The 6337 is for cars with catalyst
And from AJ6 Engineering:
16CU Part Numbers and Applications.
All are preceded by DAC.
4118, 4585, 6335 - Lucas ignition, Cat
4119, 4478, 4586, 6336 - Lucas ignition, non-Cat
6337 - Marelli ignition, Cat
6338 - Marelli ignition, non-Cat
Cheers
DD
The only situation with no dash lights scenario - would be main relay, not ECUâŚ
Have you been using âtrustedâ workshop âwith good reputationâ or youâve replwced these by yourself? Just sayingâŚ
Have a lookie here, one of the greatest f-ups of Bosch. These relays were critical components and were released on the marketâŚ
The main relay has no bearing on the dashboard warning lights.
The lack of dashboard warning lights (oil, brake, etc), since they all apparently fail at once, suggests lacks of voltage from the ignition switch. And of course voltage from the ignition switch triggers the main relayâŚwhich in turn powers the ECU and injectors and triggers the fuel pump relay.
Experiences vary. They were no worse than what anyone else was offering, in my experience. Loads of oldie Jags are still running around with them, decades on.
Cheers
DD
I donât know how different the 1990 MY electrics are from older models, but like Doug and Janusz, I also see no connection whatsoever with the dash lights and the ECU or the ECU main relay.
The dash lights would be the first thing to investigate:
Dead battery because of parasitic drain or a failed alternator
Ignition switch
Bad contacts somewhere
Failed load relay
The ECU main relay is worth investigating though, easy and can be substituted with a normal SPDT relay with an addition of a diode as per the relayâs diagram printed on its casing.
The Resistor pack if missing will damage the injectors and not the ECU
Donât know what would happen if itâs shorted , but for sure the ECU will not be very happy.
Other things to check:
Does the fuel pump runs for 2 seconds when the ignition is turned on?
Do the injectors see 12V?
Is the injectors ground shorted?
Does the ECU have 12V and good ground?
Doug
When the string of dash lights are off it will not crank over so no start.
I am now convinced that the issue is loose or failed connection anywhere from the MAIN RELAY to the battery via the ignition and inertia switches.
Since the ECU is tight next to the main power and fuel injection relays, its likely something got disturbed while changing out the ECU, so things started working again, for awhile. There are also chassis grounds in the area, as well as connections on the positive battery cable that could be disturbed.
That is my suspicion as well. I am going to see if I can hard wire the inertia switch despite multi meter test it seems to be working fine.
First thing I would check would be for 12v out if ignition switch when on.
If no, check foe 12v to ignition switch.
If no, make sure your tester has a goid ground, check for 12v at fuse box, some have 12v from battery (Iâm not sure what might be between fuse box and battery, Iâm thinking nothing without looking at electrical wiring schematic).
If yes, 12v at fuse box, you have a good ground for tester.
If no 12v to ignition switch, then work back toward battery, there is not much between the ignition switch and battery.
What a weird situation, this one.
To me, if the dash lights are out and the ECU is deadâŚdoes the engine crank at all?
Usually, when one sees complete loss of electrics either the battery is dead, or the ground(s) have lifted somewhere.
There is a braided stainless ground strap that connects the engine block to the suspension cradle- if that is loose, you will get problems- while 12v is brought explicitly, the tie to the ground side is sometimes perilous, especially when strap bolts loosen.
On my car, I had severe corrosion (long but interesting story) of the 12v buss cable that runs along the firewall- the battery feed to the engine compartment comes to the firewall on the left side (driver side) and is bussed along the firewall to the right side, where, among other things, ECU power for the ignition is provided (IIRC), and the lead to the starter (heavy lead, that is, not just the solenoid lead).
That corrosion resulted in the symptom of a completely dead battery, by the way.
I will be very interested in what the resolution to this one is.
-M
I added a secondary ground strap from the engine to the flange with the chassis number stamped into it.
Use a multimeter and check for DC voltage from chassis ground to engine, the meter should read 0 volts.
I will post results as I go. Currently checking for 12 volts at the Inertia switch and every ground points.
A few remarksâŚ
As you go thru the checks donât forget the â+â voltage junction posts on the firewall.
Checking and cleaning grounds is always a good idea. Whenever you come across a ground point take a few minutes to clean it. Even if not related to the problem immediately at hand youâll be pre-emptively solving most of the much ballyhooed âPrince of Darknessâ problems.
Ignition switch failures are not unsual on these cars.
Cheers
DD
My parts book is often misleading, it gives differing numbers which I will ignore as the 6337 seems to work well.
In WW2 German pilots had no Radar so they would look for factory lights over Birmingham and Coventry. That is why the Lucas factory was never bombed.
You sure the Lucas factory lights werenât flashing morse code which saved them?
The DAC6337 16CU is for Marelli cars with 11.5:1 compression and O2 Sensors
Your original ECU was the DAC4586 16CU for Lucas cars with 12.5:1 compression and no O2 sensors
I donât know what the implications on different compression ratios and Lucas vs Marelli would be.
Did you ever measured emissions?
I presume slight fuel trim differences, high compression versus low compression.
Not sure about Lucas versus Marelli. I donât see how or where the Marelli system talks to the fuel injection systemâŚexcept for engine speed signal. Maybe thereâs a difference there?
In any case, Rogerâs problem seems, at this point, to be a supply voltage issue.
Cheers
DD
In my ongoing search for a fault in the intermittent ignition issue I need some interpretation. Page 182-22 of the XJS Service Manual volume 2 has trouble shooting the MAIN relay. A B C D E sequences.
A
Check voltage at terminal 86, if yes proceed to test B Check voltage at terminal 87. If yes proceed to test C. In my case it is NO.
Check voltage at ground lead from terminal 85. IF SATISFACTORY renew relay.
My question is what does SATISFACTORY mean, Is 12 volt or no voltage satisfactory???
Roger
âVoltageâ = existing system voltage
A charged battery will have about 12.7 volts
Terminal 30 is constant voltage supply so it should have the same voltage as the battery, or very close.
Teminal 86 is voltage from the ignition switch when switched âonâ and should also be at or close to battery voltage
Terminal 87 is the relay output and should also be battery voltage when the relay is triggered
Terminal 85 is the ground. If tested with a voltmeter it will show â-â battery voltage rather than â+â battery voltage
The relay itself will operate at less than 12.7 volts, perhaps quite a bit less. However, the components it sends power to might not
In this type of situation youâre preliminarily looking for voltage in a yes/no sense. If you donât have any voltage at all you look for an open circuit.
If you donât have open circuits you then start looking at how much voltage youâre getting.
Cheers
DD
Thanks again Doug
All of the main relay terminals show battery voltage as they should except terminal 87. I then move to next step which is to c heck ground voltage from terminal 85. It reads 6.2 volts which seems to gel with your number. Question. Do I ground the 85 wire on its own and then ground the other three battery voltage wires on one ground bolt.
Before messing with it the terminal 85 6.2 volts wire shared a ground bolt with a battery voltage wire.
Thanks again
Roger
