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MG MGB Technical - Ignition circuit problem

I had an interesting thing happen this evening as I was cranking over a newly rebuilt engine to get oil pressure prior to start up (77B). While cranking the starter in 20 second bursts (with 20 seconds cool down in between), I noticed smoke from under the dash. Shut off ignition switch and the smoke stopped, but the ignition warning light remained on and the fuel pump was clicking. It was as if the ignition switch was in the "run" position even after having been turned off. Disconnected ground terminal of the battery.

Removed steering wheel and cowl and sorted wires. Brown wire feed between the headlight switch and the ignition switch had overheated and melted most of its insulation off. Physically disconnected the wiring harness to the ignition switch and reconnected the battery. Ignition warning light came on and fuel pump started, demonstrating power feed to the white wire circuit even though the power feed to the ignition switch had been disconnected. Checked for power (using a test light) on the input side of the third fuse down (white wire circuit unfused and green wire circuit fused) and there was power present. Removed the white wires from the front (input) connections at the fuse box and went to check them for power. No power present. Reconnected the wires, which had been hot before being disconnected and they again showed no power flow. Reconnected the wiring harness to the ignition switch and it now functions normally (but the brown wire feeding it will have to be replaced tomorrow).

Then, cranked the engine over some more, until it showed oil pressure, and had no problems. Put a timing light on the number one spark plug lead and had Adrienne crank the engine over while I dialed in the timing (coil lead had been disconnected from the distributor cap and grounded while building up oil pressure). No spark shown at number one lead. Checked the coil lead, which had been reconnected and it showed no spark. Test light showed power input to coil and power through the coil. Opened up the distributor and found the wires burned and insulation melted. (Lucas 45D4 with points.) The input wire on the 45D is part of the condensor and consists of a spade connector going to the coil wire, then a second connector which goes to both the points and condensor. Distributor was working fine when I pulled it prior to installing the new engine.

Bottom line is that the brown wire going to the ignition switch burned through its insulation when initially cranking (but worked later), something energised the white wire circuit and fed power through the system even after the wiring harness had been disconnected from the ignition switch and the wire to the points/condensor burned up.

I am thinking that something caused the ignition relay to jamb in the energized position. This would explain the power to the number three fuse. It might also explain why, when I disconnected the wires to the fuse, the relay turned itself off as there was no longer current flowing. But, I cannot understand why the brown wire to the ignition switch burned up, nor why the wires to and inside the distributor burned. Since this car will be going back to my daughter after the engine is run in, any thoughts anyone my have about what happened and what needs to be done to prevent it happening again would be appreciated. Les
Les Bengtson

Les,

You want a Paul Hunt answer but while your waiting...

In 24 years of MGB ownwership I have found that only 3 circuits tend to overheat and other electrical shorts in the wiring are often a consequence of these.

Firstly, the headlamps. This is why many people on this board have fitted relays so that the switch and loom only carry a switching current.

Secondly, the horn. When operating this draws a high current. The wiring's ability to loose the consequent heat satisfactorily assumes it will only be used for short bursts. For reliability one can use a relay here as well or disconnect one of them.

Thirdly, the charging circuit. I have found heat damaged insulation on the main wire from the altenator to the starter motor. My solution is to use the spare tab on the altenator to dual wire the run to the starter andalso to run these wires outside the loom thereby increasing their cooling.

David
David Witham

First thought is that if the wiring in the distributor burned it must have been carrying a high current. Since it only has two connections that power could possibly come from - the signal terminal of the tach and the coil -ve - it follows that there must have been some very low resistance connection to 12v on one of those wires e.g. the coil may have fully or partially shorted out or be an incorrect item with very low primary resistance. However this circuit carries double the normal current anyway when cranking because the loom ballast is shorted out, and so may get a bit warm after a long period of cranking, but I wouldn't expect it to get to meltdown unless there was a problem with the coil. And because you *do* have the ignition relay the brown and white at the ignition switch don't carry ignition loads, just the current to operate the relay, light the ignition warning lamp, and power the overdrive and fuel pump, so a short at the coil shouldn't have damaged the wiring at the switch but may have at the ignition relay. The relay could have 'latched' but would need a feed back from the white/brown at the fusebox to the white between relay and ignition switch. But apart from not being able to turn it off with the key the currents involved - in the absence of any other shorting-out problems - are exactly the same as when running normally and so should not in itself cause burned wiring. The fact that the smoke stopped but effectively the ignition was still on when you stopped cranking and switched off is also interesting, it implies that the problem was being caused by the cranking current and not the ignition current. It occurs to me that if there were a bad connection between the main battery cable and the solenoid stud but not between the cable and the brown wires, or in the engine/transmission ground strap, then when cranking the starter some of the current could be flowing though the brown, ignition and cranking wires so damaging them. I'd also check the ignition and starter relays are correctly connected if these have been disconnected when the engine was out as mixing up the winding and contacts terminals could cause strange effects. It's still difficult to imagine just one fault causing all this, so you may have more than one. Keep us posted.
Paul Hunt

Paul and David. Many thanks for the help. I believe I have the problem sorted. A combination of the car being maintained (partly) by another mechanic and partly operator error.

I found that, for some reason, the coil had been turned 180 degrees in its mounting strap. This means that the positive, input side, was closest to the distributor. The car has a Lucas 45D4 distributor and Sports Coil. The top of the coil (coil is mounted in the upright position rather than nose down) was dirty and I hooked the line to the distributor directly to the postive side. This means that, when the points were closed and the ignition on, there was a direct short to ground.

While rewiring everything from the distributor back to the ignition and lights switch, I found that the input side of the third fuse from the bottom had a black wire connected to one of the spades. This wire had a bullet connector on the end and was hanging down, touching the chassis. Thus, a second short to ground. (Do not know why this wire was there or where it came from. That position is normally occupied by the connection to the electric fans which, as is common in Arizona, have been reconnected to the bottom fuse for ignition switch off operation at some point in the past.)

Found all of this over the last two days of rewiring and cleaning. I think the next major project on this car, after the new engine is broken in, is going to be a new wiring harness. Too many strange wires going nowhere. Thanks for the help and advice. Les
Les Bengtson

More on this problem. I spent yesterday and today re-wiring where wires had burned off their insulation. Attempted to start the engine. No joy. No power to the ignition relay. Traced the white wire back as far as I could and could not find a problem. In the process, I had to remove a wire hold down that had several black wires running to ground. Still could not find the problem without entirely tearing apart the wiring harness. Decided to hot wire directly to the white wire circuit. When I did this and turned on the ignition switch to the run position, the starter turned the engine over. Thought I had screwed up and hot wired to the starter relay. (Ignition relay is forward of the fuse box while, I had always thought, the starter relay was rearwards of the fuse box.) Checked the wiring diagram. I was correct. The wires going to the rearwards relay were correct for the starter relay and the wires going to the forwards relay were correct for the ignition relay. Tried again and, with the hot wire connected to the ignition relay, the starter turned when the key was moved to the run position. Decided to study the wiring diagram again and have a beer. While having the beer, and studying the wiring diagram, remembered the wire clamp with grounds connected to it. (Thank god, and the Aussies, for good beer.) Reconnected the ground wires to the body. Turned the key to "run". The engine did not turn over. Hot wired to the white wire circuit. The ignition relay clicked and there was power to the coil now. Turned the key to "start" and the engine turned over. Everything was working properly now. Pumped the accellerator pedal a couple of times (Weber DGV) and turned the key to start. The engine started.

When my daughter got home from school, she started the engine and I monitered the front of the car. Ran it for the 20 minute break in period and all went well. The oil pressure started out at 80 PSI and settled in at 70 PSI. Engine temperature was just below the midway point (no degree markings on the RB cars). Will run it for a few minutes tomorrow and change the oil and filter.

Lessons learned. My eyes got old and I could not read the dirty markings on the coil. Things had changed since I last worked on the car a year ago. Grounding really is important. I still cannot explain why the lack of a proper ground would cause the starter relay to be energised by the ignition relay, but it did happen.

Bottom line. Double check everything and, when you get old and cannot see well, check a third time. Les
Les Bengtson

"I still cannot explain why the lack of a proper ground would cause the starter relay to be energised by the ignition relay"

Les - someone else had this just a couple of days ago, and that was the first time I had come across it in all my years of LBCing. The problem is that the ground point, when disconnected from the body, leaves several circuits connected together. Current flowing in any one of them instead of going to ground will flow through the others if it can find a ground that way.

In this case current through the ignition relay can't get to ground, so it flows via the shared ground backwards through the starter relay onto the white, and through the ignition warning light and the alternator to ground. The same thing would have happened if you had turned on the lights, which is what the symptom was last time.
Paul Hunt

This thread was discussed between 22/03/2004 and 26/03/2004

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