Saturday, December 31, 2022

Alternating Reality

A few days ago, Alice and I were happily winging our way home at about 8:00.  Suddenly, my dash lights went out. I thought I just had to fix the patched connector I had put together when my dimmer switch died. But then the radio quit. And then the lights got dim. And then I said, Uh Oh.

When I got home, I shut her off, and then turned the key to find absolutely nothing. So I threw her on the charger and went to bed.

The next morning, I took Alice off the charger and she fired right up.  I got my multimeter out, and was surprised to see 14 V at the battery. Puzzled, I reached over and turned on the lights. Suddenly I only had 12.5 V.  You know what that means - a new alternator!

I looked around and was not satisfied with the “OEM replacements” I found either in cost or output. So I figured I would upgrade a bit.  A little research on the Googles let me to the Bosch alternator upgrade by using a unit from a 1979-1980 Ford Fiesta. Unfortunately, the actual Bosch alternator (13107) is extremely rare. But I found a clone on Amazon for 70 bucks, and I figured I’d take a chance.

There is only one problem with this upgrade for my application. The connector on the back is not the same. For 1975, British Leyland decided a “little – big – little” connector was necessary, unlike almost every other year that used a “big – big – little” connector.  (I don’t understand what it is with 1975. Everything has to be different.)   Thankfully, the correct connector is available online and rather inexpensive. So I bought one for a tractor that happens to be exactly the same and saved a few bucks.

I was pleasantly surprised when I received the alternator. It came with a test sheet showing what my amperage would be at different RPM. Note that the RPM listed is the revolutions per minute of the alternator, not of the engine. On an MGB, the crank pulley is 6 inches in diameter and the result is a 2.4X multiplier at the alternator. However, the replacement alternator has a larger pulley and the resulting multiplier is only 1.7X by my measurements. I was hoping that would be sufficient because the Lucas alternator has a different shaft size and the pulley is not compatible.

Removing the alternator is simple. First, you must do this.

This is NOT OPTIONAL.  There is a lot of current that goes to the alternator. You will create a big mess and probably kill yourself if you do not disconnect the battery.

To remove the alternator, remove the connector from the back, disconnect the lead to the distributor and the temperature sensor, and move the harness out of the way. Then remove the three bolts holding it in place and take it out.

Fitting the replacement was just as easy. Transfer the adjuster bracket from the old alternator to the new one and install the alternator.  The fit is exact.

There is one small problem with this conversion. You will need a larger belt. The stock belt will not quite make it. Fortunately, it’s easy to find a replacement. A Duralast 15385 does the trick and is only $12 at your local parts store  

For most cars, you can simply plug in the connector and fire things up. However, I had to convert to the newer connector. A little digging on the MG Experience gave me what I needed.  The replacement connector was easy to install.

I hooked everything up and did a quick test before installing the connector body.  I’ll skip ahead a bit and tell you that everything went fine, so here’s a picture of everything tidied up.

I hooked up the lead to the distributor and temperature sensor, reconnected the battery, and turned the key.  Here is the before and after at the voltmeter.

Then, I turned on the lights.


This was an easy conversion with a good result.  It cost me less than $100 including the connector and the belt. The replacement puts out an extra 12 A.This is good as I have installed a couple of things that use a bit of juice.  I’d recommend this conversion if you still have the original Lucas alternator.

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