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I have installed a new GM alternator rated at 55A, the question is, the ammeter is rated at 40A max according to the Bentley Manual, which should be an issue if the battery is in good condition, but on a cold morning start up the potential for the alternator to supply more amps than the gauge can take is an issue. Has anyone found a 60A ammeter of the same vintage or a volt meter? the car is 1969 with the early black bezel on the gauges and the narrow text on the gauge face.

There are new Smith's classic gauges available but they look more like the later style.

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I spoke to the local Lucas service department about the very thing.He reckons he can upgrade the original Ampmeter to 60a and of course service it at the same time.I hope to drop it off in the next day or so and will let you know the outcome and cost.

Regards Harry.

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I put a lot of extra power sapping kit on my car eg halogen headlights and driving lamps, tow bar lighting kit, kenlowe fan, emergency flashers and cigar lighter. I was concerned that at tick over the power load showed a discharge. Being ignorant of the potential problem with the ammeter limit, I decided to replace the alternator with an 55amp rated upgrade just in case the original was about to fail. Its been on the car for the last 2 years and the ammeter works perfectly. At start up it goes off scale for about second but cuts back on scale and goes to centre position very rapidly.

 

I assume that as long as the control circuitry works the spike does not last long enough to damage the ammeter - at least in the short term.

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My six is fitted with a fair number of the usual extras, halogens, fan, etc. But when the heated windscreen was fitted i put a mega on to measure the current draw. It indicated, if i remember right, 35 amps'ish. So at the same time the alternator was up rated to a 65 amp unit. Neither wiring or gauges have proven to be a problem. Two years running on this now.

 

Noel

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I have purchased a 45amp unit Lucas replacement for my TR% and havent yet fitted. I have noted that various experts recommend that you upgrade thge wioring from the alternator thru to the ammeter and to the battery or solenoid. Notably the argument appears that wiring designed for 28amps may burn out and start a behind dash fire without upgrading. Judging by the comments here this isnt the case .

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Ampmeter is u/s,so I have ordered a 60 amp Smith one.To uprate a standard amp I was told a shunt is fitted between the terminals' so it can be done.

Regards Harry.

 

Correct.

All ammeters use a shunt.

A shunt is basically a resistor mounted in parrallel with the meter.

e.g If a meter is rated at 1 amp FSD (full scale deflection) and you have 60 amp source to measure you will need a shunt (resistor) that will take 59 amps.

The formula is Resistance = Volts/Amps.

So by varying the size (resistance ) of the shunt you can use the same meter to read different currents. As in a multimeter when you switch to a different current scale.

Regards

John

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Correct.

All ammeters use a shunt.

A shunt is basically a resistor mounted in parrallel with the meter.

e.g If a meter is rated at 1 amp FSD (full scale deflection) and you have 60 amp source to measure you will need a shunt (resistor) that will take 59 amps.

The formula is Resistance = Volts/Amps.

So by varying the size (resistance ) of the shunt you can use the same meter to read different currents. As in a multimeter when you switch to a different current scale.

Regards

John

That sounds like a plan, where does one get a 'shunt' from.......

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Fitted a 60 Ampmeter from Speedy Cables.It even has Smiths logo on it and looks the part.It cost's £29.55 plus vat,post and packing all in £43.53.

It has 9.5mm spade terminals.I connected up the cables and the extras on the minus side with drilling the terminals and bolting through these with eye terminals on the advice of our electronic electrician at work.I believe you can not get piggy back female spade terminals this size,unless someone tells me different.At first glance you would not know it was a pattern part.Well pleased.

Regards Harry. :rolleyes:

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When I did put my 6 on EFI I fitted a 65A alternator and fitted a shunt to the Ameter. I used a 2cm long copper wire as used in buildings

But this does not solve the eventual overload problem on the rather underdimentioned TR6 loom. The most easy thing is to add an additional cable from alternator direct to the battery, what will also solve the Ameter issue with concern to charging ;) and needle deflection

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