward of the ballast resistor and to the ON circuit (as well as the hold in front of the ballast resistor). I guess technically it could be drawn there wired across the OPEN pins (which would put it in front of the HOLD circuit when the harness is connected). If the Zener were shorted it would hold BOTH the OPEN and HOLD sides grounded with the harness connected, but would show connected to ground ONLY on the OPEN side with the ECU disconnected (both subject to the 0.33 Ohm resistor)… IF this diagram is accurate.
I commented about this on another thread- this is why you cannot run the engine with the resistor pack disconnected; there is no protection for the “hold” transistors, as they get the protection through the pack. Lucas was too cheap to have a second Zener, or run the extra wire.
The failed 6CU that I looked at failed in the mode you describe, with the “on” transistor shorted, and the “hold” still functional. With the “on” transistor shorted the injector is powered and what the “hold” circuit is doing doesn’t matter. Same would be true if the Zener shorts out.
Hi Paul & Robert,
Thanks for your thoughts to date…AJ6 Engineering is slammed with work and wont be able to look at an ECU until end of August. ( This is car is my daily so I’m lost for summer ) I called ACT Drivetrains LTD who tested my original unit to see if they can repair it or the donor unit. Donor unit primes but injector fault is still present ( Injectors & ECU case get hot at key on ( No Crank )
** Got Load Probe leads to do voltage drop test from InJect. Plug… would get 12.11v drop to 11.56v when loaded **
Unless you can get someone to tackle repairing the ECU itself, I think at this point you’re reduced to pulling the covers off the ECU and probing components to see if you can find a fault (likely the Zener diodes, and RCA transistors). I haven’t fully investigated, but there might be an analog of the pressfit (I think it’s pressfit) diode here: https://www.wontop.com/pressfit-diodes.html
Replacement transistors have been theorized and discussed in a couple of other threads.
Of course this is still totally speculating on likely failed components.
Terrible options but:
These guys down in AU advertise on EBay to repair XJS ECUs:
It sounds like these guys owe you at LEAST a phone diagnosis if not a re-check of the ECU they supposedly looked at given all the diagnostics you’ve been through:
IDK if Walter Petermann of Corsair Electronics in Houston Tx still exist, but in the past he did do repairs on the 6CU ECUs… but he may have retired. You can still look up a phone number for that search, but IDK if it’s still any good.
These guys have fixed more modern Jag stuff and ABS modules. Don’t know about earlier ECUs:
Thank You all …for your suggestion’s for a ECU repair specialist. I contacted ATC Drivetrain UK ltd again and took my Original & Ebay ECU’s in for testing.
Text Messages back from Steve at ATC Drivetrain UK ltd
ATC Text Response (1 )
Ref ECUs for test.
The ECU you purchased from eBay has failed on circuit fault - this is a fundamental failure and would almost certainly not start the car.
The ECU we previously tested has failed on a list of specification errors, including fuel pump output.
This is the same test rig / harness as used to test it originally so our best explanation is that it’s just a genuinely intermittent / faulty ECU
MY reply
What are my options & the related costs?
ATC Text Response (2 )
I can schedule the ECU for rework. This would be £295 plus vat. However, as you’ve paid a test fee previously we would reduce this to £225 plus vat.
Please confirm,
Thanks
Steve
I told them to go ahead and remanufacture one of the units. Hopefully once back I’ll be back on the road before summer ends.
Good luck on that, but I’d keep in CLOSE communication with them as there is STILL no explanation on why there is injector control malfunction with your original ECU (unless that’s part of the list of specification errors). And THAT has to be explained. Your testing has ruled out everything but the ECU that can currently be explained. Possibly need to talk over any possible test that could have been missed in our check regime (I can’t think of any at this point).
Hooray!!!
Hi Paul & everyone
Got my original ECU back yesterday ( Friday ) now fully rebuilt and with a lifetime warranty. First I connected and keyed on to check if fault had gone or was it something in my looms…Fuel pump primed 3sec and injectors stayed closed. I’d then fix cold start injector wire an fit new boot :proceed to refit injector loom properly under & around manifold.
Now refit injector rail to manifold & connect loom do KEY ON voltage test…all good. Now clean and refit all spark plugs ( I had oiled bores and left plugs out with towel over head hoping to dry out bores ) connected plug leads and checked all related ignition wiring.
Time to crank…shed spin but wouldn’t fire. Fuel? yes gauge was still connected inline and holding pressure Air /Compression ? pulled all six plugs and checked…Low 80-100 across the six. Pumped clean engine oil into all six bores and cranked engine for about 5 cycles and let sit for 15mins and repeated oiling of bores. Checking the pulled spark plugs ,fuel was present so injectors had been firing (forgot to check with rail out…phew! ): cleaned all spark plugs with brass brush and brake cleaner they had been gaped recently.
Compression check now 130+ across all six ( had to use extension on old gauge setup AJ6 engines have such deep bores & I think my gauge is on its way out ) and engine spin had slowed indicating better sealing of rings.
…
Plugs back in and booster pack attached on second attempt, she fire up after a couple of stabs on accelerator. Seem like she need more air…checked butterfly …ok Then remembered I had swopped in a spare AAV for my rebuilt unit. I’d check the associated air pipes to manifold… all clear, so I adjusted butterfly out beyond spec to get her to stay running ( had to burn of all that oil ).
Fully warmed up I’d adjust AAV and butterfly to get a reasonable idle 850rpm running there was a bit of a shake in block…this would clear up a bit. Now to see if she’d stay running under manoeuvre’s …back and fort on a gradual incline drive. Clutch up slowly …car loads up as revs drop …We’re MOVING YES!!! So I do a few runs and all seem good , fuel lights been on the whole day…I’ve only kept 5-10 litres in her the past month. Its getting late for cranking & revving so I call it a night after getting a lift to gas station and putting a10 litres night cap in .
Saturday …after the rain and helps from the booster pack she fires up. Stays high in revs 1100is after 7mins so I fiddle with the throttle butterfly & AAV…check Fuel Trim on ECU and set two clicks forward of centre. Warmed up Temp needle at centre I’m getting 800-850rpm after a throttle blip.
Its time …for a road trip . I do a route around 8 square blocks, a mini roundabout ,four junctions and a slight climb to drive. Car pulled well and revs would settle ok ( need a new TPS just to be future proof replaced about three years back after Bournes unit failed on arrival to London , that unit develop cracks in the wire at base )Think I need to do a decent run to charge up battery and ensure all well before a long trip.
Anyway THANK YOU ALL for you advice & help , seem this is just the way of Jag Life.
