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MG MGB Technical - Problems with Electric Harness refit

Hi all,

Hope someone can give me a pointer to this before I start to mess things up!

Have recently finished an engine bay repaint and refurbishment; so had stripped out everything apart from the engine. Everything now back in and would be ready to go apart from some problems with my re-attached electrics!

Two main things I've found wrong:

a)With the electrical system ready for testing, the alternator warning light is on even without the ignition switch being turned on.

b)fuel pump works, again without the ignition switch activating it.

c)There is no light at the headlights at all.


Indicators work fine, fan works, windscreen wiper works.


P.S. I have the harness protected by a headlight bulb setup at the battery.

Thanks for any info.

Iwan
IA Jones

Hi Iwan,
I have recently completed the same exercise as you,having removed everything from under the bonnet apart from the engine block,brake & clutch master cylinders & pipes. I disconnected the main harness from body & gearbox harnesses & then wrapped the main harness during the painting process.
You don't say which year your car is or whether it has a two fuse or four fuse box.
The fact that your alternator light & fuel pump work without the ignition switch on suggests you have connected them in some way to the brown supply that is live all the time rather than a route that goes through the ignition switch.If you use a circuit diagram car relevant to your car ,then you should be able to follow through your connections & establish where you went wrong!
I assume you have cleaned all the bullets & replaced all the sleeeve connectors.
A test light & voltmeter are very helpful in sorting out the various issues you probably have.
Cheers
Charles
Charles9

Hi Iwan,
If you have a 4 fuses box have you put it back in the car upside down? The tabs that are joined by a bridge should be at the top as the top fuses serve the side lights.
If it is upside down then it would create the issue you have.
Cheers
Charles
Charles9

Hi Charles,

Thanks for that, just went out to check. Noooo, I thought I'd made sure it eas the right way round.

Well done mate ...... how much bother have you saved me; how chuffed do you feel :)

Cheers,

Iwan
IA Jones

Hi Iwan,
That's good news.
My silly mistake in putting it all back together under the bonnet was bolting the heater valve to the block the wrong way round.
Have fun in starting up & checking for leaks!
Cheers,
Charles
Charles9

Are the headlights working now? Fusebox error shouldn't have caused (I was going to say 'be connected with' ho ho) that.

With a headlight bulb in the battery earth lead (I always recommend that when testing a new or refitted harness) everything should work - to some extent - except cranking. With two headlights at the front of the car in series with the protection one at the battery you will only have about 1/3rd of the voltage at the front and 2/3rds on the test bulb. So the ones at the front should glow a bit, and the test one quite bright. If the test one doesn't glow at all then there is no connection though the switch to the headlights, and from there to earth. If there is no glow at all, is the main beam tell-tale glowing on dipped beam? If so the earth is missing, another easy mistake, and on cars with a starter relay the earthing point is near the relay i.e. black wires with a through-hole terminal to a threaded hole on the inner wing.

If the test bulb is at full brightness when you switch on the headlights then that indicates there is a short-circuit past the headlight switch, and your test bulb is preventing your harness from burning up!
paulh4

HiIwan,
I agree with Paul that orientation of the fuse box shouldn't affect the working of the headlights. On my 1980 roadster the earthing wires are screwed with an eyelet tag under the brake master cylinder which is both easily forgotten and not the easiest to get at.
Cheers
Charles
Charles9

Hi,

Thanks Paul and Charles,

Yep, headlights were another matter. The earthing was ok. Checked the power from the ignition switch & that was ok, got to the headlight switch ..... hmmm.

I had replaced my old switch (paint a bit faded) with a brand new one as part of a general refurb of the dash of my gt. Guess what, the dreaded - poor quality of new items gremlin - had hit again!! It had disintegrated, not into bits but everything loose and wobbly. Can't remember where I had ordered it from, but it was not cheap.

Replaced it with the original item, hey presto, everything working fine.

Will probably put back the other two switches I swapped out.

Thanks once again.

Iwan

IA Jones

Had the same problem. Replaced it as the logo was a bit faded 28 years ago. Some years later it was intermittent, and by now the logo on that had faded as well, so the original went back in!

Opening it up the contact surfaces had been gouged from wear (and this a car with very little night-time work), but the design is such that one part or another can be turned round to operate on 'new' surfaces. Greased, reassembled, and ready for the next failure. That was an early rocker, the later 'squarer' ones are a lot trickier to work on.
paulh4

You can inadvertently make the, modern made at least, rocker switches come apart without notice at the front when trying to remove them from behind the dash. (Don't ask how I know).

The 'new' Lucas switches (that might not be that newly made, look at the date on the switch, in with the switch number) can re reassembled and work for many years (not decades perhaps like the original switch).

I had a 'new' wiper switch that had come apart two or three times (fat hands groping blindly behind the dash on various jobs) and it still lasted 10 years of regular use before it fell apart again.

Which was whilst on a touring holiday as it started to drizzle, by the time I'd found the spring and worked out which way round the components went and reinstalled the switch it had stopped drizzling but the embarrassment of having to stop as we were traveling with friends meant that switch had to go.

I can't remember when the latest switch was made but it wasn't in recent years.

ETA: It's a good thought when something plays up to think about if any new/different parts have been added.
Nigel Atkins

Three things on the light switches, 1. You can actually remove the rocker itself using 6" engineers rules, and thus repair and clean the contacts.
2. When the back separates from the body or bits break on disassembly because the plastic has become brittle, a really sound, re-assembly, repair can made using a hot glue gun.
3. You can take the electrical load off the switch contacts by using relays on the dip and main beam. This has the added benefit of making your lights slightly brighter!!
Allan Reeling

I fitted relays for someone with very uprated headlights. He said that before when he switched between main and dip he was momentarily plunged into darkness while the new filaments 'warmed up' like an old valve radio. Afterwards it was instantaneous as well as them being significantly brighter. They take the dip-switch out of the equation as well as the main lighting switch and myriad connections. It does mean that in the event of a short-circuit you will fry the wiring much quicker, so fusing is highly recommended, one per filament so you don't lose them all in one go.
paulh4

Hi,

Checking through all the electrical stuff and found that it all seems ok; however, there is a problem with the fuel gauge.

This seems to be on (showing as full) all the time, even without the ignition keys being on; it also felt to be quite warm at the back when I pulled out the connections.

Any ideas?

Thanks
IA Jones

Hi Iwan,
As i am sure you are aware the fuel guage & temp guage share the same light green & green wire from the voltage stabiliser via a multi connector behind the dash. This is in turn supplied by a green wire that after a couple of double sleeve connectors is fed as one of the green tabs on the output side of the second fuse up on the fusebox.This output should only be live when the ignition is on.If it is live all the time then your fuel guage will register full as the earth return is effectively from the guage via a green black wire through the sender unit on the side of the tank. As your earlier problem of the fusebox being the wrong way round has been resolved as evidenced by the fuel pump no longer being permanently on (the fuel pump being supplied by a white brown wire that is paired with other whitebrowns on the supply side of the second fuse up ) then the green output should only be live with the ignition on.i would be suspicious of where you have connected the green supply to the voltage stabiliser. I assume all your indicators work properly either as turn signals & as hazaards.Have you disconnected the flasher units & stabiliser unit during your refurb process or left them alone? Worth checking your sleeves which join the main harness to the body harness with a circuit diagram to hand.
You may have conected a permanently live feed (such as the boot light wire into the sleeve where the greens are) by mistake.
I'm sure others with greater knowledge will be able to assist!
Cheers,
Charles
Charles9

As you say, if everything else on the ignition works OK i.e. goes off with the key then for the fuel gauge (year? Elec or capillary temp gauge?) to be at full scale all the time two things have to be wrong. The first is that there must be 12v on one side of the gauge, and the other is that there must be an earth on the other.

The first could be 12v coming from a purple circuit or a brown circuit, so the first thing is to pull the purple circuit fuse and see what happens then. Also check the interior lights and horn i.e. are they working with the fuse in and not working with the fuse out? That's how it should be.

Also as said check where the greens are connected on the back of the fusebox. They - depending on year you may have three or four wires but only two spade connectors on them - check they are in line with the second fuse up. Then check the white wires on the front of the fusebox, again several wires but only two spade connectors, again they should be in line with the second fuse up. Similarly brown wires in line with the front of the bottom fuse, and purple with the back of that fuse.

The second is possible if the tank is completely full, otherwise there must be an earth on the green/black to the tank sender.
paulh4

Hi,

Thanks for the comments. 1972 gt. Only one white wire to connect at fusebox.

When purple fuse taken out, horns etc. don't work, but still 12v at gauge.

I have not previously touched the connections to the voltage stabliser; had not touched gauge either.

All the colours/connections at the fuse box and harness connection is as supposed to be on the diagrams etc.

Hmmmmm ....... see below


Well, guess what ..... typed in the above then went back out to garage to take a photos of harness connections etc. Fiddled with the wires to spread them out a bit for photos and hey presto 12v at gauge had disappeared.

So something must have been touching??? It's a bit disconcerting; because, I had been fiddling with the harness connections a number of times before, checking the connections and noting if the wire colours matched and nothing had happened. Will go through them again, anything I should look out for?

By the way, the 12v was coming in on the black green wire.

Ta

IA Jones

Hi Iwan,
A puzzle indeed.
On my 1980 roadster i have 4 double sleeve connectors & 5 single. (You may have one more for the HRW).
My sender unit is supplied by the green black wire joined by a single sleeve.All other connections have matching colour wires each side .On mine the red of the green red wires looks (due to ageing) very similar to the green brown.Not sure if that helps you at all.There is a purple double connector there on my car which only had 3 wires in it (i have used the spare bullet hole for the memory function of my radio).However, as you have tested this goes dead when the fuse is removed i don't think there can be any touching here. Has the original harness been hacked around in any fashion?
Cheers, Charles
Charles9

Hi Charles,

Thanks for all the info, its been very useful. Wish I could work electrics out the way you can :)

The harness is in very good condition, it is original and with all the protective tape in place, no bare bits and none of those dreaded 'clasp connectors' (don't know what they are called) have been used. The only change/addition I've made is to put in an inline fuse on the fuel pump; following Paul's advice ages ago (no bare wires on this either).

Yep, I tested the purple by taking out the fuse as advised by Paul. No horn etc. but still there was a current at the gauge green/black at that time. I can only think that somewhere there was contact.

I'll go through the whole area again, make sure all the colours match (mine are quite colourful/obvious) and ensure all bullet connectors are covered. Hope this little gremlin has gone for good though.

Cheers,

Iwan
IA Jones

For there to be still voltage at the gauge with the purple fuse out it must be in contact with a brown somewhere, but there should be no browns in the mass of connectors where the main harness joins the rear. So odd that it went when you joggled wiring in that area, and the chances have to be that it will come back at some point. It also means that the bullet connections are probably as they should be and it is a stray connection that should not be there, and not a wire plugged into the wrong bullet.

If the 12v was on the green/black (confirmed by taking the wires off the gauge and testing them I assume?) then it would 'see' an earth through the connected gauge, the stabiliser and the green circuit to the many components that are powered when the ignition is on. Which means that when the gauge does show full with the ignition switched off, that current through the gauge is powering all the ignition components including coil, fuel pump etc. to some extent, so not something to leave unfixed.

It should also mean that when you turn on the ignition the fuel gauge should drop below E as there should then be 12v both sides of the gauge.

If it comes back part the connector joining the green/black in the main harness to the green/black in the rear harness and see which of the wires has the voltage - that is, if the problem is still there after joggling the wiring again!
paulh4

Hi Paul,

Problem came back. Did as you said and parted the connector at the main harness/rear harness junction.

With ignition off.

1. There was 12v at the green/black wire leading back to the gauge. There was nothing on the green/black leading to the tank.

2. There was also 12v on the green black at the gauge. The gauge is disconnected at the moment.

3. I then took the time to check the other connections at this point with the ignition off. There was 12v on the purple circuit; however, the white wire on the main harness showed 0.5v as did the green wire

With ignition on.

4.There was 12v on the light green/green wire leading to the gauge. So if the gauge was in it would be getting 12v from wires both sides.

I suspected there is a problem with the harness, I checked this but could not see any obvious wear, melting of the outer tapes.

I've had the car 20 years and don't remember any major electrical problems in my ownership. Would it break down internally without external signs?

Thanks for any info.

Iwan


IA Jones

Hi Iwan - thanks for the comprehensive info.

With both wires still connected to the fuel gauge, with the wire to the sender disconnected, you will see the same voltage on both gauge wires as the gauge has a low resistance, so we don't yet know which side the voltage is coming from.

Paradoxically had you checked both sides of the gauge with the other connection in place, and the gauge showing full with the ignition off, then there would have been a difference in voltage which would have told us which side it was coming from, but there we are.

So now you need to remove at least one wire from the gauge, and see if the voltage is on the wire you have removed or the wire that is still on the gauge. I suspect it will be on the green/black, based on you seeing 0.5v on the white and green wires, which will be from that voltage going through the gauge and the stabiliser (backwards, so to speak) and from there onto the green and white circuits. That can be confirmed by removing the green circuit fuse, which may well remove the voltage from the white side, and increase the voltage on the green.

As you have previously taken the purple fuse out and still had 12v at the gauge when the horns etc. stopped working, then the green/black (I'm assuming at this stage) must be in contact with a brown somewhere, which has to be somewhere between where the green/black goes into the harness at the gauge end, and where it comes out at the master cylinder end. The most likely place for damage is where the harness goes through the grommet at the bulkhead, but to be damaged enough to bring two wires into contact like that would be unusual, and also concerning as the brown could come into contact with the bulkhead which could cause a lot of damage. Do you have a cut-off switch? If so always disconnect it before leaving the car, and if not always disconnect the battery earth cable.
paulh4

Hi Paul,

I was just going to update on my findings today and saw you had replied. Yes, you got it right. I was baffled by this 12v either side of the bulkhead but the harness looked good puzzle and I'd decided the problem was certainly there as that's the only place the green/black could pick up the power.

I'd looked at the harness from the gauges to the bulkhead abd it looked dusty but pristine. So, took off the pedal box assembly cover, loosened off the accelerator cable and lo and behold a small gash in the taping.

I say gash, it was more like a small slot, nowhere near where some abrasion might be. So it looks like one of my friends from the countryside (driven into the garage for refuge by our cats!)had had their own back. Yes, ..... a mouse had chewed a brown wire and a green and black wire with the filaments touching.

Managed to get the tape off in this constricted area and one handedly put some back round the wires. No more 12v on greenblack; re attached gauge, it shows tank just under half full. Result!

Will have to make a neater job of it tomorrow.

Thanks to you and Charles for all the advice, It would have taken me far longer to get to this point.

Cheers,

Iwan

IA Jones

Hi Iwan,
That's brilliant! - any credit must really go to Paul who manages to shine a light on many of our electrical problems!
When you get a chance it would be good if you could upload a picture of your under bonnet efforts!
Cheers
Charles
Charles9

Excellent news Iwan, and just goes to show that electrical gremlins really do need to be bottomed out or far more serious consequences can arise.

My son has a classic BMW in a 'temporary' shelter consisting of heavy gauge sheeting over a tubular frame, and another outside under a waterproof cover. They obviously preferred the added comfort of the shelter as he had his harness chewed on the car in there. He now has a drive-in inflateable inside the temporary shelter!
paulh4

Hi Charles,

The under the bonnet work was just a quick tidy up and repaint. I'm not into doing fancy/chrome/super detailed etc. stuff and licking the engine bay every other day to keep it clean :) I've got an over-bored engine, tuned head, fast road cam etc. all nicely hidden away with a coating of dust and slight rust with the odd bit of flaky engine paint here and there.

Not worth a photo!!

But I know it works; drives and handles well.
IA Jones

Hi Iwan,
No sweat - cars should be used & enjoyed & not wrapped in aspic!
I only did mine up as it was the first time i had done any restoration under the bonnet, other than essential maintenance in nearly 38 years of ownership & as the car has not been modified under the bonnet apart from a 123 Distributor & copper fuel & brake lines I wanted to get close to what it looked like when I bought it!
Cheers,Charles
Charles9

This thread was discussed between 22/05/2018 and 31/05/2018

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