An Alternator Conversion for the G8 to ‘74 MGB By Bob Muenchausen Many folks probably wonder why an MGB owner would put a different alter- nator into an already alternator- equipped MGB. Actually, the reasons are pretty simple. The old Lucas units as used on 1968 to 1974 MGBs were designed to provide adequate battery charging and electrical system capacity. But very quickly, additional electrical accessories could eat up this marginal capacity if they were on all at once (as sometimes happens in cold climates at night), or newer accessories (big blast stereos, etc.) added to a low (35 amp) amperage system. My 1968 MGB-GT had been known to drop its voltage at idle (900 rpm) in the aforementioned winter evening sce- nario to as low as 8.5 volts. The engine idle would stumble, the lights would dim, the heater fan would slow, and the wipers would groan. The problem? The early OE Lucas alternator voltage regu- lator was set up to cut-in at too high an rpm and with all this stuff on, those 35 amps were simply not available. A cold battery was not enough for the load. The Bosch and Lucas units seen here have the double advantages of a lower cut-in rpm level (approximately 400 rpm) and a basic higher capacity (55 amps). Those of us who have converted to this or other, similar alternatives (such as the AC Delco alternator) have noticed that our cars now run much bet- ter in many subtle ways and that the electrical system in general seems to be less strained for any given task. Getting Started The Bosch alternator has five con- nections on the rear: three contained in “plug” receptacle well, one is a threaded post, and the other is marked (+) or bat- tery as shown in Figure 1, page III. The Lucas has just three in the same well, same configuration. Please be sure to disconnect the battery grounding wire before beginning work! Use two 10-gauge wires. Use the existing large OE brown wire and then add a second 10-gauge wire from the sec- ond spade connector to the battery con- nection at the solenoid. Use a 3/8-inch should look like (view left). female connector at the alternator output spade connection. 1. The final fan belt required may be different from the original. 2. The lower adjustment arm require- ment may be different or need minor modification to clear the alternator fan. 3.Be sure the rebuilt alternator comes with a pulley and fan as some don’t. Usable pulleys can be from later Lucas units, Delco (internal regulator units, keyway requires Locktite), or from a wrecker or auto electric supplier. The only connections to the Bosch (Lucas 18ACR) unit that need to be made are in the plug cavity. You can carefully pull the existing connectors out of the connecting plug(s) of your early B by inserting a very small, thin tool into the plastic connector(s) and compressing the “ears” or barbs that Rear view showing wiring. Large red and brown wires go to battery post on sole- noid; small brown/yellow to dash lamp. Note insulation on unused posts. hold them in place and pulling the con- nectors out. The two large male blade connectors (B+) go to the battery wire connection on your starter solenoid. Use the exist- ing large MGB brown wire and a sec- ond 10-gauge wire, and route along the same basic path. Only use automotive or marine quality wire. You will need to buy a larger 5/16-inch female spade lug connector to connect the new wire to the alternator output (B+) spade lug. The small (1/4-inch) male blade connector (D+) goes to the small brown/yellow wire for the indicator light on the dash. The small female connector in the MGB plug with two brown/yellow wires is from the dash. Strip the tape on the har- ness back to see which is the main small brown/yellow wire and which is just the loop. Use the main wire. 13107 alternator. If this part is missing…then connect the thinner brown wire. Cover this post with heat-shrink Wire Color Codes N—Brown—Battery conections at solenoid NY—Brown/Yellow—Dash warning light W—White—Switched power from ignition switch Fig. 1— Wiring connections for converting from a Lucas 16ACR to a Bosch Double, brown-yellow pf
sulate this “pigtail” and secure it back with tape or a plastic wire tie. New 10-gauge wire to starter solenoid. Thick brown wire.