Possible burned ignition amp?

So long story short I had a small electrical fire in the vee of my pre he xj12. Turns out something arced on the valve cover and started some grease on fire. Thinkinh it was a faulty spark plug wire.
Anyway. The damage was pretty much limited to the injector harness, sensor harness, and the plug leads. I also noticed that some of the leads to and from the opus ignition amp were melted. This was my biggest concern as these are hard to find and expensive. To test this evening, i cut off the damaged portions and spliced in new wire temporarily.
I turned on the ignition and tested the positive post of the coil with a multimeter and got a reading of 4.9v with the battery at 12.5. Im not an expert on these things but as far as i know its supposed to read battery voltage at the positive coil post no?
Should I start worrying that my amp is bad?

In principle, ‘yes’, Alan. With the engine not turning there is no connection between coil ‘-’ and ground - and both coil ‘-’ and ‘+’ should show battery voltage with ign ‘on’…

The ign amplifiers function is to ‘make’ and ‘break’ coil ‘-’ connection to ground - which alters coil ‘-’ between 0 and 12V. However, there are some pitfalls with the OPUS layout; if you disconnect the leads between dist and amp - coil voltage should indeed be between 4 and 6V…so…?

However, the best amplifier test is simply to run the engine - if it runs OK, everything is presumably OK…

xj6 85 Sov Europe (UK/NZ)

Unfortunately the engine won’t be able to run for a little while. As i have the intakes, cam covers, and fuel rails, off of the car.
The left side cam cover is being replaced as it was damaged in the mishap along with the harnesses. I bought quite a bit of polyethylene insulated wire and a terminal repair kit. I’m rebuilding the damaged harnesses from scratch. The fire didnt damage my injectors but im not taking chances and will be replacing any rubber hoses that were near the flame.
Also a goal is to move the ignition amp to a more cooled location than the vee after the harness is repaired. Im thinking maybe the core support or inside of the car under the dashboard.

Then I think the problem with the amplifier should be deferred until the engine is in running order, Alan - even comprehensive amplifier testing cannot fully substitute the verification of a running engine…?

xj6 85 Sov Europe (UK/NZ)

Yes I agree. I just always think two steps ahead. I had a test light on one of the plugs last night to determine if i had spark but im reluctant to turn over the engine without the intakes attached lest i pull something into one of the cylinders.

That is so very unlikely to happen, of course you can hang cardboard in front of it to keep birds out :smiley:

My sense is that the wire at the ignition switch is at battery 12 +. So, the other end at coil _ should be almoat the same. Slight loss as to resistance is normal.

but, this is nt an amplifier issue. Tehe amp is between distributor - and coil -

Will the car run

use a strobe on anmy spark plug lead. with ignition on, crank. does the strobe flsh? if so, the amp is doing it’s job.

if not, mebbe not. As it coluld be awire ssue, some hwre.


I have really bad luck.

Im planning to focus on just making sure all the wires are repaired from the fire properly. Once thats complete ill be able to test it for sure. I suppose getting some voltage at the amp is better than nothing.

So I finally had some time the last two weeks to get into the garage. I sorted out the amp problem. turns out it was wiring issues. the amp itself tested great. i soldered in brand new polyethylene leads and the car started and ran great. spark is better than its ever been.


Good on ye’, Alan - good work, and thanks for the info!

xj6 85 Sov Europe (UK/NZ)