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MG MGB Technical - Electrical problem

I have a strange electrical problem on my 78 B roadster.
I start the engine normally and the tacho is working fine. As soon as I use the indicators (either left or right turn) the tacho drops to zero and the indicators don't work. I turn the indicators off and the tacho is still on zero until I stop the engine and restart.
With the engine off I checked the hazards and they work fine. I also connected the hazards to the flash unit for the indicators, still good.
Then I started the engine, tacho working fine, turned on the hazards and the tacho kept working (and the hazards worked too). Tried the indicators again and the tacho dropped to zero.
Any ideas?
D O'Brien

I am no expert on '78 B's but from a pure electrical standpoint it sounds like something is wired wrong and the indicators are shorting the tach.

Is this a recent issue? Have you done any work electrically to the car recently?

When these odd "interactions" between components take place my first guess is a poor earth. Check the earth spade on the back of the tachometer. Not surprising the Hazards work as they have a different, always live supply. The taco and the indicators are fed through an ignition live circuit (GREEN) from the fuse box. Check/clean these connections too.
Allan Reeling

Definitely a weird (not wired ...) one if the hazards don't do the same thing.

The only earths that the indicators use are at at the corners of the car in the light units (mounting points at the rear, wired shared with headlights at the front) and the tell-tales on the dash. The tell-tale ones use a common earthing point in the dash harness that feeds no less than seven components including the gauge illumination, tach and main-beam warning as well as the tell-tales, with the left indicator earth being daisy-chained off the tach earth. So if none of the other circuits cause the same problem it's difficult to see it as common earthing problem. The tach and tell-tale earths could have become separated from the others, but that still doesn't explain why it doesn't happen with the hazards.

It's more likely to be a fault in the 12v supply, but that same 12v supply also feeds the heater switch by being daisy-chained off the tach. So does turning the heater fan on cause the same problem? Or does it fail when the tach drops?

The 12v supply for those comes through one of the multi-plus behind the dash, and the main harness side of that goes to the wipers, instrument voltage stabiliser and handbrake warning light. So if none of those are affected or or the wipers don't cause the problem, and the heater fan is also affected or causes the problem, then I'd say the problem lies in the multi-way connector.

If the wipers cause it and/or are affected at the same time, then it's more likely to be at the fusebox, but there are also a couple of 4-way bullet connectors between the two.

Before I read all these replies I did a bit more investigation.
Pulled the steering wheel off (just for easier access to everything) and then removed the temperature gauge and tacho so I could get enough slack in the wiring to pull it out and have a look. Tested power and earth leads one-by-one and everything seemed fine. Reconnected everything to the tacho and temperature gauge and tested it. Everything was fine. Put the tacho and temperature gauge back in the dash and the fault returned. I reached behind everything and just gave the whole bunch of wires a bit of a shake and the problem is fixed (well, sort of fixed).
Definitely a short somewhere in there. I'll have to pull it all apart and check things more carefully because I don't like the idea of driving down the road, hitting a bump and the problem comes back again.
I'll definitely check all the things Paul mentioned.
D O'Brien

"Definitely a short somewhere in there."

Unlikely to be a short, which you could expect (unless there are bad connections between there and the fuse) blow the fuse. More likely just a weak connection without a short.

"then I'd say the problem lies in the multi-way connector"

That's what it looks like.

Checked everything you suggested.
Wipers, stabiliser and handbrake warning light neither cause the problem nor are they affected after the problem occurs.
Heater fan switch is affected.

If I turn on the heater fan (low or high) everything seems fine. With heater fan still on I switch on the indicators. They work fine and keep working without failing.
But as soon as I turn off the heater fan the indicators blink once (or twice) then fail. The heater fan also fails to restart after that.

I turn off the ignition and try again. This time starting with the heater fan OFF and just the indicators turned on. Indicators blink only a few times then fail. Then I try the heater fan and it doesn't work. I switch off the indicators and the heater fan still isn't working.
D O'Brien

The common point of failure which would affect both your indicators and the heater fan and tachometer is the hazard switch, where two green wires (according to the diagram) terminate on pin 8 (page34 in the link below):

Have you checked the hazard switch wiring and all the bullet connectors on the green circuit?

Andy Robinson

I think the hazard switch was the culprit.
I removed the dash to expose all the wiring then reconnected the switches, the tacho and the oil pressure gauge so I could do some more testing.
I used my multimeter to test resistance and continuity on every green wire and a few others that were involved. Everything seemed fine.
Tried the indicators and everything was not fine. Blinked half a dozen times then stopped.
I disconnected the heater fan switch and tried again. Still no good.
Next I tried with the hazard switch disconnected. No good again.
With nothing else to think of I stripped the wires off the back of both switches and resoldered and put new heat shrink insulation on them.
Problem seems to be solved. I have pushed and pulled at everything and it is still good. Put everything back together and went for a 15 minute drive. Still good.
Fingers crossed.
D O'Brien

FWIW the hazard switch only interrupts the indicator circuit, not the heater fan or anything else. If it did, and the fan switch was on, it would be pulsing on and off with the hazard warning lights, and the flasher unit may not like the extra current.

However on a UK-spec 77 and later the supply-side of the hazard switch IS used as a teeing point, with the supply coming in on one green and going off in another green on the same terminal (not to be confused with the single green on the other terminal that feeds the indicator flasher unit) to the heater fan switch and the tach. But those two greens would normally be spot-welded into the same spade connector, which would mean that even if the spade connector was making a poor connection to the spade only the indicators would be affected, the other two components would work normally. If that spade terminal had been replaced though, then yes a bad connection in that spade could affect all three things.

But if it's a US spec, the tee is at the multi-plug behind the dash, with one green going to just the hazard switch for the indicators, and the other green going to the tach and heater switch. In which case a poor connection on that pin in the multi-plug would affect all three circuits.

This thread was discussed between 23/07/2017 and 18/09/2017

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