No Spark

[Lotus 50]

On Saturday my engine started missing and losing power. At the same time the radio began cutting on and off. And the tach started swinging to 8000 rpm, sometimes staying there. The real rpm was around 3000 and falling as the engine lost power.

This didn't last long, and everything shut down. A quick check showed that the battery connection had vibrated loose. It was easy enough to re-attach once my wife arrived with my tools. This brought back normal electrical function, except for spark.

I changed the plugs, but still not spark. And when I crank the engine, the tach rotates a little bit in the wrong direction.

Suggestions are most welcome.

[Andy8421]

Do you have a dynamo or an alternator, and do you have an electronic ignition system of some sort?

Alternators behave reasonably well if the battery is disconnected with the engine running, but the very basic electromechanical 'voltage control' used to control dynamos really needs to have a battery in circuit to maintain voltage stability. While it is tough to be specific, it is possible that the dynamo output went well above the normal battery voltage and has fried some of the electronics in the car - this would have been more likely if you didn't have lights, or wipers or the fan on.

[Lotus 50]

The car has an alternator and conventional point ignition

[Andy8421]

Well, my first guess was pretty wide of the mark.

You will have to work through the ignition system systematically. If the points / coil aren't doing their thing properly, then the rev counter won't work. First step is to confirm you have a supply to one connection on the the coil with the ignition key switched on. If that is OK, check the points are 'making and breaking' a connection to ground on the other terminal of the coil when the engine is turned over. A low wattage bulb and a couple of bits of wire will do if you don't have a meter.

If that is all OK, remove the centre lead from the distributor cap and hold it close to an earthed lump of metal, and confirm you are getting a spark from the coil when the engine turns over.

This will test the low voltage side of the coil and the associated wiring, and will also confirm the coil is working OK.

Let us know how you get on.

Good luck.

[2cams70]

You may have fried something in the rev counter and since the rev counter is connected to the ignition system it may be preventing things from happening properly there. I suggest you disconnect the rev counter and then try starting the engine. Alternators don't like this kind of thing happening is my understanding. You may have blown the output diodes in it too. It shouldn't affect starting though. You'll know for certain once you have it running.

[Lotus 50]

I am thinking the tach as it was behaving weirdly before everything shut down.

Thanks for the ideas

[Lotus 50]

It's back from the repair shop but just temporarily. No start was due to bad points. But it still has an intermittent misfire and a tach that swings randomly in both directions. The shop told me that the distributor is worn and should be replaced. No surprise, it has 175,000 miles on the original distributor.

I have a new distributor on order. Once that's in place they will re-assess.

[2cams70]

A worn distributor will normally not cause a misfire. The usual symptom is timing scatter and this will result in engine knocking at high RPM. If a new distributor fixes the problem it still probably is not the root cause - more likely it's due to something like cap, leads, condensor, etc.

[Sadbrewer]

Lotus50...We were in the Garage trade for over 40 years...My old man was capable of solving and repairing any fault whether mechanical or electrical that arose...sadly I am not.
We all like saving a few quid and there is always a buzz from fixing your own car ...but.. your car is worth X1000 pounds dollars euros ...unless you've owned it from new you do not know what a previous owner has done to 'modify' your electrics...I would seriously recommend you abandon the self fix principle, bite the bullet, and take it to an auto electrics professional to sort it out. ( if of course professionals who understand points, dynamo's, voltage control boxes etc still exist).