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MG MGB Technical - Lost dash electrical, turn signals, brake lights

All of a sudden today while driving I lost my fuel gauge, tach, temp gauge, turn signals, brake lights...Fuses all appear to be good...Any thoughts on where to go next?

Thanks
John Staub

"Fuses all appear to be good"

How did you check the fuses. For the record, the MK I eyeball has a terrible reputation for diagnosing the health of a fuse. You need to use a meter to check for voltage in and out of the fuse. The fuse clips are often loose, as are the rivets on the fuse block, along with the spade lug connectors. Cheers - Dave
David DuBois

What year, 2 or 4 fuses? All those circuits are on ignition switch controlled circuits. I presume, because you didn't say, that the car continued going? If the wipers, reversing lights hazards and heater motor are all dead then fuse 2 (white in, greens out) is the culprit or the white feed to it from the ignition switch. Check that this is live when the switch is on run.
Allan Reeling

Thank you for the quick responses - The car continued to run, the wipers and heater motor work but no reverse lights, brake lights, turn signals and dash instruments as described previously.
John Staub

David - I run the recommended checks...The Mk1 eyeballs are most definately not what they used to be :-)
John Staub

Sorry - It's a 73 US spec. During the recent rebuild, the entire wiring harness was replaced.
John Staub

John. First, get a copy of the wiring diagram for your car. It will tell you that all of the nonfunctioning components are part of the green wire circuit. Most common problem is bad fuse, bad fuse holder, or loose connectors at the fuse box. While Dave's suggestion of a volt meter works the best (allows you to see voltage drops indicating poor connections), a simple test light makes for a very quick and easy go/no-go system.

I have the wiring diagram expanded then laminated at Kinko's. Have been using the same two for over ten years and find they are quite useful when the electrical problems begin to crop up. Dan Masters has color coded electrical diagrams available for free on line.

Les
Les Bengtson

Thanks Les - I have a color coded diagram, but do need to have it expanded!
John Staub

Thanks to all....It was a blown fuse..David, the ole Mk1 eyeballs are not as good as they once were...The test lamp worked well Les ('course, you both knew that :) )

Now to figure out the "why"...
John Staub

John. The first thing to think of is old fuses with many life cycles on them. The seal of the metal caps to the glass tube can deteriorate, allowing air into the capsule. If my understanding is correct (not a fuse maker), the wire is like a light bulb and operates in a vacuum. If the vacuum is breached, the fuse will burn out just as the light bulb will.

Second thing to consider is some part of the system having a short to ground and the fuse is doing its job preventing a possible fire.

I do not have a wiring diagram for your year. But, I had a similar problem with my 68 which has a two fuse system with the bottom fuse powering the green wire circuit. I removed the various connections from the rear (output) side of the fuse box and wired in inline fused to each of the wired coming off. Female push on connector to attach to the fuse box spade, male connector to plug in the factory wire, and a lower amp fuse between the two connectors. This allowed me to determine which circuit the problem was on and trouble shoot only that circuit rather than the entire green wire circuit.

But, Paul Hunt, in his Pages of Bee and Vee, used to have a good deal of information of electrical trouble shooting since he, like Dave D, is actually trained in such things.

Les
Les Bengtson

Just returned from a 30 mile cruise...Now the turn signals aren't working..The four-ways are, everything else is...Looks like I have more work to do!
John Staub

That could be down to the hazard/4-way switch.

Try flipping it on and off a couple of times and see if your turn signals 'return'.
Dave O'Neill 2

Hi David - tried that....I think I'm going to have to tear into the console to get to the switch...It'll have to wait a week or two unless I can steal a few hours in the evening this week..
John Staub

Okay - I have 12v to the hazard flasher and if I jumper across the two contacts on the flasher, the lights come on but do not flash...

Any thoughts now?

Thanks
John Staub

Got it...Bad ground at the turn signal flasher...I can't for the life of me imagine why they put the turn signal flasher under the glove box, but it is there....

Thanks all!!
John Staub

John,

It sounds like you need to remove every grounded piece of electrical and clean the ground area with a slight abrasive, and then smear with a dielectric (non-conducting) grease. The grease will inhibit the three corrosion enemies of electricity; oxygen, moisture and dirt. It will squeeze out between the contacts allowing good contact and sealing the connections. Then disconnect every bullet connector on the car and do the same. Dave DuBois has some good information on the subject. Don't be tempted to use a conducting grease as that can cause stray electrical activities.

Good job on your troubleshooting to date! But it is more fun to drive the car than to trace gremlins.

warmly,
Dave
Dave Braun

There is no ground used at the hazard flasher, or the turn signal flasher. The only grounds used are at the lights that they flash, it is a simple series circuit from the battery, through the flasher unit, switch to the lights. Which was it, hazard flasher or turn signal flasher?

If the hazard flasher then bad grounds are only ever an issue with one bulb not working, they should always flash even if only one bulb is functioning.

If the turn flasher than a bad connection anywhere in the circuit will cause lighting but non-flashing. But if the only time the lights even came on was when you jumpered across the two terminals, then the flasher unit is bad, or there is a bad connecting between the wiring connectors and the spades on the unit. The turn flasher contacts are closed normally, when you first operate the stalk the lights should come on immediately, then after a short pause start to flash off-on-off-on. The hazard flasher is different, that is normally open and when you operate the switch nothing happens immediately then after a short pause they start flashing on-off-on-off.
PaulH Solihull

Paul - >There is no ground used at the hazard flasher, or the turn signal flasher.<

I removed the turn signal flasher (located under the glove box on my 73), jumpered across the two wires and the lights came on steady. The flasher is encased in a metal "holder" that is in turn screwed into the body. It showed signs of oxidation so I cleaned it somewhat, and then replaced the flasher. The turn signals now work, albeit somewhat faster than before. The flasher "came apart" during this process. I was able to put it back together properly, but ordered a replacement just in case.

I found it rather an odd place for the flasher, and thought the mounting method even a bit stranger, but then again, it is by no means the strangest thing I've ever come across in all the cars I've owned.
John Staub

Pure coincidence, and the fact that the flasher came apart and was reassembled speaks volumes! Look at the schematic, unlike the instrument voltage regulator there is no earth symbol shown. That's why the turn and hazard flashers simply push into a spring-clip, whereas the regulator is screwed to the firewall. The flasher units will work exactly the same dangling on their wires, except for clicking quieter.
PaulH Solihull

This thread was discussed between 14/08/2011 and 18/08/2011

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