Ignition Doing My Head In!

[davemar 16]

I still haven't gotten to the bottom of my ignition problems. I've got a 205 Mi16 with the Jetronic ignition system, so it's indepedent of the ECU, so just a distributor, ignition amplifier module, coil, leads and plugs to go wrong. Took her out for a spin yesterday and after about 5-10 mins, the firing hiccuped a couple of times before giving up. Fortunately I was going down hill so had enough momentum to park safely. Walked 3 miles home (still warm yesterday evening, and I need the exercise!), got the other car with tools and spare bits. I checked the spark, and there wasn't one. There was a signal going from dizzy to the amplifier, but nothing across the primary of the coil, so it looked like the amplifier module had given up. I changed the coil (a newish one I had spare) and the module (one I've used before, but not sure about), and she started first time, and I drove her home quite easily.

 

Today I thought I'd take her out for another spin, but staying within walking distance of home! For 15 mins she drove perfectly, so I started to venture a bit further, and then the hiccups started again (rev counter dropping to zero), before conking out. Again had enough momentum to park safely and legally (got just out of the restricted parking zone!). Walked 1/2 mile home in freezing weather...brrrr. I haven't bothered to go back and trace the fault or restart her, I'll do that tomorrow.

 

But I'm at a loss to why the ignition system is being so unreliable. Have I just been unlucky with the amplifier modules? I've ensured a good film of silicone paste between the metal and the module, and it certainly wasn't even warm when I removed it. Does there need to be a good electrical (rather than just thermal) connection between the heatsink plate and the metal mount? I've recently rewired quite a bit of the circuitry for the ignition circuit to try and overcome this problem. There's now a very short direct route for both the ground and positive supplies, rather than the old setup which went in through the bulkhead and out again. The only thing I haven't tested is whether the rev counter which connects across the output of the ignition amplifier is causing problems; but I've never heard of it doing so.

[Miles 331 1 Cars]

Replace the Dizzy, these are now prone to breaking down, The paste on the amp does nothing really and I;ve run car's for yr's without any and never had any problems at all.

[davemar 16]

Replace the Dizzy, these are now prone to breaking down, The paste on the amp does nothing really and I;ve run car's for yr's without any and never had any problems at all.

It's quite a new dizzy, and I've had one breakdown before which gave different symptoms (gradually worse firing, but still doing so, rather than a binary firing or not). The signal from the dizzy pick-up was absolutely fine viewed at input pins of the amplifier (I've got a hand-held oscilloscope) when I was cranking the engine but getting no spark. Unless the signal's waveform is fine, but just getting too weak maybe (the sensor faulty?)? What sort pk-to-pk voltage should I expect at cranking rpm?

[davemar 16]

I've just come across this link:

BMW Jetronic

which suggests "As the distributor spins, and the reluctor lobes are rotated in the magnetic field, an AC voltage signal is generated to indicate the ignition point. The peak to peak voltage of the signal (when viewed upon an oscilloscope) can vary from 5 volts at idle to over 100 volts at 6000 rpm. The amplifier converts the AC pulse into a digital signal." I assume this sort of voltage is what can be expected from a 205 distributor? I'll have to check my levels again.

 

Maybe the levels are just dropping below the amplifier's input threshold when the car warms up, and replacing another amplifier just may have had a lower threshold that made it work a little longer. Certainly when the car started to hiccup, if I revved hard it would kick in again for a minute or so.

 

I might have to make a dizzy spinning rig to test it off the car.

[davemar 16]

Went back to the car this afternoon, and it started first time, and I got it home OK.

 

OK, here's some waveforms. The first one is at idle, when the engine is pretty cold. The second one is after a few minutes when things have warmed up a bit. It is clear the second waveform is smaller, and it kept reducing until the engine finally cut. So Miles, your suggestion looks promising. Could it be the sensor pick up on the dizzy, or the internals? I'm hoping the former, as it easier to change, and I might be able to find a spare one.

 

[Rob_the_Sparky 9]

My guess would be sensor pick-up. I've also had problems with the wire from the dizzy to the ignition amp (as well as the amp and coil on other cars). However, I'd expect a wire fault to show a on/off failure so this is not likely. My experience of an ignition amp with an insensitive input was that it would fail to start but ran fine once started hence my opinion that it is more likely the pick-up (hall effect) in the dizzy.

 

Rob

 

N.B. I never measured the signal on my cars though so can't be 100% sure...

[davemar 16]

The magnitude of the signal dropped away quite steadily, not a loose connection type behaviour. The wire passed the wiggle test, so that's probably OK.