View Full Version : Old Jeep Generators; An Introduction

08-24-2018, 09:22 AM
A recurring subject on the Kaiser-Willys Forum involves old Jeep generators and voltage regulators. For the first half-century of the automobile’s history, the heart of the electrical system was the generator (a DC machine) and the regulator that controlled its output. But, for about the last 60 or so years the generator has faded into history as the alternator (an AC machine) has replaced it as the electrical power source for vehicle systems.

The same questions keep showing up and after discussing things with a couple of the other moderators, I decided to try to put together a simple paper that can help folks who are struggling with their old Jeep electrical system. We’re going to deal with the 6-volt and 12-volt generator systems that were controlled by the three-relay voltage regulators. We’re not going to try to figure out 6-volt to 12-volt conversions; just about every one of those becomes a special project in some way or another.

The Generator

I had planned on writing a section describing Jeep generators, but as I researched a number of sources, I discovered that someone had already done a much better job of it that I ever could.

For one of the best discussions of Jeep generators, here is a link that will tell you 90% of everything you would ever need to know:


My thanks to the CJ3 guys for taking the time to put all of this into one place, in a manner that lets us understand it.

In addition to the summary information that I will show below, this site gives you information of how to figure out if you have a Type A or Type B generator and how to properly polarize either type if you are resurrecting an ancient barn find that you have no history for.

Type A and Type B Generators

The biggest take-away from this CJ3 article is that there are two types of generator used on the old Jeeps. The original MB and GPW used the Type B generator. As a summary, in a Type B generator, the field coils are connected through the regulator back to the battery. In a Type A generator, the Field coils are connected back to ground through the regulator.

A Diagram of Type A and Type B Generators goes here. It has to be attached separately because of Forum format protocols. Look at the attachment, TypeA_TypeB

In a Type B generator, if you disconnect the lead from the Field terminal of the generator, then tie the field terminal of the generator to the positive terminal of the battery, the generator will, if it is operating properly, show a full charge on the ammeter. (Of course, the engine has to be spinning the generator – clockwise when viewed from the pulley end). Military Jeeps used Type B generators – the 24-volt generator used on the M38A1 is a Type B unit.

For a Type A generator, if you disconnect the lead from the Field terminal of the generator, then tie the Field terminal of the generator to ground, a properly operating generator will show full charge on the vehicle ammeter. Civilian Jeeps used Type A generators, as described in the CJ3 link.

You do need to pay attention to which type you have. I led a gentleman astray (I just plain old screwed up) and had him polarize a 12-volt Type A generator using a Type B method. It gave him a positive ground generator! No permanent harm was done (I believe); the solution is to just try again, do it correctly and go on with life. Big, heavy rotating machines are a lot more forgiving of mistakes than solid-state electronic controls.

TM 9 - 1825B

Now, I mentioned that the CJ3 article would give you 90% of what you would ever need to know about generators. If you want to know everything, there is an encyclopedia of old vehicle electrical knowledge out there.

In January 1944, the Army put their entire vehicular electrical system component test, overhaul and repair information into one book, a War Department Technical Manual - TM 9 - 1825B; Ordnance Maintenance, Electrical Equipment (Auto-Lite). This book does not concern itself with the particular vehicle a component was used on, just the item itself.

Now, this book does not address components used in newer vehicles such as the M38, M38A1, or M151, but there is enough information here to help you understand the newer stuff. My looking indicates that a later version, circa 1955, exists (TM 9-1825C) – but I’ve not seen or found one. Use your Internet browser to find TM 9-1825B. It appears to be available in a number of formats and you may prefer to download it in a format that suits your computer configuration.