MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Fuses blowing, odd behaviour, strange solution.

Hello All,

Big day today - restoration complete, engine in - MOT!!

Anyhoo, I got the car out of the garage and whilst I was doing a last check of lights etc, the indicators started behaving oddly.

First, both turn signal lights on the dash would flash together, even though only one side of the car was flashing. Then the fuse went - glowed red, then redder and finally burnt out in about 5 seconds. This happened about 4 times, eventually the fuses were just going immediately - always when I turned the indicators on - Left or right.

Called the garage, put MOT on hold. I spent four hours disconnecting testing for grounds on the green circuit and traced the problem to the indicator assembly itself.

It turned out that I had fitted the locking collar on the steering column so that the grub screw was on the right - and the spring tines on the stalk were hitting it whenever I indicated left or right.
It also broke my new flasher, so I used one from my SD1 donor car, rigged it up with an earth and its fine.

Odd, but I was pleased with finding an answer, bought a handful of spare fuses from my lcoal shop, took it down the the garage for an MOT. After 5 minutes on the road the indicators went again, I stuck in a new fuse right outside the garage, and it held up through the test, but then fused again on the way home.

Tonight I have not been able to replicate the fault at all, even with everything on the green curcuit switched on at the same time (e.g. in reverse, braking and indicating left with the heater on).

Now then, my local parts shop only had 20 amp fuses - thats all it says on them - 20 amp, I know the originals were rated 35 amp blow/17 amp continuous.

Could it be that my original fuses were being blown by the short in the stalk assembly, and now that the problem is fixed the 20 amp fuses are not up the job?

If I am buying fuses with only one rating printed on them what should it be?

1972 GT

Thanks,

Liam

Liam H

Try a local garage that services MGs, Jaguars, etc. to see if they have the correct fuse or if one of the other circuits has a 35amp fuse (ie. side marker lights) and switch it to see if the fuse blows. If the correctly rated fuse blows, then you still have a problem. It's possible a wire was pinched or damaged when replacing the cowl. Also check the wiring in the circuit to make sure the insulation hasn't melted exposing the wire and causing a short in the circuit.
Kimberly

I'll tell you something else as well...

when I put the loom in I can't recall ever seeing anything that looked like a voltage stabiliser.

Could this be causing my problem? and were these fitted to 72 cars - if so what did they look like? and where behind the dash would one find it?

Regards, Liam
Liam H

Liam - If the voltage stabalizer were missing or not operating correctly, you woud see some strange indications on you fuel gauge (always being pegged against the full mark) and, depending on the year of your car, the temperature gauge will always read high. Cheers - Dave
David DuBois

Wiring diagrams are available at http://www.advanceautowire.com click on stock schematics
Find the drawing that best matches your car. Look at the wiring going to the fuel guage. Then verify the wiring in your car is the same. The voltage stabilizer is located on the firewall under the dash, probably behind the guages.
Kimberly

HI All,

Thanks for the input.

The fuel gauge is stable and consistent with what I would expect, although I suppose need to get under and have a good rummage around to verfiy what is actually there. BTW It's diagram 11 on the above link. 72 GT.

Can anyone tell me if a UK 20 amp fuse is too light, and whether I need to get one rated at a straight 35?

The 17/35 taxonomy has always had me scratching my head a bit about what exactly the current rating is.

Liam
Liam H

Hi Liam

The 17/35 designation indicates a rating of 17A continuous and 35A fast blow. It could be that the 20A repalcement is blowing when some of the auxiliaries are switched on - especially motor loads such as the heater blower or windscreen wipers.

A 35A continuos rated fuse might not offer protection for the loom in the long term. I think Kimberley's suggestion to locate a garage that services our cars to find an appropriate fuse replacement would be the best approach.

Regards,

Barry
BJ Quartermaine

Forget the voltage regulator, you would have had to conenct two spades together to bypass is, and it would just cause the fuel gauge to read high (not necessarily pegged). Forget also the small difference in rating of the fuses, if the fuses were blowing it is because there is a short in the wiring or a component. Forget also bad grounds, they cause circuits to stop working, not blow fuses.

The blowing with the indicators *could* be a red-herring, another circuit could be taking way too much current but not enough to blow the fuse, but adding the current from correctly operating indicators becomes enough to blow it. But if the indicator flasher burned out at the same time it could well be a short on the green/red and green/white wires out to the corners of the car. It could be the two connected together, which could explain why both dash tell-tales were coming on together with the indicators, the indicator flasher can only handle two bulbs, not four. But if only one side of the car was lighting up at the same time as both dash tell-tales, it could be you have trapped the wiring near the steering column and some are cut and some shorting to ground or each other.

Bypass the indicator flasher for the following tests i.e. connect the green and light-green/brown wires together) which will stop you burning out any more indicator flashers. Also replace the 2nd fuse up (which I presume is the one blowing) with a high wattage 12v bulb like an old headlamp bulb, so save burning out any more fuses. If a short is present the bulb will glow brightly, and none of the fused ignition circuits will work properly. When there is no short the bulb will go out, but turning on circuits will cause the bulb to glow dimly. Note that some circuits will not work quite as they should with the blb in place of the fuse. The bulb also ensures that in the event of a full short to ground the current is limited to what the wiring can safely carry.

With the bulb lit brightly i.e. the fault present you can disconnect various components on the green circuit until one of them causes the bulb to go out ort get very dim, and that is the faulty one. However the green circuit is the most difficult to diagnose as it has lots of branches and components. Even worse if the problem is intermittent, the only thing then is to split the green circuit into two (starting at the fusebox) and fuse each branch separately. When one fuse blows slit *its* circuits into two lots and fuse *those* separately, and so on.
Paul Hunt 2

That was really cool Paul. I saved it for the future as a text document in my electrical file folder.

thanks
dave
Dave Braun

As a rule of thumb a fuse will open at twice its constant carrying capabilty. so 17/35 is a beleivable number. Our sars need a 35A continuos which will open immediatly at 70A
Stan Best

I was under the impression that it was 17 amp continuous, 35 amp blow, hence the 17/35. May be slow blow at 35 and quick at 70, but still only 17 continuous.
Paul Hunt 2

Thanks for the input, Paul that is very useful indeed.

I was pretty sure the fault is in the indicator circuit as I unplugged everything else on the green circuit and still had the issue.

Tonight I tested all the feed wires to 4 indicators individually for earth and they were OK. The light units are all new.

I suspect I broke the flasher unit by forcing it into its holder too hard, there was a big dent in it - I think that was a side issue.

Two new things I found tonight :

1. One of the contact springs inside the indicator assembly was bent and was just touching one of the contacts when indicator was off, and may have bridged both when indicating left. I'm not sure if that could cause aan earth problem or just a lights not cancelling problem. I replaced the whole unit with a spare.

2. The 4 way green wire junction under the dash had one of the wires in a very loose connection. The junction fell apart when I took it out. I put a new one in place. Can a bad connection on a live circuit cause a fuse to blow?

Let's see how that all works out. Naturally, the thing only fails now when I am out on the road and not in the garage!

Liam

Liam H

Unless the loose spring in the switch was touching something that was at earth potential it wouldn't blow the fuse, but if it was touching one of the indicator contacts it could well mean that side flashed continuously even when the stalk was in the off position.

If the loose spring caused the indicator switch to light both sides that would probably damage the indicator flasher unit as I mentioned. But at 84 watts instead of 42, and 7 amps instead of 3.5, that should be nowhere near blowing the standard 17 amp rated, 35 amp blow fuse. Unless there was something else taking a lot of current from that circuit, and probably more than it should.

A bad connection *at the fuse* may cause it to blow because of localised heat, but not anywhere else. A bad connection *reduces* current in a circuit.
Paul Hunt 2

I think I have isolated the problem.

I thought that the indicator cancel lug might be touching something it shouldn't as I noticed the problem was only ever when turning (d'accord!) otherwise why would I be indicating!

I took the cowl off and jacked up the car and examined lock to lock what was happening.

Turns out the grub screw was making its way back around and earthing the springs, I don't know why I thought that just rotating the collar would solve the problem.

I put a shorter flush grub screw in and it seems to have done the trick.

Still some good problem solving advice though, thanks.

Liam
Liam H

This thread was discussed between 08/06/2007 and 14/06/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.