Alternator Advice

[Gary71]

Driving back last night my alternator light started to glow bright each time I dropped below about 2k rpm.

It’s always been a bit reluctant to go out, needing a blip first thing, but with the load of lights and things it tipped over the edge.

As a dumb mechanical engineer is this a simple ‘buy new alternator’ moment or should I be investigating something else first?

For context all the earth straps/points are clean and new and I’ve two new batteries.

[stretch]

A voltage check on the battery whilst reving the engine to 3k would be my first check, other person needed. If it exceeds 14.4 volts then it might not be regulating the voltage..... Think that's right from memory. As the 72 on has the regulator in the alternator it would be an easy swap. I purchased one for my 70 car via ebay last year. Got the right bulb in the gauge Gary.?

[Lightweight_911]

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I've used a digital voltmeter (with built-in USB lead) plugged in to the cigarette lighter socket via a USB adapter - allows you to monitor voltage output 'on the go' when travelling solo ...


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[Gary71]

Thanks guys, I’ll do the voltage check tonight.

I should have remembered that my USB socket fitted under the dash has a voltage meter built in!

Doubt it’s the last word in accuracy, but should give an idea.

[PeterK]

As the 72 on has the regulator in the alternator it would be an easy swap.

SCs still had an external regulator in 1979

[Gary71]

Excuse the old crusty picture, but I think the highlighted part is the regulator?

[Gary71]

12.8v Ignition on - Engine not running
13.1v - Idle (1000rpm ish) - Alternator light on
14.4v - 2000rpm - Just as light goes out
15.1v - 3000rpm and above - Light out

I'm guessing it should be 14.4v (+/- small variation) all the time when the engine is running?

[stretch]

Yep, that's the bugger. I first tried replacing mine with a solid state regulator, no difference on mine so i replaced with the ebay unit The danger is fried batteries. We have had a few on DDK.

[inaglasshouse]

Yep, that's the bugger. I first tried replacing mine with a solid state regulator, no difference on mine so i replaced with the ebay unit The danger is fried batteries. We have had a few on DDK.

Yep


https://www.ddk-online.com/phpBB2/viewt ... 05#p616281

(I bought one of Jonny's uprated alternators to replace mine, on the basis that I knew what I was getting, it is massively over-specced so should have a nice easy reliable life, and I couldn't be bothered to faff about).

[Gary71]

So measured with proper meter directly at the battery rather than the nasty USB feed hacked into the radio 12v feed...

12.6v @idle - Light On
13.7v @2k - Light just out
14.9 @3k - Light out
14.5v @4k - Light out

[stretch]

My light only came on when i turned out the ignition. Readings seem ok Gary.

[Lightweight_911]

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I wouldn't have expected the voltage to decrease at higher revs - I would have thought it would 'peak' & then remain at that figure as revs increased further.

Idling speed voltage seems low too ...
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[Gary71]

Definitely low at idle, and up to 2k, hence the light being on.

I’m going to do a voltage drop test later and make sure the earths are all good before I start pulling anything apart.

[deano]

I think the regulator should output its nominal voltage at idle (yours is not, hence why your light is on I think). It is normal for the voltage to go up or even down slightly with higher RPM (non-linear output with change of input frequency/RPM). The voltage that you measure will also depend on where you measure and which gadgets are turned on.

I would say that, if you are measuring less than about 13.8 directly on the regulator output terminal (with negative/black probe to a nearby earth), either your regulator or the alternator itself are the main suspects. Obviously, change the regulator first. I found that the regulator is used on many vehicles of similar age and can be found more cheaply using the manufacturer part number...

As usual... I take no responsibility for any advice... blah, blah, blah - we all know that its always something unexpected

[Gary71]

You mean like this one?