Converting an MGB from a Lucas alternator to a Delco alternator

By Larry Hoy

Sunday, August 03, 2003

This information came from my experience in converting my 1969 MGB from a Lucas alternator to a Delco alternator. I had plenty of help, mostly from my Internet friends.

The first step is to locate a Delco alternator. These are available rebuilt "everywhere" for about $30.00, expect to pay about $15.00 more if you don't have a core to turn in. Many places will accept a Lucas alternator as a core but remember the Lucas core may be more valuable $25.00?. The Delco alternator I used is a 63 amp internally regulated alternator. It fits 1971 through 1985 GM products. The person at the parts store will probably ask you what year and model car the alternator fits, if this happens just name a GM product . . . how about "Camaro", and pick any year between 1971 and 1985. It won't really matter what model you name, I'm pretty sure GM made only one internally regulated 63 amp alternator during that time period.

While you are at the parts store you will need to buy two additional items. The connector that plugs into the Delco alternator, this is a standard item, probably found in the "Help" section of the store. It will cost about $2.00. You will also need a new adjuster arm, the adjuster arm I used is a universal type; somewhat "J" shaped with two holes at the top end of the "J" and a long curved slot at the other end. It will cost about $12.00

The mounting points on the Delco alternator are different than the Lucas alternator, so you will need to mount the Delco alternator in what might best be described as an upside down position. By doing this the large mounting ear on the Delco alternator is mounted to the adjusting arm (more on this later) and the small mounting ear is mounted to the front side of the water pump. Mounting the small ear to the water pump is a critical mounting point. It is very important to make sure there is NO movement front to back at this point or you WILL break the "ear" on the water pump (does it sound like I'm speaking from experience?). The small ear on the GM alternator is threaded with metric threads, that means the bolt supplied with the alternator is metric. I don't suggest using this bolt. The metric threads are very close to our standard course threads. I used a standard bolt and screwed it into the metric treaded small ear on the alternator so that the threaded end of the bolt points to the rear of the car. It doesn't matter if you screw up the threads on the alternator because you will not have to remove this bolt again. This bolt (which now is better described as a stud) will now slide into the mounting ear on the water pump.

Now you must fabricate a mounting system to connect this "stud" to. I used a bolt which I modified; the length of the bolt you use will be unique to your installation, but remember it needs to be able to reach from the standard MGB mounting bracket to the backside of the water pump ear. I used a grade five bolt (three "marks" on the head); I cut the head off, and threaded that end of the bolt. Now I have a "bolt" without a head, threaded on both ends, with no threads in the center. All Thread could be used here but I have found the bolt is much stronger than the All Thread, and as a result provides a more durable installation. On one end of this "bolt" you will install a standard nut, on the other end you will install a "long" nut, I don't know what this type nut is called, but they are about 1.5 inches long. You need to secure this long nut by using a jam nut with it. Now you will slide the end of the bolt with the standard nut through the front side of the rear mounting bracket and screw the "long" nut on the bolt that is attached to the alternator (which is now through the hole on the water pump). Next, install another nut to the end of the "bolt" which should now be extended through the backside of the rear mounting bracket. For now just "snug" all these nuts by hand, they will be tightened a little later, after the alternator belt adjustment is done. All of this may sound confusing, but in reality, if you could see this installation, it is quite simple.

The larger mounting ear on the alternator is attached the adjuster arm. You will need to modify the adjuster arm by cutting 6" off the top of the "J"; since this eliminates the mounting holes you will have to drill a new mounting hole. This hole needs to be drilled as close to the "inside" of the curve as possible without sacrificing the integrity of the arm. The arm is then attached to the standard mounting point on the MGB.

On the back side of the alternator you can attach a strap that is fabricated from 3/16" by ¾" flat stock. Just make some minor bends, and drill two holes at each end of the strap. One end is mounted to the standard alternator bracket on your engine and the other to the standard mounting bolt on the back of the Delco alternator. This bracket helps eliminate vibration, which can break the ear on the water pump. This strap is probably not necessary if you follow the above procedures carefully and/or have a cast iron water pump.

When you have completed the entire installation and have adjusted the alternator belt tension it is important to tighten the new mounting "bolt" correctly. First you will need to tighten the long nut against the water pump ear, then adjust the nut that is on the front side of the MGB rear mounting bracket so you have the "proper" tension between the water pump ear and the rear mounting bracket. Then tighten the nut on the back side of the rear mounting bracket. This will insure there is no movement front to back on the water pump ear.

Don't panic yet, you're almost done! Now it's time to connect the wires to your new alternator. If you haven't done so already, this would be a good time to disconnect your battery ground connection. You will probably have three wires to work with, a large brown one, a small brown one, and a small brown/yellow one. On the large brown wire you will need to install a 1/4 inch ring connector, and attach it to the stud labeled BAT, which is located on the back of the alternator. The small brown wire goes to terminal 2 of the plug. The small brown/yellow goes to terminal 1 of the plug.

The above connections work for my car, but as always, especially with British electrical systems, there are several exceptions. I cannot speak to all of these exceptions but I know someone who can. The following four wiring scenarios are complements of Dan Masters, Dan is a well known British Car enthusiast who is considered an expert in the electrical aspects of these cars. Dan is too modest to claim the title "expert", but I am here to tell you, he is one.

1) If you have two wires at the alternator; connect the large brown wire and the small brown/yellow wire as described above. Connect another small wire from terminal 2 to the large BAT connection, along with the large brown wire.

2) If you have five wires at the alternator; there will be a large brown, a small brown, and what appears to be 3 small brown/yellow wires. In reality, there are only two small brown/yellow wires, one you will use; and another which is really a short wire that goes from the alternator into the wiring harness for a very short distance and comes right back out again to terminate at the alternator. Remove this short piece of wire, and wire the remaining 3 wires exactly as stated above.

3) If you have two large brown and one small brown/yellow wires; connect the two large brown wires to the BAT terminal. Connect the brown/yellow wire to terminal 1. Connect one end of a short piece of brown wire to terminal 2, and connect the other end to the BAT terminal, along with the two large brown wires.

4) If your MGB was equipped with an externally regulated alternator; at the alternator, connect the brown/green wire to terminal 2, the brown/yellow to terminal 1, and the large brown wire to the BAT terminal. Discard the black wire (or connect to a good ground point). At the regulator, connect the brown/green wire to the brown wire and connect the brown/yellow wire to the brown/black wire. Discard the black wire (or tie to a good ground point). You can either discard the regulator or gut it and use the terminals to join the wires as described above.

5) If your MGB is equipped with a generator then at the alternator, connect the existing brown/yellow wire from the old generator to terminal 1, and the brown/green wire to terminal 2. Using a 1/4 " ring terminal, connect a wire of at least 10 gauge to the terminal marked "bat" and connect the other end to the starter solenoid terminal with the battery cable. Route this new wire along the existing wiring harness as much as possible, using cable ties liberally. At the regulator, connect the two brown yellow wires together. Connect the three brown wires to the brown/green wire. Discard the black wire (or connect to a good ground). You can either discard the regulator or gut it and use the terminals to join the wires as described above.

The cost for this conversion is approximately:

• Alternator $30.00 (if you have a core to trade in)
• Plug 2.00
• Bracket 12.00
• Steel strap 2.00
• Total $46.00

This is a "one size fits all" article on alternator conversion; so if any of this doesn't make sense, if you have questions, or if you have suggestions for this article please contact me (Larry Hoy). I will try to help you, and revise this page so it will be of more help to others in the future.

In June of 1999 Ernie Higham wrote to me, I have included his comments here.

Good luck! If you have any suggestions or comments about this information please contact me, Larry Hoy.

BACK TO MGB FAQ PAGE