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MG MGB Technical - Troubleshooting Wipers

Over the past 8 years, I have infrequently used the wipers on my 1972 B. Typically, once or twice/year & at the annual inspection. Had it out today, it stated misting, and I found they no longer worked. All fuses are in order. Can anyone tell me a step by step guide to trouble shooting? I am capable to get to the switch, but what wires should I be testing to determine if it is the switch, or the wiper motor? It would figure that the inspection is due this month!
Joe Dufresne


I had mine quit in action in a downpour. For me it was a matter of lubricating the spindles and linkage with a shot of WD-40 to get them going again.

C R Huff

Are both speeds defunct? My roadster is reluctant to start on normal speed after not being used for a long time but will start on fast, after that it starts as it should on normal. Until after the next period of no use.

From 71 to 74 the power for the wipers, heater fan and electric washers came from the accessories position of the ignition switch, a white/green wire, an in-line fuse below the fusebox, and a green/pink wire. If these others don't work either then check this fuse and the circuit back to the ignition switch if there is no voltage on the green/pink side of the fuse.

If only the wipers are affected it is a matter of getting the voltmeter out. Slow-running wipers are a pain to diagnose (probably more than indicators/turn signals) as they take quite a bit of current, and the slower they go due to mechanical drag the more they take, which drops the voltage. But then if it is bad connections it is low voltage which causes the slow running, which ups the current again, so it is a chicken and egg situation. If they don't work at all it should be a simple disconnection and relatively easy to find.

On the back of the switch you should have green/pink, red/light-green, blue/light-green, and brown/light-green.

Green/pink is the 12v supply for running, the switch connects this to the red/light-green for normal speed and to blue/light-green for fast. Brown/light-green is to do with parking and is connected to red/light-green when the switch is off, but I won't say any more than that as it gets complicated.

So the first thing to do is check you have 12v on the green/pink with the switch off, and still have 12v with it switched to either of the on positions.

If you don't see 12v, or it drops right down when switched to the first position, then there is an open-circuit or bad-connection leading back to the fuse i.e. the green/pink wires and a couple of 4-way bullet conenctors behind the dash.

If the voltage drops a little when switched to the first or second position, then power is being connected to the motor, so if it isn't then running then either it is mechanically siezed, or there are bad connections between the switch and the motor, in the motor, or from the motor to ground.

You can take the motor out or disconnect the con-rod between the crank pin and the rack, and if the motor then runs like billy-oh it could well be stiffness in the rack and gearboxes. Unfortunately taking the load of the wipers off the motor makes life easier for the motor, so unless you can monitor the voltage right on the brushes with the wipers connected, then you still aren't going to know whether it is excess mechaical load or insufficient voltage/current, or indeed a combination of both.
Paul Hunt

After inspecting the on-line fuse (it was not blown!) I have found the wipers started working again. I would suppose the spring or contacts in the fuse holder are suspect, and will add to my list of things to replace. For now, if they are working I can pass inspection and be good for another year. Paul, thanks for the details you provided

My 74 wipers sometimes do not work, however if I jiggle the key in the ignition switch they come righ on

The wipers are on the key-on circuit, and if they aren't working probably there are other things inop as well. I would try some contact cleaner on the ignition switch (battery off), but you might eventually need a new switch.

Does it ever rain in Tennessee?
Dan Robinson

Depends on the year Dan. From 71 to 74 they are powered from the accessories position of the ignition switch, along with the heater fan and electric washers. Which is why *they* can fail but nothing else, and why jiggling the ignition switch can bring them back.
Paul Hunt

I had intermittent wiper failure on my 73B roadster. It was in conjunction with intermittent heater fan failure as well. When one stopped working so did the other.

The fact that they failed together made the fix fairly easy. Since other items on that same fuse still worked that ruled out the fuse. The wiring diagram showed a junction point where the two circuits came together. Turns out it was behind the radio and I had been working on the radio prior to the problem starting.

When I pulled the radio out again I found the wiper/heater fan bullet connectors had been pulled slightly apart....thus giving the intermittent contact problem.

BH Davis

This thread was discussed between 02/08/2009 and 06/08/2009

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