Advice on testing for ignition faults

[Mark.T]

Hi

I've recently had some intermittent ignition faults on my BX 19TRS, and would appreciate some advice on steps for testing the ignition next time it fails.

So the car has stopped dead a couple of times recently. Using a spark plug tester showed there was no spark at the plug. I have recently repaired a couple of breaks in the ignition and engine sensor looms, but I'm not sure a further break/bad connection was the cause this time. Re-seating connectors and "joggling" wires did not help, but after waiting a while the engine could be re-started and run long enough to get home.

The ignition module is new - Magneti Marelli brand - and I used heat sink paste when I installed it.

Distributor cap, ignition leads and rotor arm are new. Coil is maybe ten years old.

I'd like to know how to test the ignition for faults if the same problem occurs back at home. The ignition module is mounted directly on the distributor (not on the firewall, like my Series 1 16TRS) - so it is not easy to test the distributor sensor with the engine running. But is it safe to test output from the ignition module with the engine running, by back probing the connector, or putting a T-piece into the output wires? I have read that the output from this to the coil could be about 100V. Or can the distributor sensor be tested with the module removed, just cranking the engine over?

Is it worth purchasing a basic USB oscilloscope for testing the ignition, e.g .the ignition module output? Anyone here use one?

I also see from the "Haynes" manual that the BX series have diagnostic sockets... that was news to me, as neither of mine do...

Also, I've yet to find a BX wiring diagram that quite matches my car. Was the wiring for the BX changed on a monthly basis?

Finally - what does the crank shaft sensor do? I thought it was part of the ignition timing set up, but in the various BX wiring diagrams I have it is either not shown, or it is simply connected to the diagnostic socket. As a test of the functioning of the crank shaft sensor on my car I have just tried disconnecting it.... and nothing changed. The engine ran fine, and the tachometer works OK. Bit of a pain as I recently bought a spare off E-bay... So what is it there for?

Any thoughts on how I should test the ignition components next time it dies on me would be very welcome.

Cheers
Mark

[Jaba]

One good diagnostic trick is to note whether the rev counter freezes or not. This will then point you to either the low voltage or HT side.
That diagnostic socket is a joke. Its a crank position sensor but I dont know if it measures TDC, BDC or something else. To my knowledge they were never used.
The wiring diagram (1987 all models) is quite good usually - although it is a generic schematic and includes all the BX features which may or may not be present on your car.
There was a post recently about new ignition modules playing up. I would be suspicious of the HT coil cutting out when warmed up too.But it could be so many things. Brittle cables broken internally are likely culprits.
Hope my ramblings help.

[xantia_v6]

An easy first step is to connect a test light of some description into the 12V feed as close as you can get to the ignition module, but visible while you are driving the car, so you can see that there is still power present when it misbehaves.

[Mark.T]

Thanks for the helpful pointers on ignition testing. Should help get me started. Still puzzled by my redundant crankshaft sensor...

[Tinkley]

Coils do degrade slowly, but usually you get poor fuel consumption as an early warning. I've personally had more trouble from HT leads usually but not exclusively the distributor to coil one. If water gets in there, you can get quite a bit of corrosion.
Equally the carbon inside them can just fail without warning especially if the lead is removed and replaced often ie changing the spark plugs. Make sure the distributor cap is sound, sometimes even new ones have a hairline crack in them and this can be very hard to see - maybe test in the dark? usually any external sparking shows.
I've had coils fail on my old 1.4 and one that was weak on one of my 1.6s'. The 1.4 coils failed on getaway from standstill ie maximum load even though loads of 'troubleshooters' say they fail at high speed.... Same with ones on m/cycles only ever had one fail at speed.

[benofbrum]

I have suffered coil failure on my Midas kit car, based on a 1990 metro. The Metro spares dealer told me I needed a 9v coil which was wrong so it only lasted about 120 miles before cooking itself and damaging the amplifier. Since replacing both last November, it has been fine.