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MG MGB Technical - No power (electrics)

Hi again again....suddenly no power ie. dead electrical system. Went to start the car (after fiddling with the web carb to get it right)first time - turned over no problem but would not fire (I've been adjusting the choke). Went to try again but this time no power - everything dead. No turnover of the engine/starter motor, no lights, interior light, hazard warning, whole system has gone out.
Have checked the battery connection, rocked the car backwards and forth in gear (to unjam the starter motor if this was the problem), checked all the fuses and connections but cannot see anything wrong.

What could the problem possibly be?
/Moss
Moss

Moss

Get a test light, start at the battery and work your way from there.

Bob
R.A Davis

This has usually been the battery connections when its happened to my car. They may look OK and be tight, but clean and reseat and away you go. As ever a smear of grease over the terminals afterwards will keep corrosion away. I have been able to measure 12.7v between the battery post and the clamp with the headlights on. I thought you had bought a meter Moss?
Stan Best

Moss,
Good advice about the battery terminals - the suddeness of the problem suggests the terminals have moved - the earth can also be a problem and it is important that there is good metal-to-metal contact between the earth lead and the carbody at the back end of the battery box AND between the gearbox and the gearbox crossmember. The gearbox earth is vulnerable and most of the old lags here have added a separate earth strap between the body and the engine in a place where it can be inspected - the engine bay.

Roger
R Walker

It must have been the earth terminal as twiddled with it, took out the battery, looked at the wiring, could not see any problem, put it back in, changed the position of the earth lead to battery and instant power! Why on earth (!) this would make a difference is beyond me...
Thanks for comments
/Moss
Moss

"Why on earth (!) this would make a difference is beyond me..."

???

If the earth, or any other connection, isn't making a good connection where it is supposed to, then you will get restricted or zero current. This is the metal parts that touch each other when connected so you can't see what is going on while they are connected. Looking at the wiring, i.e. the plastic covered bit, is only likely to show you something useful if you are getting short-circuits i.e. damaged insulation. You need to look at the metal parts that touch when they are connected, when they have been disconnected. They should be clean and shiny. If there is corrosion then wiggling things can bring power back, only for it to go again some other time, until you clean them properly.
PaulH Solihull

Further, "looking at the wiring", and removing/replacing/disconnecting parts, usually involves twisting, turning, pulling etc. This can cause bad internal connections or broken/corroded cables to remake contact for a time. Any crimped wire ends, and even factory made battery cable ends are prone to this. Battery cables corroded inside the jackets do it.

RM
FR Millmore

I expressed myself incorrectly 'why on earth' etc...all the metal to metal connections looked ok ie. from the earth lead connection on the battery to gearbox earth strap. I took the engine and gearbox out in January and made sure these connections were good - they are in fact new - so nice and shinny. I also installed an earth strap on the engine mounting.

A new development since earlier. The front right indicator unit (rubber bumper type - new in February) is no longer working and the left rear light has also stopped working. The interior lamp has also given up. BUT the rest of the electrics are working and engine starts with no problem (aside from the carb issue)

So two seperate problems, one of which is solved, or are they linked? My first thought is a short somewhere - but then the four 35 amp fuses are good (also new in January).
Moss

I wouldnt think that these are linked. A fuse will not just affect one lamp in a circuit. I would suggest checking the voltage to the offending item bulbs and earths in that order.
Stan Best

"made sure these connections were good"

You missed one then :o) On my roadster it was where the battery cable and brown wires joined together on the starter solenoid, failed with no prior warning while it was having it's annual safety check - totally stumped the garage concerned.
PaulH Solihull

Chunk missing.

If all number plate lights are still working then the fault with the left rear is where the tail from the light unit connects to the bullet connectors nearby, or the bulb, or its connection to the holder, or its earth. There can also be a bad connection where the bulb holder mounts in the metal body of the light unit.

If the number plate light(s) are also out on the same side you probably have four red wires on the back of the top two fuses - one for each corner for the parking lights, if so one of these could be making a bad connection. If not that you could also have a bad connection where the two spades are riveted on the back to the fuse holder.

For the indicator there should be two green/white wires coming from the main harness by the fusebox, going into a 4-way bullet connector, and a third wire from the rear harness. Of the two from the main harness one is from the switch (which if the rear light is working that must be OK) and the other goes to the front light so it could be that bullet. There are also bullet connectors by the headlight that side, and for the earth, and it could be several things inside the light unit. The fact it is 'new' butters no parsnips.

For both faults, with the circuit switched on, lights work along the connectors from the supply to the lights, with a test-lamp or meter, looking for 12v. When you pass the fault the voltage will drop significantly or to zero.
PaulH Solihull

Paul. I must indeed have missed one or possibly more! As with all things technical there must be a rational explanation and solution ie, a piece of technology with a disfunction which can be repaired after the irrational human being (myself) has identified where the disfunction is!

For the record the left hand side number plate light is also not working....

/Moss
Moss

OK, 4-way bullets by the rear light or 2-way by the fusebox then, or one of the four reds not connecting to the fuse.
PaulH Solihull

Paul , your knowledge going right down to the bullet connectors in the loom is amazing. I have only ever tracked them down using a meter. When our car failed some years ago , I pressed on when the tacho failed and later ran out of fuel because the pump was on the same circuit, it turned out that a rather buried 3 way with switched ign in and 2 outs had gone. That was original and the feed sprung cylinder had corroded lenghtwise meaning it no longer made ohmic contact with the 12v feed. I put the fuel pump onto 2 good ones to get home. I'm sure you can tell me exactly where I had to reach into to do this fix . Dec 1966 built car ;-)
Stan Best

The one where the rear and main harnesses join, in the mass of the others by the fusebox I'm guessing.

I've been fiddling with electrics for 55 years now, both employed and hobbying, and if I include the plugs and adapters my parents had to buy me to stop me fiddling with the 240v ones on the walls before I could walk, then you can add on another ten years. It's just a quirk of nature and in my genes somehow, maybe my grandfather being born where one of the main Lucas factories was subsequently built had something to do with it. At least it isn't immoral, illegal or fattening. Funnily enough a pal came round today saying his horn kept failing, and even though he had tried to clean and tighten up the brown feed to the fusebox which fixed it for a bit it always failed again. As I recommended to Moss I tested on the brown spade while pal held the horn button down and got 12v, ditto the brown side of the fuse and holder, but nothing on the purple side of the fuse and holder. The fuse was one of the 'waisted tape' and hadn't blown, but as soon as I removed it the end cap fell off and you could see where the end of the tape had been arcing against the inside of the cap because it hadn't been bonded properly. Took longer to get my meter ready than find and fix.
PaulH Solihull

:-) Yes,exactly. It is under the bonnet in the mass of wires in front of the firewall below the pedal box.
The comment about slow blow fuses is a good one. I lost the switched ign feed once and a visual check of the fuse showed it good. I eventually swapped it out anyway and problem solved. The fuse as you noted had not blown but the beam like slo blo link was detached from one end cap. I looked at it when we got home out of the rain. It stayed in one piece, looked fine but measured open. It turns out that is a known "feature" and I avoid them where possible now.
Stan Best

Hi Paul and Stan, thanks for your extensive advice. I'm working my way through the electrics, slowly but surely. Bit of a time issue what with work and children...so apologise for not responding quicker.

I can now conclude the bulbs for lights (front indicator right hand side, the red rear light, with dual filiment bulb, left hand side, and the interior light) are functioning as I have tested them in another circuit. So the problem must be in the wiring and or fuse connections as you both indicate (no pun intended!).

/moss

Moss

A non-related issue re: electrics. As is known I am looking at the electrics and whilst I'm at it I've taken the clock out as it has never worked since I bought the car - thought I would look and see what the reason could be. Not yet taken it apart (if at all possible) but the 2w bulb is blown (I pressume has been for a long time as never worked since owning the car, as said).

What could be the possible reasons for the clock not working? Could the blown bulb be the cause? There is a black wire and 2 purple wires (as one) going into the connections on the rear - obviously the power source.

And then I thought about the voltage stabiliser. I've seen pictures but cannot seem to find it! Where is it actually located? My workshop manual names it with a picture but no pictures or description relating to location.
/Moss
Moss

Highly unlikely the bulb would affect the clock assuming the bulb only comes on with the lights (normally).

If you have 12v on the clock spades carrying those purple and black wires then the problem is internal to the clock, so could be absolutely anything particular that particular mechanism.

The voltage stabiliser has nothing to do with the clock, it only feeds the fuel and electric temp gauges. It can be either on the lower edge of the bulkhead behind the dash in front of the driver on an RHD on mid 70s models, much higher up on later models, I think. It *may* be the same on LHD as the same harness was used after the first few years.
PaulH Solihull

This thread was discussed between 16/05/2012 and 24/05/2012

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