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MG MGB Technical - Wipers gone to war with tacho

Hi,

New problem on me: when the wipers start their stroke the tacho drops like a stone (74 CB GT), immediately bouncing back off its stop and running normally until the start of the next stroke.

I've got a Smartscreen intermittent wipe unit to slightly complicate things, but it shows the same problem when the wipers are run on the normal setting. I can't detect a misfire and the ignition light doesn't seem to come on. Any ideas? Thanks!
SteveP

I think your car's just got a sense of humour Steve. Just having a little fun and games.

Don't worry, somebody knows the cure to this one.

Keep smiling.

David
D Balkwill

Steve,

Generally when unrelated parts start having relations, it is a problem with a ground somewhere.

Keep in mind that I don't really have the wiring in my head, so they could be related and I wouldn't know it.

Charley
C R Huff

Anyplace the circuits are common, most often the fusebox or wires there, or the big group of White wires junction by the steering column.

FRM
FR Millmore

Sounds to me like your tach is just trying to keep the gauge glass clear in the rain !! Can't be too upset with that !

On a less blatantly bad humor note the first thing I think I'd do is get the intermittent wiper out of the picture if that is easy to do. Also, check the tach conection at the coil (that's where it is on my 73 and I think it's the same on the 74). A corroded connection there might give just enough resistance to kill the current flow to the tach under the heavier electrical draw moment of the wipers. The wipers are on because it's raining. That dampness could easily make an electrical connection weak....at the coil, fusebox or elsewhere.

BH
BH Davis

Ordinarily I'd say check the fusebox for bad connections if all the fused ignition circuits fail as well, or certain bullet connectors in the green circuit if not. But this era of MGB powered the wipers (and heater fan and electric washers) off their own in-line fuse from the accessories position of the ignition switch (white/green unfused side, green/pink fused side), which bypasses all of the white and green circuits. It can't be the brown circuit or the switch itself or the engine would be cutting out as well. So that leaves, unusually, a earth problem as mentioned. This is shared with the tach (and heater fan, dash lights, courtesy light on manual) but they only come together right at the point where they bolt to the body so I suspect that is loose or corroded. This is probably very high up on the firewall behind the dash close to the wiper motor, may even be concealed by it.
P Hunt

Thanks for the suggestions. It has, of course, stopoped doing it now that I've got time to look at it. I will try the earth first.
SteveP

This happened to me once, for about half an hour, then ever again. Big generous sweeps of the tacho'.
Very heavy rain, on an up hill grade. Water was running onto something and caused a short.
It is the PRINCE toying with us mere mortals.
P.N. Sherman

Oh well! Whilst I was watching the synchronised dance of my tacho, my diff melted. I'm going to have to fix the large cogs first and will oil the little ones later.
SteveP

This thread was discussed between 23/11/2009 and 28/11/2009

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