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MG MGB Technical - Melted Overdrive switch 69B

I am currently working my way through the electrics on my 69B replacing and cleaning parts that require it. I came across something that I have never seen before and was wondering if this is a common problem. The OD switch is the standard crook type of that year but the white wires going to it (I think they are white) have melted and worst of all the actual crook of the switch has melted with the black plastic bubbling. Also my ignition died when I turned on my headlights, but after wiggling the wires in the 2 fuse box the ignition came on again and the fuel pump started clicking again and I could start the car. Is this a symptom of bad connections or worse? Hope someone can shed light on this problem. (Pardon the pun!)
Albert Gate

Yes, o/d wires are white there is no fuse in this circuit although some owners have fitted them since. It sounds as though the switch deputises in an emergency. clearly there is a short downstream from here and you will need to find it. The switch controls the relay so maybe thats short, try measuring it to ground.
Perhaps cleaning the spade terminals and mabe retensioning the fuse holders and spades will solve your headlight problem
Stan Best

Albert

As Stan said you must have short in either the wiring or the OD solenoid to melt the switch. When it's working properly the OD should take 1amp or less which is nowhere near enough to melt the switch. I've fitted a fuse in my OD circuit. I don't want to cast aspersions on your abilities but if you're not confident with car electrics I'd seek help, on these old cars with un fused circuits you need to be careful or you could have a fire on your hands.

Bob
R.A Davis

Hi,

A trick I use to safely work on electrics, is to make up a short length of heavy wire, with a 20 amp inline fuse in it and appropriate spade connectors. emove the brown wire from the solenoid and connect this in series with it.
Should anything go awry then the fuse will blow.

Herb
Herb Adler

Since the LH overdrive solenoid only draws .800 ma, there is no need for a relay in the OD switching circuit. One of the white wires has probably shorted to ground and has melted the wires and switch. RAY
rjm RAY

Question . O/D works well cold. When hot it takes 6 to 8 seconds to engage , engages cleanly and switches out instantly. Could it be simply an oil problem as the only change seems to be tempeture, but maybe the solenoid. Sean
S Sherry

As the solenoid ages, it can become slightly distorted over time. At higher temperatures, the problem would be exacerbated. After 41 years of use, it might be time to replace it. I replaced mine, last Summer, and it made a world of difference. RAY
rjm RAY

Ray,
Good advice from you. I think I will replace the OD solenoid as a matter of course. They are not expensive and as you say this one could be 41 years old. Am i right in thinking my OD is a LH and not D?

Albert Gate

D type OD units were fitted up until '67, on 3 synchro transmissions. From '68 to '80, the LH type unit was fitted on the 4 synchro transmissions. Yours should be a 4 synchro/OD unit that uses the internally mounted solenoid. With the removal of 4 small bolts, the solenoid will drop right out. RAY
rjm RAY

Albert
Refer to thread "Overdrive slips in reverse" for a good link, which shows Tom Twist removing the solenoid from an overdrive unit.
David Levy

Waste of time and money replacing the solenoid if the problem is shorted wiring, which is the more likely. Diagnose properly before replacing parts, otherwise it is frustrating as well as expensive. The problem is that unless you were investigating a faulty OD or smoking wiring, or the car has come to you as a non-runner, it is almost certainly the case that the fault has been and gone. Both my MGBs and two I have worked on have shown heat-damaged wiring to the fuel pump, but it was past history. I've fused both the OD and the fuel pump circuits, as neither are fused. With the manual switch on the dash it is easy to insert an in-line with male and female in the white on the back of the switch. With the later column switch it is easiest to use an in-line with bullets where the yellow wire from the main harness joins the gearbox harness in the mass of connectors by the fusebox, which is also the most convenient place to insert a fuse for the pump, in that case in the white where it goes from the main harness to the rear harness. Gear lever switches, and later American cars, are a little more complicated to describe.

The headlights, ignition and fuel pump are not powered through the fusebox, the car should start and run with it missing altogether and the wires sticking up in the air. It's more likely to be a bad connection where the brown connects to the solenoid, or possibly the battery and its earth strap connections. Waggling the wires at the fusebox unless you were really pulling them about and moving the wires at the starter motor as well, as that is where the solenoid was from late 1967.
PaulH Solihull

Thanks for the advice Paul. I will be fitting in-line fuses. You may also be right that the OD melted wires are historical and probably OBE, as I found more melt on other wires, lengths of which have been replaced by the PO. This weekend I fitted a new fuse box, cleaned the connections and replace a few spades and bullets. I now have services working that did not work before. My Moto-Lita horn now works (thanks to advice on Paul’s site and this forum) but now the headlights only work on full beam. I will look at past threads for answers to my electrical woes as it is a common MGB problem. However should the horn and headlights still work when the ignition is off? Mine do!
Albert Gate

Yes horn, parking and head lights, and headlamp flasher should all work with the ignition off.

If mains work but dips don't then the blue-red circuit is broken, which could either be the dip-switch contact or the 4-way bullet connector by the right-hand headlight for a single point of failure, but there could also be other faults affecting each individual headlight.
PaulH Solihull

Paul, Checked exactly where you said and my 4-way bullet connectors were rusted out! I have replaced them with new ones and coated in Vaseline. All works well now. However my front side lights/indicator housings are rusted out. I have ordered new ones. Every time I make progress something else always pops up! Anyway thanks again for your expert advice.
Albert Gate

Glad to hear you are getting there. With chrome bumper parking/indicator light clusters all four corners earth through their mechanical fixings to the wing panels, so you can get problems here as well as within the clusters themselves. Rears are generally OK as the inside of the boot is relatively clean and dry, not so the fronts of course. These are supposed to bolt through a foam pad onto the wing then have a nut behind, so rust or good paint can stop an earth connection. I didn't get the earthing problem but I did find that the chrome trim round the lens would have cut into my brand-new paint (something I've seen on a concourse winner in the past). I've no idea whether this was due to a problem with the new base, or the new lens and trim, but up with it I would not put. So I ran a nut onto each base stud first to hold the base away from the wing panel far enough for the trim to clear the paint. This did mean I had to put about three foam pads on each to fill the gap, but the double nuts meant I could get a good earth connection, Front and back of the wing (to offset the effects of the nuts cutting through the paint) and both sides of the lamp base plate (not the reflectors of course) were liberally coated in Waxoyl before fitting.
PaulH Solihull

This thread was discussed between 17/03/2011 and 27/03/2011

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