Alternator Performance, Test and Upgrades

[In ref. to a question regarding low alternator output at heavy load and low RPM]

Dan Masters writes:

It is normal for the smaller, underpowered, alternator on a TR6 to experience a small voltage drop with all loads on. Try a voltage reading with all loads off (rpm above 1500). If the voltage stays around 14 volts, you are OK. There is quite a bit of variation in alternator output voltage from one unit to another, but theoretically, they all should produce 14.6 volts under light load.

When the alternator is loaded so high as to drop the voltage to the battery level, what you are doing then is drawing power from the battery. Theoretically, a good battery should produce 12. 6 volts. If for example, it takes 30 amps to bring the alternator voltage down the battery voltage, and you have applied a 35 amp load, the extra 5 amps will come from the battery.

Check the voltage again with no loads, and if it is around 14 volts or more, then no reason to worry. If it is less than 13 volts, then you may want to consider getting the alternator repaired or replaced. In between, it's a judgement call.

That is one of the advantages of the larger GM unit. Even though the Lucas has sufficieant capacity, when fully loaded the voltage goes down, causing a corresponding reduction in light output, etc. The 55 amp GM will provide the typical TR6 load and still maintain voltage.

I bought three GM units in the last year or so, from two different stores, and they all three had the 7127 model number, so I assumed that must be a universal number. Evidently not. Anyway, the model you want is called a "Delcotron SI", and was used on GM cars from the eary '70s till the mid '80s. They are almost universally used in Street Rod applications, so if you can find a store that caters to that crowd, you should have no problem getting the right one.

I can get them locally for about $24 -$30 dollars, plus a $15 core charge. [...] They are fairly light, so shipping costs should be small. They can be sent UPS or USPS.

Hope this helps.

Dan Masters,
Alcoa, TN

Date: Wed, 7 Oct 1998 06:09:57 GMT
From: Egil Kvaleberg
Subject: Re: TR6 - Alternator Output

On 7 Oct 1998,[Dan Masters] wrote:

> It is normal for the smaller, underpowered, alternator on a TR6 to experience
> a small voltage drop with all loads on.

Small?

If I remember correctly, we are talking of more than 1 volt below proper charging voltage, which is not what I would call small. For winter use, such a mediocre voltage means constant reliance on home charging for trouble-free starting at all temperatures and long battery life (a good battery should last 10 years, insufficient charging will half that).

The problem is inherent with the design of the built-in regulator. I would imagine the early, seperate regulator units to be better, but I've never tried them. The Lucas DC generator with relay-based regulator does definitely maintain the charging voltage over load much better then do the Lucas alternator! With the exception of when the engine is running at idle, of course.

I should hasten to add that is not only Lucas alternators that have this problem, most brands of the 70s and 80s are just as bad (I'm not really familiar with 90s).

Not being able to leave things alone, I've replaced the poor built in regulator with a seperate, adjustable PI regulator measuring actual system charging voltage (PI in this case meaning Proportional/Integrating). With such a regulator, the Lucas 18 ACR alternator is able to keep correct charging voltage even with all loads applied (PI pump, heated rear window, headlights and extra driving lamps, fan and wipers).

Now enter the following tale of car electricals engineering: It turns out that the rectifier diodes have been dimensioned with the limitations of the built-in regulator in mind! With the increased output commanded by the new regulator, the diodes get so hot that over time the solder starts to melt! After having replaced the diodes once, before I understood what the problem really was, I resorted to installing proper Schottky rectifier diodes instead. They have the additional benefit of increasing useful alternator output by an additional 50W due to significantly lower losses.

Of course, fitting a big GM unit is probably more inexpensive and simple, but I'd just wanted to say that you it is in fact possible to have satisfactory charging with a Lucas unit (much modified, admittedly).

A much simpler (and quite customary) approach to increasing poor alternator output is to mount a silicon diode in series with the regulator, increasing the charging voltage by 0.7 volts. Just make sure it doesn't bring the charging voltage sans load too high, though.

Ensuring first that the brushes are OK goes without saying - they do not last particularily long on the Lucas unit, but are cheap and easy to replace.

Egil