Coil voltages confusing

My 1977 pre he with opus ignition has me baffled.
I have a weak spark, just enough to get it kinda running, extremely low rpm.
I have 12.5 volts coming out of the wire that connects to the coil positive terminal.
The second it is connected to the coil the voltage drops to 4.5 volts. Engine cranking voltage about 8. The negative side of the coil shows 1.5volts in on position.
The coil shows 1 ohm across the terminals with 4.7K across the outlet. Lucas coil.
The ballast resistor has only two wires to it per side instead of three as shown on schematics, having been modified per Jaguar. The coil positive is coming from the lower pin with upper two on each side empty.
I’ m confused. Could buy coil but if anyone has an idea of what it might be or what I should check first I’d appreciate it.

Put a good 12v on the coil, seeif it makes a difference

Thanks for response.
I have done this direct 12 v from battery using meter/probe. Its really only way it starts. I have done this leaving stock wiring as well as disconnecting feed from ballast. No noticeable difference. . Low idle like 300rpm.then (I assume progressive fuel loading til) it dies. Little throttle response.
I have had rebuilt transmission installed and I also removed fuel rails to replace all hoses. I also removed air injection and most emissions equipment.
It started and accelerated easily before these endeavors. It did not move tho do to frozen brakes and rotten tranny.

I have found a Lucas Fault book.
The coil voltages are indeed this low.
I now have a procedural checklist to follow…
I was able to get it to rev up with throttle linkage disconnected from pedestal.
At least i know engine still works.

So, it revs OK if the throttle switch stays at the idle position? That’s a big clue! Try simply disconnecting the throttle switch and put the linkage back together, see if it works that way too.

Thanks for reply.
I’ll try that and let you know. Revs ok is my optimism. It very slowly increased to around 3500-4000 from sound of it with a few hiccups and pops. But yes i see ECU would still be thinking its in idle or?.
On a separate note I decided to adjust linkage while at it. There were no, I assume bronze, bushings to hold butterfly shaft solid on the end . I found some rubber bushings to temporarily stabilize. Are fiber or bronze bushings still available?
When the manual states to hold pedestal stop 2mm off of idle stop while adjusting the two longer rods is this just for that process? I mean at idle should that 2mm gap be there and I adjust cable accordingly?
That’s where I left it last night but on waking that seems wrong. as cable would always be under tension. How should i proceed?
Thanks again.

Engine does indeed run with throttle switch disconnected. It will also rev up to about 2500 using only pedestal. . For one brief moment it sounded sweet. Then it began to act like the ignition timing is very retarded. Backfire in exhaust etc.
I cannot figure out why engine ran great before transmission was installed and now runs so poorly…

Well, I guess the first thing to check is the throttle switch itself. Unfortunately, I’m only barely familiar with it. IIRC, a wiper passes over a “comb” of contacts, each of which causes an injector pulse and so acts as an accelerator pump. Whatever, I’m sure there’s a test procedure somewhere. Go over the thing with a VOM and tell us if it looks good or not.

I have my doubts that will yield answers, though. I’m afraid there’s something wrong in your ECU. But we need to eliminate all other possibilities.

OK Lets say switch does the on/off resistance thing its supposed to do. Is there a way to determine if that signal is reaching the ECU. The connector/wire harness into the switch looks like it could cause problems as well. I’ve wiggled the wires a bit but hard to tell if that’s an issue when its not running. Its not visibly broken but looks delicate.

If your throttle switch is unplugged, you’d have no accelerator pump. Hence, you’d have to accelerate very gently. But doing so, you can rev the engine up. With the throttle switch plugged in, what happens? Cannot rev at all?

BTW, if you haven’t already, please check that your engine ground strap is properly connected from engine to subframe to chassis. That’s the kinda thing that can get messed up when the tranny is out. And the symptoms can be all over the place. And unfortunately, lack of a good ground might do permanent damage to something or another, including the ECU.

I’m not super familiar with the pre-HE or Opus ignition either, but per the 79/80 diagram, there doesn’t appear to be a throttle position sensor (thus wiper as you described)… just maybe a switch on some cars.

1979/80 3CU portion of the circuit illustrated below & colorized for wire-tracing / checking w/ multi-meter from ECU connector to sensor terminal:

Note the connector is a 25pin unit, not the later 6CU 35pin unit.

~Paul K.

I can’t even find the throttle switch on that diagram!

Thanks for diagram. It does seem to not show the swiper under the throttle capstan. That is definitely part of my 1977 system.
A new thread details my startup and now running v12

There are some small mistakes in the diagram, 310 is the throttle switch and 298 is the thermo-time switch.


Thanks for pointing that out. Yep… had that wrong in the diagram. Guess I got crosseyed while making it. Have corrected and updated it to V 1.1 and will soon upload the correction to the FBook Jaguar Owner’s Group files section.

Thanks Tobmag

//never-mind… realized I did this diagram twice… and the one in the files section of that FB group “pre-HE XJS D-Jectronic Schematic (Colorized)%20 from AKM 3455.pdf” is correct for the two pointed out mistakes.

~Paul K.