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Thread: Voltage Dropping

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Voltage Dropping

    This is the second urgent pre-event thread I have!!

    I noticed when trying to fix my oil pressure issues that as I turn on more and more electrical items the voltage on the volt meter drops. A few revs gives a little more voltage.

    With the car idling, as I turn on heavy loads (lights or heated front windscreen) the car seems to run a little rough and the tacho needle starts to jump. Giving a few revs causes the tacho to jump to 5000rpm and hold there. ie it starts doing strange things.

    Any ideas?

    Is this likely a regulator issue? Expensive item to replace 'just in case'.

    I measured the voltage feed to the tacho and it dropped as I turned on electrical loads and got as low as 12.1v. This could cause the tacho to act strange?? How will the coil handle low volts? Will that cause poor running???
    1970 Mk1 Escort Tarmac Rally Car

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    Mechanic

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    Re: Voltage Dropping

    "Weird" electrical issues are almost always earthing problems, so check you have good earths first, starting at the tacho.
    It is normal for the voltage to drop (and the revs to drop) when a lot of electrical items are on at the same time. Have a read here:
    http://www.aa1car.com/library/charging_checks.htm

    Another point to note, your heated windscreen draws a LOT of power, more than anything else probably on your car. i know my car with a modern 70amp alternator doesnt like it one bit when the engine is idling and i turn on the windscreen, its idle revs drop right down and if the alternator drive belt isn't bastard-tight it slips like mad trying to drive the alternator... thats how much strain the screen puts on the system. (i avoid using the screen sitting in traffic as a result...)

    I dont know what alternator you are using or what its output is, but if its an older pinto/crossflow alternator its probably less than 40amps, if thats the case you'll only be able to use the screen for short bursts.

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Voltage Dropping

    It's an aftermarket jobbie and I think 100amps. IIRC the screen is either 25 or 35amps so a big draw. Useful article ta
    1970 Mk1 Escort Tarmac Rally Car

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    Pit Crew Decade Plus User MK1_Oz's Avatar

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    Re: Voltage Dropping

    I took a close look at the earthing of the tacho. I had connected the gauge lights, gauge power and tacho earths together. Several sections of wire were a little thin so it is possible that the earth was insufficient even though the gauge lights and power would not be a huge consumer. I added an extra earth to the tacho and the needle seems happier although not perfect.
    1970 Mk1 Escort Tarmac Rally Car

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