Intermittent ignition fault - diagnostic advice.

R

rottenlungs

Member
Hi. My usually reliable Elegant Thirsty Dinosaur (75 3500) has started misbehaving following a bit of a lay-up. It was off the road for about 9 months. Now, there is an intermittent fault with the ignition. Large hesitations occur. They are abrupt and the tachometer jumps down to zero and back up.
My diagnostic train of thought so far is this.
1) its not a fuel issue, as fuel delivery problems always come on progressively as one then the other carb runs out of fuel.
2) The tachometer jumps down to zero and back up. This behaviour supports my theory that that it is not a fuel problem and is a likely to be low tension issue rather than a high tension problem.
3) It seems to get worse as the car warms up - this can suggest a coil problem, right?
4) I have an accuspark electronic ignition module fitted. It is probably about 6000 kms (and 5 years) old. What is the reliability of them like?
5) I have wiggled all the low tension connections to the coil and the distributor with no obvious effect

I suspect a loose connection somewhere, however, it could be either the coil or the accuspark giving up the fight.

Its a suffix E RHD car. I`m struggling to understand the wiring diagram as to what fuses / circuits supply the low tension from the battery to the coil? Can anyone point me towards any circuitry I can follow to find the supply to the coil (I`m guessing from the ignition switch somewhere?)

Thanks

James
 
I don't believe the ignition circuit is fused, white wire from the ignition switch feeds the ignition and the tacho. Does the ignition warning light come on when the tacho dips as that is in the same circuit?
 
Thanks guys for your comments.

Roverp480 , thanks for the information regarding the circuit feeding the low tension side. I`m surprised there`s no fuse. I would have thought there would be some protection for a short in the coil or similar. The ignition light stayed off when the hesitations occurred, I guess the engine was being dragged around by the car coasting at fast enough a speed to keep the alternator delivering (power steering still had some authority also) . All the other electrical systems in the car were unaffected. I was driving at night and all the lights etc stayed on. I didn`t think to look at the ammeter when the hesitations occurred, I was starting to look for places to pull out of the traffic!

Butterfingers, I do have some aerosol contact cleaner in my garage. I will go through the terminals , clean and then nip them up. The spade connectors on the coil felt tight, as did the inline ones where I hooked in the accuspark in lieu of the points.

The car was off the road for , amongst other things , a steering idler replacement. Getting the ball joints off the idler was a complete pain the arse and I wonder if there is some wiring in that area between the wiper motor and the back of the engine which I might have dislodged? I discovered some new swear words while I was in there with the ball joint splitter!

I hate mechanical breakers with a passion but I do have a new set of points and condenser that I could swap out for the accuspark , at least as a dignostic test.

Thanks again.
 
To test for a loss of ignition supply as a cause of the problem, connect a test light to the supply side of the coil and earth, and with a lead long enough to get the testbulb inside the car. If you lose the supply the test light will go out and if it's on the dash in front of you you can't miss it. Then you can drive the car and wait for the problem to occur.
 
Well, it appears to be the accuspark module dying. The fault seems to be on the -ve side of the coil. Waiting on a new one from Simon bbc.

To add to my woes, now the front driver`s side brake caliper is leaking! No rest for the wicked..
 
I also had problems with my Powerspark ignition module and replaced the distributor by a Range Rover one. I fitted the Range Rover ignition module on a heatsink on the inner wing. It seems these cause problems sometimes due to the engine heat and i believe the Defenders hat it factory fitted on the inner wing

Peter
 
Thanks for all your help everyone. I finally found the smoking gun with my ignition issue tonight, I think.

There is a fuse on the ignition circuit, labelled 19 / 20. I discovered I was able to make the engine cut out by jiggling the fuse around. This fuse is adjacent to the headlight fuses on my car (near the door end of the fusebox). Those being the ones which had already melted in the past and been replaced by a small , more modern blade type fusebox.

When the car was off the road I refurbished the suspension, including removing the top links and replacing the bushes. I must have disturbed the connections when I removed the glovebox to get access to the toplink studs.

I cleaned up the fuse and holder and tightened up the connections on the back of it. Took it for a 15 k drive tonight, all behaved perfectly, even in the rain.

So much for my theory that the fault was on the -ve side of the coil, it was a +ve supply fault all the time.

Given that all the ignition consumables were 14 years old I replaced the coil / dizzy cap / rotor / leads anyway. It felt smoother than it has for a while.

And the brake caliper leak was just a union that had I hadn`t tightened enough after rebuilding the front sus. Win win!

Thanks again.

James
 
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