[Mgs] alternator wiring quick question

PaulHunt73 paulhunt73 at virginmedia.com
Thu Aug 13 08:45:33 MDT 2015

Is your alt like this?  http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/alt16acr.jpg

That is the 4 wire system with two plugs.  The upper one is pos (thick brown) and neg, although there may not be an earth wire on the neg if it makes that connection through the mounting ears as on all the other types.

In the second plug B+ should have another brown going back to the solenoid battery terminal with the brown from the first plug.

A brown/yellow should go to the IND terminal in this second plug, then loop back into the harness and out again to a 3rd terminal alongside it in the same plug.  I think this is to protect the electronics in case that plug becomes detached.

Is your clear plug like this?  http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/altbee3.jpg

If so then it sounds like your harness has the plug for the later 2 and 3 pin alts, but you still have the 2-plug, 4-wire alt.

This shows the three types, how they were wired, and how to convert between them http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/altconv.jpg

If yours aren't like the first two images then without pictures of your alt and plugs it's a bit difficult to work out what is what.

Coming on dimly as the revs drop is actually what happens with a dynamo.  On an alt the light should switch out as the revs go above about 1000rpm, and switch back on again if they drop below about 600 rpm, with little changes in brightness.  In practice the idle should never drop that low with an alt, as that stops it charging, and one of the benefits of the alt is that it should be charging at idle, unlike a dynamo.  If yours isn't coming on with the ignition but does come on dimly as you describe, then it is not wired correctly.

  ----- Original Message ----- 
  Thanks Paul. It is a 16ACR, I had it rebuilt about 5 years ago. The only connection that fits it is the old clear 3 prong plug with the heavy brown lead and 3 brown/yellow leads. The plugs are not like your photos though, its 2 side by side and one below on the right side? 

  Also the large brown “mystery” lead on the other 2 prong plus is not live, and I’m not sure if it hasn’t been hanging off for years or not...Also my red ignition warning light doesn’t come on either when the key is turned and I believe it never has since I’ve owned the car, I did see it come on dimly last night when the revs dropped low, which I know is typical?
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