The V8 Owners Forum -

I've been having a few problems with the 24V ignition on a 101 today - hopefully someone can suggest where the problem lies.

Spent yesterday getting the engine running again after the vehicle had been laid up for a number of years, and by the end of the day it started and ran fine (though it did need some tuning)

Today, after doing work on other areas of the vehicle, I tried to move it but it wouldn't start. The work that I have done today cannot have disturbed any ignition components. The problem was eventually tracked down to a very weak spark, but I am at a loss as to what is causing it - timing and points are set correctly, condenser is fine, rotor arm is making good contact in the cap, the power feed to the coil is fine and the connection between engine and battery is suitably heavy and in good condition.

My thoughts are that the coil may have expired. Since these particular coils are quite pricey, I want to check that there isn't anything obvious that I have missed.

Since this is one of the Rapier FUT vehicles, and as such very rare, I am not willing to fit an aftermarket ignition system

Hi Bryan!

Does the 24V system have a ballast resistor?

Can you try a different set of leads?, incase they are breaking down?

Chris.

It has a voltage regulator rather than a ballast resistor. I can't be sure that it is putting out the correct voltage (I can't find anything that tells me what it's output should be), but the output seemed sensible and was stable.

I checked the spark at a few plugs - all were weak. Would have thought it unlikely that they would all fail overnight, though it could be the coil lead. Unfortunately, we don't have any spare leads. Is there any way of testing them off the vehicle?

Hmm, a voltage regulator eh? Does that mean that it runs at 12V?

Chris.

when you say you diagnosed a weak spark as the problem, what colour is the spark? ARE you SURE it is a weak spark causing the problem?

jam one of the plug leads straight into the coil, and a spark plug in the other end and crank it over. You will get 8x the number of sparks as you do at one plug, but that will eliminate the coil lead if it is faulty.

What voltage are you seeing at the coil when the points are closed? (ie. not cranking)

with the same plug lead in the coil, remove the spark plug, and jam a screwdriver in the plug lead. (You can hold the handle of the screwdriver, as long as it is plastic!) Get someone to crank the engine, and see how far you can get the spark to jump to the engine. Should be at least 6mm.

The spark is so weak as to be barely visible. The previous day when I checked before starting the vehicle the spark was exactly as you would expect from a correctly functioning system. Since this is 24V EMC shielded system, the plug leads cannot be connected to the coil as the screw on connectors are different on the coil and distributor cap. I am getting just under 14.5 volts out of the voltage regulator (before anyone starts talking about this being close to the normal charge voltage for a 12V, that would be 13.8V for a vehicle from this era)

Is that this setup?



If it is, you can borrow it to see if it makes any difference. I would think 30 year old leads must be questionable by now.

Chris.

14.5 volts out of the regulator sounds spot on. can you connect ANY old plug lead into the coil to test output? It doesn't have to screw in, just the centre make contact.It is possible the problem might still be on the primary side. Otherwise, find yourself an ohmmeter and test the plug and coil leads you have at the moment.

Check your earth strap connection - battery to chassis and also chassis to engine.

I'd bet that when you crank you are getting a massive voltage drop that then translates to the weak spark

Ian

I suppose another idea might be to power the ignition from a separate supply to the starter.

Chris.

but the starter will be 24V no?

This should be an easy one to diagnose. Don't let the fact that the vehicle is 24V with a screened ignition baffle you, this is a very basic 12 volt points ignition system.

With a 14V supply to the ignition system, you should be able to swap out the ignition system either entirely or piece by piece with standard 12v parts to figure out the cause of the weak spark. Or else test the parts individually on the bench until a fault is found.

Chris - unfortunately, that isn't the one. As far as I can make out, the system I'm trying to sort out is unique to the 101 and certain recce vehicles of the same era. I have tried powering the ignition separately from the rest of the vehicle but it didn't help

Conrod - This is NOT a basic 12V system. It is a very specialised military system running multiple spark. The components cannot simply be swapped with those from other systems. Replacing the system in it's entirety with a different type will not solve anything because I will still not know what is wrong with this one

Bryan,
Do you want it original or running? If original, then go to a forum called ARRSE (http://www.arrse.co.uk/forum.php) and ask around there for help or info.