Ignition Puzzle

[Howard]

My TR7 V8 has a very odd problem with the ignition system that has left me totally perplexed. There have been a number of electrical issues which have required a lot of rewiring and some replacement parts to address the various defects, but now I am certain they have been sorted, there is still a problem.

When starting from cold the car starts easily but dies when driven off - usually at the junction at the end of my road - however it will restart after a short time and then runs without any further problems.

The configuration is Newtronic/Pirana ignition with optical sensor, Lucas 35DE8 distributor with the integral electronics replaced by Piarana sensor and disc. Coil is NOS Lucas unit for use with a ballast. HT leads have all been replaced. All other wiring has been replaced and checked except for the ballast resistor (which is on order). The issue is definitely with the ignition as when it stops the Tacho immediately drops back to zero and this takes its signal direct from the coil LV supply.

Electronics don't usually fail like this - they either fail or work - so I don't think its the ignition module, but has anyone come across a similar fault and how did you solve it?

[RobPearce]

45 minutes ago, Howard said:

Electronics don't usually fail like this - they either fail or work

Woah! Stop right there! Electronics can be very weird in its failure modes and should not be discounted. The point at which it fails is a candidate for thermal gradient issues, which can expose dry solder joints and the like.

[Howard]

Thanks for the suggestion. I carefully resoldered all the connections on the PIrana PCB (one power transistor & a single IC)  and discovered a dry joint between a wire and the PCB track which may have been the problem. I will test drive the car over the next few days and see if the problem has been resolved.

Cheers

H

[thescrapman]

I have Piranhas in a couple of my cars, all have needed dry joints dealing with.

the big transistor has also failed on one of them, easily replaced 

also make sure it’s getting 12v if you are using a ballasted system, easily wired into the feed wrongly.

[Howard]

Sorting the ignition out has been a drawn out saga.

First a wire to the optical sensor had broken internally, so replacement optical sensor purchased from Newtronics, then found that ballast system had been bypassed and NKG coil installed which may or may not be designed for use with a ballast circuit ( no data available on the internet! ), so replaced with a new NOS Lucas coil and supply wiring completely replaced. At each step things improved, but still had a problem when initially driven, which probably was the dry joint I found after Rob's comment

Hopefully that's it finally sorted 🙂