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MG MGB Technical - Windscreen wipers

I recently replaced the parking switch on the 1971 MGB wiper motor as reading an archive thread this seemed to be the solution to my wipers not always self parking - they parked OK when the wiper was used for a short period but once the motor got hot they would not self park.
It was a fairly easy job and when tested all seemed OK - wipers worked and self parked.
First time in the rain the wipers moved just a few degrees of arc from the parked position and stopped.

I have now found that the 35A fuse had blown. I relaced it and it blew again - what have I done wrong??!!

John Turner
John Turner (Midget & MGB)

Funnily enough that is what my V8 did yesterday, fortunately when I used the wiper stalk instead of the indicator after switching over from the ZS. In my case it isn't the fuse as the other things on that circuit still work.

If a fuse blows it probably means there is a short. If a replacement fuse of the correct size blows immediately then you definitely have a short!

The important question is when does it blow? If it only blows when you turn on the wipers then it must be the wipers. If it blows straight away then it could be anything on that fuses circuit. However if the wipers aren't parked then they will try to do so as soon as the circuit is powered, and so the wipers could still be the cause!

Secondly which fuse blew? According to the schematics a UK 71 has an inline fuse for the wipers and heater fan plus a spare connector for a radio, as all these things are functional with the ignition switch in the 'accessories' position as well as the 'run' position. If it is this fuse that is blowing then there is a limited number of things to check. But if your wipers and heater fan only worked when the ignition was on i.e. it is the 2nd fuse up in the fusebox that is blowing then there is a mass of other stuff that could be causing the problem - *unless* you can reliably and consistently isolate it to just one circuit being turned on that blows the fuse.

Shorts can be tricky to trace, I have found replacing the fuse with a high-wattage 12v bulb works best. When the short is present the bulb will glow at full brightness, when it isn't then it will only glow dimly or not at all. Some circuits will work OK like this, some not, which is why it should only be used for diagnosis and not as a 'fix'. With the power on if the bulb glows brightly you can go through the circuit disconnecting things until it goes out or dims down, then concentrate on what lies beyond the last connection you broke.

If the fault is intermittent it is even more tricky to find, and in that case break the circuit down into branches separately fusing each branch temporarily, then which ever fuse blows shows you which branch is causing the problem.

OTOH the plug to the wipers may have fallen out and the 12v pin is shorting on the motor.
Paul Hunt

Paul,

Thanks for a comprehensive response.

As I said the odd thing is that after replacing the parking switch on the motor the wipers worked OK and parked OK. It was only a couple of weeks later when I turned them on for the arms to move a few degress and then stop.

I had thought of the in line fuse - do you know where it is?

As I only changed the swithc I am wondering if the plunger which operates it from the wheeel within the motor has to be fitte at a certainpint in the rotation of the drive wheel. I thought I had done this as I changed the switch with the wipers parked

Thanks

John
John Turner (Midget & MGB)

The parking switch and connector block on the 2-speed motor (which I assume you have as they were standard from 1968) can only fit in one position. It is a notch in the drive wheel which operates it, so that has to be in the correct place for correct parking, and I assume you didn't open up the motor to replace the switch as it isn't necessary.

If you have an in-line fuse (i.e. the wipers and heater fan normally operate off the accessories position of the ignition switch as well as the run position) then on my cars (73 and 75) it is underneath the fusebox. But if you say you replaced the fuse, but don't know where the in-line is, then I suggest you have the earlier version where the wipers and heater fan only work when the ignition is on (it changed for the 71 model year) and you changed the 2nd fuse up in the fusebox. In that case it could be anything on the green circuit that is blowing the fuse, and you could have trapped and damaged the harness when working with the motor.

But the parking switch contains both earth and 12v terminals, and so it is possible that it is faulty and bridging the two, so blowing the fuse as soon as you power the circuit i.e. turn on the ignition. By removing the U-strap you should be able to move the motor enough to unplug the harness from the motor, which should eliminate or prove this as the cause of blowing fuses.

I've had my motor off today and it looks like worn brushes are the cause of my non-operation, new ones on order.
Paul Hunt

Paul,

Thanks again.

It looks as if I shall be doing more upside down crawling in the footwell next weekend to track the fault down.

Regards

John
John Turner (Midget & MGB)

Paul,

Fault found - it appears to be the 'new' switch I bought fom Moss that had failed causing a short - I blew 2 more in-line fuses (found it beneath the fuse box as you said). I put the old switch back in (which was not 'parking') and it works fine (still not parking) - but not blowing fuses. Its back to Moss for a replacement

Thanks again for some good info

John
John Turner (Midget & MGB)

All too common these days, I'm afraid, and Hunt's Fifth Law of classic car ownership. Fixed mine too - new brushes. Although I then had the really weird situation that when I fitted the motor back to the gearbox it still wouldn't run, although this time it was mechanical as the flying leads I was using to bench-test it were sparking merrily whan tapped on the spades (they hadn't been with the old brushes) - a strong recommendation for bench-testing things before refitting them if one were needed! Eventually I had to wedge a small washer between the motor and gearbox casings in one particular position.
Paul Hunt

Aaaaarg - I don't belieeeev it! Just fitted a replacement switch from Moss and it blew the in-line fuse straight away. Put the 'old' switch back in and the wipers work fine - including parking which was always the case until the motor got hot.
I have double checked the catalogue to see if there are 2 types of 'clip' switch but there is only one.

Antbody any idea what is going on?

Regards

JohnT
John Turner (Midget & MGB)

Sounds like there is a batch problem, if the first replacement worked for a short while. If both had blown the fuse straight away then it's possible there is a wiring discrepancy between the switch and the harness plug. There were several motors and wiring arrangements up to 68, but after that only one, which your 71 should have of course.

If you take the 2nd replacement switch off and can do a continuity check, test between the single spade that is 90 degrees to all the others (which is the earth terminal) and the two upper ones in the bottom photo on this web page http://www.mgb-stuff.org.uk/ws.htm. They should have black on the single spade at the end, brown/light-green on the top one next to that, and green or green/pink on the one next to that. The bottom two spades go straight through to the outside for the motor slow and fast speeds, as does the earth spade. The two top spades and the earth are part of the change-over park switch, which connects the brown/light-green wire to the green (green/pink) wire when not parked, and to the black earth wire when it reaches the park position. If the switch contacts (see bottom picture on the above web page) are badly set and the black spade comes into contact with the green (green/pink) then it will blow the fuse instantly, i.e. before the wipers are even switched on. This will also happen if the green (green/pink) wire in the harness is earthing somewhere.

OTOH if the brown/light-green wire in the harness is earthing somewhere then that will blow the fuse as soon as the motor moves from its parked position i.e. as they are switched on.

The problem is that if the short is intermittent then you aren't really going to know if it is in the switch or in the wiring, unless you remove the brown/light-green wire from the harness plug. If the fuse doesn't blow at all then the problem is on that wire. If it still blows instantly it is on the park switch or green (green/pink) wire, and you would also have to remove the green (green/pink) wire from the harness plug to find out which, if it still blows it is the wire, if not the switch.
Paul Hunt

Paul,

Thanks again for an excellent reply

I am home tommorrow so will check things out as you suggest and report back

John
John Turner (Midget & MGB)

Hi,

I am still having problems!

I have now tried 3 'new' switches (the last one an original Lucas) and they have all blown the in-line fuse when the original dosen't!

The original switch works OK (parking correctly) until the wipers have been running for sometime then they won't self park and shut off - the original fault I have been trying to cure.

I have compared the circuits of the new and old switches with a continuity meter and they are exactly the same (both when switched and unswitched) - this has got me beat. Any further advice?

Regards

John Turner
John Turner (Midget & MGB)

John,
I had a problem with my wipers not self parking, I found it was the earth wire (black) from the motor, it was loose & fraid...rewired..bingo. On my 73 bgt the earth wire runs very close to the flasher unit wires(& others) worth a look to see its not in contact with a live wire in that area.
By the way I have taken on board a lot of Paul Hunts'
comments from all the threads, he makes more sense than most.
Good Luck
Bill
Bill Griffiths

When does the fuse blow? Slow speed? Fast speed? When switched off? Does it blow all the time or only when it stops parking?

A switch itself can only cause a fuse to blow if its spades come into contact with the body or if there is some other metallic path from the internal contacts to the outside, and there shouldn't be much chance of that with a plastic-bodied switch, no chance if the fuse still blows with the switch pulled forwards from the dash.

That means the problem is on whatever the wiring from the switch goes to, or the wiring itself if it is chafed or any bullets aren't pushed fully home. It's quite possible the old switch was slightly high resistance which could well have limited the current to something below what is necessary to blow the fuse, whereas the new switches are (rightly) low resistance so now the fuse is blowing.

The park switch on the motor is fairly delicate, I've been able to repair most switches of that age on the MGB but I had no luck with that and had to replace it.

You can do a certain amount of diagnosis of the park system at the manual switch. With the wipers switched off and stopped but not parked i.e. the origional fault you have been trying to cure test the brown/light-green and red/light-green wires for 12v. If you see it on both then the motor should be running so under the circumstances I think we can discount that.

If you see it on the brown/light-green but not the red/light-green then the manual switch is faulty or you have wires on the wrong spades, with the switch off these two should be connected together.

If you don't see it on the brown/light-green then the park switch in the motor is faulty, or that wire between motor and manual switch is open-circuit somewhere, but they should be going direct, or the green/black 12v supply to the motor is missing. That *does* go via several 4-way bullet connectors behind the dash so one of those bullets could be partially pulled out and intermittently shorting at some times and going open-circuit at others.
Paul Hunt

This thread was discussed between 31/08/2008 and 09/08/2009

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