ignition coil cutting out due to overheating

[mikeeskriett]

Hi,

the reference to degrees is the ambient temperature.

Assuming the data from a 1986 1.3 van - Trust me there is logic, it's a 1275 with the later style loom but not electronic ignition so should match your setup the dizzy is a Lucas 59D4.

Nominal resistance at 20degrees C is 1.2 to 1.5 Ohms coil power consumption at idle is 2.4 to 2.9 amps

Are you using a pre engaged starter motor...again there is logic. In the later loom there is the option for a cold start wire for the 998 cc models not using electronic ignition which is white with yellow I have had occasions on a 1275 where this being connected can be an issue. It's normally a white/yellow wire to the starter solenoid. If my understanding is correct this bypasses the ballast resistor on cranking to give a bigger spark.

If you we're running a ballast resistor I would expect the coil to be circa 9v your drop could be poor earthing or something else. Normal running voltage with the alternator charging should be 13-14.2 volts if you are seeing that it suggests that you don't have a ballast resistor installed. the other thing you can try is tapping the voltage at the anti run on valve as that is on a similar circuit so may be good to compare voltages.

Not one for you but with the stock rover electronic ignition incorrect coil selection will give you the same issue as it basically overheats the module on the dizzy so the engine will stop until it cools.

Cheers,

Mike

[Hans Efde]

Hi Mike,
I checked the bills and the wiring loom came from GTM, so no history. I checked the Haynes and found that the last 6 years in the Metro production ballast wires were not used anymore. I noticed the scheme you mentioned with the ballast wire bypass. But in case of the turbo, one white wire is just the power, the other white wire goes to the computer. So it's a direct power line and there is no resistor wire. I'll check the connections to see if there is a bad ground.
In the documentation from Aldon they advise for longer life of the coil to install a 1 ohm ballast resistor for normal driving. The remark is that for racing it is not necessary. I suppose they know the coil may run hot (also less air cooling at lower speeds) and it can have a melt down.
Friday I'll have a day off, so time for further trouble shooting. (and for Easter shopping ). I have a temperature sensor I use for espresso machines. I'll take a reading of the coil running without the resistor and then solder one in and take another reading.
Thanks for your help so far.

[mikeeskriett]

Hi,

Looks like you have the proper loom for the Turbo. It's interesting because the Turbo always had electronic ignition from the word go which was a special Turbo only thing until 85 when it went on to share the same system as the regular 1275 system (not 310 van though as that was still points). I'd venture that you would just need a post '85 coil for a 998cc Metro or the dizzy and coil from a post '85 1275 or a plug in inline ballast resistor.

The race guys run a 9v coil on 12v so you get a big fat spark but coil life isn't the best.

Always the fun with a Midas you never quite know what the loom came out of!!

Cheers,

Mike

[Hans Efde]

Yes I just bought a 1.6 Ohms ballast resistor on Ebay. I couldn't find a local garage that has these things in stock anymore, so thank God for Ebay.

[mikeeskriett]

eBay has rescued me many times, nobody seems to stock these things now.

these might be useful....

[stan1396]

Hi Hans,just been reading about your problem.Have you checked your engine earth strap? I had some strange ignition problems with my daughters clio and it turned out the strap was absolutely rotten. I just made a new one up from some heavy cable and its been ok since. Stan.

[Hans Efde]

Hi Stan,
Yes the earth strap is fine. My battery is in the boot and the negative cable runs straight to the top of the clutch cover. All earth is taken from there.

This afternoon I tested the Aldon flamethrower coil I had on the shelf. Although it gives the same 3 Ohms resistance reading as the Bosch coil, it gives different readings in the car. Engine on contact gives 10.5V. I wonder if the voltage drop has to do with the 5m long leads from the battery in the boot to the front of the car.
When running at fast idle the voltage drops to 7.7V. This I can not explain. Possibly when it runs warm its resistance increases? Temperature of the coil after 10 minutes running on fast idle is 59 degrees Celsius. That's just touchable. No cutouts so far, but I haven't driven it fast yet. I'll do but will stay in the neighbourhood!
I wonder if the Mini specialist sold me a coil that is perhaps suited for a Mini, but has a too low resistance for a Metro. Or it just broke.

[mikeeskriett]

Sounds interesting. Assuming you can get to the connections can you take the battery to the front of the car and hook it up with jumper cables to test the theory? While by no means perfect you might be able to rule out your cable length.

If you can tell me exactly where you are probing the voltage i.e. where you're tapping the positive and negative I'll nip out and take a set of measurements from my two Midas's again not perfect as they are electronic ignition but may be helpful to see if you're in the ballpark.

Cheers,

Mike

[Hans Efde]

Hi Mike,
I take the reading straight on the + and - of the coil. I'd be very interested in your type and brand coil and the voltage with the engine on contact and with the engine running.
I will take a voltage reading from the starter motor as well. The plus cable is running directly from the battery to the starter motor. If it reads also 10V it's the cable, if it reads 12V it's in the wire loom to the coil somewhere.

[Hans Efde]

It's 12.5V at the startermotor, so the voltagedrop is coming from the wireloom itself.