Jaguar E-Type Series II Bosch Alternator
Best Alternator Swap Yet!

Last year, we documented the swap of a Nippondenso alternator for the stock reverse-mount Lucas unit on S2 cars. I came across a Bosch alternator that makes for a significantly simpler and more powerful swap. It's a 90 amp alternator from an '88-92 XJ6, Lester# 13247. This particular alternator had been rebuilt by Lucas (A2515) and possibly misidentified as an XJ6 alternator. The alternator has a W~ connector, which doesn't appear on the XJ6 schematic. I'll give instructions for connecting an alternator with or without this connector. Instructions apply to E-Types with reverse mounted alternators. This alternator can also replace forward mounted alternators, by using a regular clockwise fan.
Bosch alternator is very similar in appearance to stock unit. Clocking is identical to the Lucas.

As was discussed in the Nippondenso article, the Lucas alternator had several unique features:

  • Left hand (CCW) rotation. Very few alternators come with fans designed for left hand rotation. I'll discuss how to find a suitable fan for the  Bosch
  • Mounting ear at 10 o'clock. The Bosch alternator has exactly this configuration.
  • An alternating current output ("AL") for the ignition warning light. It's also possible to eliminate the AL connection and 3AW, in which case it works like any modern L circuit alternator. Both wiring plans are presented.
 Automotive alternators are designed with the diodes located in the rear  (not the pulley end). The reason is that semiconductors can be damaged by heat, and there's more room in the back for heat sinks and such. To ensure proper cooling, the fan is usually mounted near the pulley, and it's designed to pull air from back to front, so that the cool air hits the critical diodes first. The fan is usually “clocked” to maximize airflow. If you simply turn an alternator around backwards (pulley facing the engine), it will spin counter clockwise, and the fan will pull less air. A 90 amp alternator produces about as much heat as five light bulbs, so the air hitting the diodes would be hot. Series II and III E-Types had reverse mounted alternators. To accommodate this, the fans are designed to rotate counter clockwise. Here's a comparison of CW and CCW fans:

Counter Clockwise Fan
Note: Blades Point Left
Clockwise Fan
Note: Blades Point Right

The Bosch alternator usually comes with a clockwise fan. This should be discarded and replaced with an omnidirectional fan or CCW fan. I used an omnidirectional Prestolite (Electrosystems) part #90-2241. These are common aircraft components which can be obtained at reasonable cost. If you don't have a local aircraft supply house, try Flying Colors Air Parts.  The Prestolite fan includes an aluminum baffle. Another option, with possibly greater availability is Bosch part#1126610046, which is a CCW fan. It can be obtained through automotive electrical rebuilders or from Summit Racing.  (Note: these fans CANNOT replace the fan on a stock Lucas alternator. The hole is 17mm, while the Lucas shaft is 5/8")  If you're trying to replace a regular forward mounted alternator, the XJ6 stock fan is fine.

Prestolite fan works either CW or CCW. Note: Blades are not angled.

One of the issues with the Series II alternator is that it's mounted very high in the engine bay. To clear the bonnet, the tensioning lug is located at ten o'clock. The Bosch alternator (left) is perfect for the application
The Bosch mounts with a 3" double foot. The Lucas 45 amp mounts with a single 2" lug. Lucas 60 amp alternators have a double foot, similar to the Bosch. It's easy enough to adapt the Bosch unit to a Lucas mount.
Wiring plan: actually, very easy to adapt to the E-Type harness. The W~ connection doesn't appear on XJ6 wiring schematics. This unit may actually be a 90A alternator from a 1990-93 VW.

The Bosch alternator has a very adaptable wiring plan.. It has a large B+ terminal for the battery connection. The D+ lead is used for the warning light in it's original environment, but we'll simply connect the F+ lead in an E-Type. The small B+ lead is for a filter capacitor, which can be mounted to the case with the screw on the right. It has a connection (W~) which is similar to the AL lead on the Lucas alternator. From the viewpoint of the 3AW relay, it's an AL lead which can be used to control the ignition light in the dash. Naturally, we recommend replacing your 3AW with one of our solid state units.

When installing a larger alternator, it's important to think about whether the wiring can take the extra load. The key issue is the wire between the alternator B+ terminal and the battery. I ran a second 10 gauge wire, leaving the original wiring in place. Alternatively, you could run a single 6 gauge cable and replace the original wire entirely.

The pulley is a single V pulley, 2.75" in diameter. A local alternator shop or junk yard  should be able to supply something suitable. The pulley from a Delco 10SI should be just about right. You can find one at Quick Start. It will be necessary to cut a groove for the Woodruff key on the Delco pulley.

The original and modified circuits look like this:

Bosch Alternator requires minimal wiring changes.
Alternative wiring: If the Bosch alternator doesn't have a W~ lead, then a little more work is needed. Completely disconnect the output from the alternator relay. Remove the AL and WL leads from the 3AW,and connect the wires with a short jumper. Optionally, add a small resistor in parallel with the dash ignition warning light. (The resistor will prevent non-charging in the event the bulb blows.) This has the virtue of eliminating both the 6RA and 3AW relays from the circuit.

The physical installation of the alternator is straight forward, although some light machine work is needed to make it work. Since the Lucas mount is different, a spacer needs to be fabricated to attach the alternator to the bracket. Simply cut one out of 3/8" pipe, available in any plumbing shop. The spacer is 1  1/4" long. Since the alternator is designed to mount on a 8mm bolt, you need to bore out the bushings to 3/8" to accommodate a larger bolt. Drive the bushing out of the alternator, don't risk iron filings getting into the core! Two bolts, rather than one, are used to secure the alternator to the mount.

Note: Spacer fabricated from brass plumbing pipe.

One of the big advantages of the Bosch alternator is ease of repair. The Voltage Regulator and brushes are packed together, and can be replaced with a screwdriver in five minutes. This style of alternator has been used on millions of cars since the 1970's, the regulators all interchange.


Bosch Regulator/Brush Pack. Typically lasts 100K miles.
I get 13.9 Volts with the A/C and headlights on at 600 RPM. At higher revs, I get 14.2 Volts consistently. It doesn't get better than this. 

Copyright © 2005 CoolCat Express Corp. All Rights Reserved. Linked vendors have no relationship to CoolCat Express. While every effort has been made to make these instructions accurate, CoolCat makes no warranty for the reader's own work.