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MG MGB Technical - Tach jumps

Maybe someone can help me with the following: The car is from 77. When one of the blinkers is on, the tach drops to zero and after a while it starts jumping in the same rhythm as the blinkers...
Maybe this is caused by a short-circuit? What does the tacho has to do with the blinkers, would not think they are in any way "connected"?
Thanks for your help!
Regards,
Richard
Richard Houtkoop

Check out your schematics available at http://www.advanceautowire.com/

They are connected to the same side of fuse 3 (third one down) but on separate green wires. Maybe this is what's causing your tach to jump.

Does this also happen if you turn on your hazards? If not, then I suspect you may have a faulty turn signal flasher module as they are separate flashers.
Jeff Grant

Thanks Jeff, will check what happens when the hazards will be turned on. The turn signal flasher has got a hole in it, so despite that it is working, this might be the cause of the problem?
If not, would it be an idea to change the connection to the fuse?
Richard

The flasher module itself has a hole in it? I'd definitely change that then.

Here in the states we can get a flasher module for as little as $2.50 from the local parts store. For that little money, I'd eliminate that as your issue right away. Any standard 12 volt 2 wire flasher should work just fine.
Jeff Grant

Yes, was about to change it. Guess it is a little more expensive here, but no problem.
It doesnot happen when I turn on the hazards, the signal on the dash is also much brighter. You are talking about a faulty turn signal flasher, you mean the handle on the steering column?
Richard Houtkoop

No, the flashers are located behind the glove box on my US left hand drive car.
Jeff Grant

This is one of the rare occasions I'd suspect a bad ground - the flasher repeaters share one with the tach (and many other things). With the flashers off the tach can 'see' an alternative ground through a flasher repeater and the flasher bulbs at the corners of the car to the grounds in those light units. With the flashers on there is 12v on that ground, hence the tach effectively has no voltage.
Paul Hunt

Well, finally all solved. The problem was that the owner (it is not my car) previously had starting problems and had it fixed by some stupid mechanic. A new wire was fitted, thus bypassing the ballast resistor (I guess this is the right name for it. It is round and black with 3 connections, located nearby the fuse box.
When the ballast resistor was replaced by a new one and the bypass removed, it started great again and all other problems fixed. I heard something about 12 volt needed for starting and, as soon as it runs, it goes back to 6 volt. Because it was bypassed, it kept 12 volts which caused the electrical problems and the engine running bad.
Thanks anyway for your help!
Richard Houtkoop

The ballast resistance is a length of resistance wire contained within the harness. Early factory electronic iognition *did* have a separate ballast resistance component, but this was to power the electronics in that version, and should be out of circuit if you no longer have that system.

All rubber bumper cars had this ballasted ignition system, whereby the coil operates at 6v for running but at 12v during cranking for a bigger spark and easier starting. Bypassing the ballast on these systems, but keeping the original coil, results in an overheated coil and premature failure. If the coil had been replaced with a 12v item then the ballast *should* be bypassed, or the new coil will be delivering a weak spark.

The only real test is to measure the primary resistance of the coil - 12v i.e. chrome bumper coils measure from 2.5 ohms to 3 ohms, 6v i.e. rubber bumper coils measure 1.2 to 1.5 ohms. With the ignition on and the coil -ve i.e. points wire i.e. black/white grounded if you have 12v on the coil +ve you have an unballasted ignition system and need a 12v coil, and if you see about 6v you have a ballasted system and need a 6v coil.

But there is no way any of this would cause the blinkers to affect the tach.
Paul Hunt

This thread was discussed between 24/06/2008 and 04/07/2008

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