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MG MGB Technical - Breakdown

Hi,

I have a 1978 MGB GT that has died today and is stranded in a car park 3 miles away!

Has been super reliable and running sweetly. Then driving along today shifted in to third and rev counter immediately showed 0 rpm, no power. Engine was running though as far as I could make out. Dropped to second, running fine. Third - 0 rpm dead again. Same in 4th. Switched overdrive on and off a couple of times in third while dead,third then seemed OK and it ran ok for 5 mins until I pulled in to the gym Car park and stopped.

Came back 45 mins later, turns over but won't start. Had a look around, nothing visibly obvious. Tried overdrive switch on and off, checked every electrical connection I could see under the bonnet

I suspect overdrive electrical somehow shorting to ground - how does that work? Any way I can disable overdrive to isolate that?

Help! Any ideas? (Going to get it recovered tomorrow if I can't sort it out!)

Cheers,
John

JS neal

Your car has the O/D switch on top of the gear knob hasn't it? The wires from that switch are loose around the gear lever and can chafe through. You need to lift the gear lever gaiter to have a look.
Mike Howlett

Could well be the OD switch wiring in the gear lever shorting out, it is a known problem. It's unfused, and comes off the ignition circuit that also feeds the coil. Theoretically a short on that wiring would burn the harness and be obvious, but it's possible that bad connections in the circuit through the ignition switch are limiting the current to a level where the wiring doesn't burn, but the short circuit is sucking away all the power to the ignition.

To get you home, and if that is the cause, there are a mass of bullet connectors against the bulkhead by the master cylinders. Look for a 4-way connector with two white/brown wires from the main harness, a white/brown going into the gearbox harness which should also have a green wire and a green/purple, and a white going into the rear harness with half a dozen others. Pull the white/brown off the gearbox harness out of the connector and that will isolate the OD circuit. Note that if you disturb the other three wires the fuel pump may not function.

In any event, I strongly recommend you fit an in-line fuse where the gearbox white/brown joins the wires from the main harness, and also one in the fuel pump circuit at the same point.
Paul Hunt

This is brilliant, thanks guys. Got the breakdown man to tow me home and shall spend next daylight following these suggestions!

Has overdrive switch on the gear shift - lifted the boot off and had a quick look. Will follow up.

Thanks again
JS neal

just been out fault finding (before the dark and rain came!)

isolated overdrive (see pic), cranked it over, still nothing

Cracked out the DVM, showing ignition switched 12V going to both side of the coil (see other pic). Tried another coil. Still 12v on both sides and no spark on plug 1. Am going to retry while its cranking to be sure points are opening and closing

I could smell fuel as I cranked it but will check fuel pump next.

Will also go read a haynes manual (!)

any other pointers as to where to try?

Thanks in advance,
John







JS neal

wrong image came out!

JS neal

John. Check for spark at the coil lead. If you have no spark, the problem is the coil or the points. If you have spark at the coil lead, but no spark at the spark plug lead, the problem lies in the distributor cap, the rotor, and/or the spark plug leads. The problem you describe is typical of a rotor beginning to burn through (rough running), then burning through (no spark at the plugs) because the high tension pulse is grounding through the distributor shaft.

Les
Les Bengtson

12v both sides of the coil indicates the circuit through the points is not complete. Normally when an engine stops it does so in one of two positions, and the points are closed, which would indicate that the wire from the coil -ve to the points has broken, or the earth wire from the points plate to the distributor body might have broken.

The points plate is continually twisting back and fore when you are driving, flexing those wires, and they can break. With the 45D distributor that a 78 would originally have had the coil wire passes through a grommet in the distributor body and is part of the condenser. The conductors inside that wire can fracture inside the insulation.

As well as making sure the points are closed, connect an earth to them when you measure the voltage on the coil +ve and -ve. If you still have 12v on both coil terminals then that wire is broken somewhere.
Paul Hunt

had a quick look, dizzy cap newish, rotor arm newish. Points clean but must be 5 years old. cleaned and moved points earth through dizzy body. Will try earthing on points next up.

Going to order up new points, condenser, coil whether I get it running tonight or not

First time its played up since I've owned it - was a one owner car from 1979 to 2010, looks like it had been ziebarted and mechanically looked after. Now its dead I don't half miss it!



JS neal

"had a quick look, dizzy cap newish, rotor arm newish"

New doesn't mean good, particularly where some new rotor arms are concerned.

Try Distributor Doctor for ignition parts

http://www.distributordoctor.com/red-rotor-arms.html
Dave O'Neill 2

I would strongly recommend fully diagnosing the fault before buying multiple parts, rather than swapping perfectly good parts for something that might not be so good when you still don't know what it is. Even Dr Dist is only as good as his source, but they are better than a lot.
Paul Hunt

yep good point

you will probably hear the cheer when I get it firing again from there
JS neal

Running!

Stuck a dmm on each side of the coil and watched it while I chased all the cabling around the dizzy.

Cleared when I moved either the black lead that goes through the case or the condenser lead.

Going to renew points, condenser and clean up everything I can see with contact cleaner.

Many thanks for all your help.
JS neal

Paul

Interested in your advice about fusing the gearbox wire. I seem to recall reading that to fuse the o/d the fuse is fitted to the yellow wire; grateful for advice.

Rod
R E Merrall

Rod,
That brown/white wire goes to a connector that has a yellow on the other side and then on to the switch.
Fusing at the point indicated in John's picture is easy to get at and will protect the cables/circuit all the way to the OD.

Best of...
MGmike
M McAndrew

Rod,

It depends on the year as to where the fuse is best placed.

For cars with the dash switch it's best done with an in-line fuse with a male spade one side and a female the other and fitted between the white wire and the switch at the manual switch.

For cars with the column switch it's best done with an in-line fuse with bullets both sides, fitted where the yellow from the main harness joins the yellow in the gearbox harness in the mass of connectors by the master cylinders.

For cars with the gearbox switch i.e 77 and later again use an inline with bullets both sides in the mass by the master cylinders, but this time in between the brown/whites from the main harness and the brown/white to the gearbox harness.
Paul Hunt

This thread was discussed between 19/09/2015 and 25/09/2015

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