MGExp

MGB & GT Forum

Alternator small large small vs. large large small

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

jdevol Avatar
jdevol Silver Member J Devol
Swansea, MA, USA   USA
I have an alternator for a 74.5 mgb GT that has the small large small plug configuration. The harness I used in rebuilding my 74 GT was from a 72 and it has the large large small plug in the white plastic housing. I have a matching older alternator that works fine.

Can I plug the Early harness plug into the later model alternator? It looks like it will fit.

The newer alternator is a higher output unit and I want to use it so I can add driving lights.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

Attachments:
IMG_3289.JPG    38.4 KB
IMG_3289.JPG

IMG_3290.JPG    54.8 KB
Sign In or Register to view this photo
ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
When I converted from dynamo to alternator I simply cut the existing terminals off and used individual crimp connectors. Yellow for the large connector / wire and red for the other.

Herb



Questions about prostate cancer? Click here to join the discussion

chris Avatar
chris Platinum Member Chris Roop
Pendleton, OR, USA   USA
As long as you have the right wiring color going to the correct spade, you're fine.


Member Services:
Used English car parts.
Be Coming Avatar
Be Coming Kelvin Dodd
So. Calif., USA   USA
The standard replacement alternators for the earlier MGB were the A1611 units. These had the small, big, big terminals and for many years the Lucas units came with a CYB400 (540-280) plug kit because so many were used to upgrade the earlier 68-69 remote regulator and later 5 wire alternators.

For some unknown reason, the 1974 1/2 - 78 cars came with a unique regulator and required a special plug. The 1979-80 cars were fitted with the CYB400 small, big, big plug for the higher output A1812 alternators.

Since this later alternator is now the most available replacement. When I build a car, no matter what year I use the CYB400 (540-280) plug kit.

This is also what the wiring harness suppliers have done, since the original plug for the 1974 1/2 - 78 design alternator is not being manufactured.

Changing the plug over is very simple, since both the large terminals are hot and the small terminal is for the smaller brown/yellow wire.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, Alberta, Canada   CAN
1971 MG MGB
Hold it! spinning smiley sticking its tongue out First, I want to be sure I understand the problem.

The small-large-small configuration is intended to be connected as follows:

small1 - thin brown wire. This was used to supply a voltage reference to the alternator. Some alternators used it, some did not (even if they had the male spade connector for it)

large - power output via thick brown wire

small2 - brown/yellow wire. This connection went to the dash lamp and supplied initial power to the field coil of the alternator

------
I think you want to use a large-large-small harness with a large-large-small alternator - if so, that will work fine.

On the other hand, if you want to use a small-large-small alternator with a large-large-small harness, here is the information you require.

The large-large-small configuration eliminated the thin brown wire, so if you want to use a small-large-small alternator with a large-large-small harness, you will need to run a wire (ideally, brown) to the starter solenoid. This will serve as a voltage reference for the alternator. Strictly speaking, you might not need the wire. Only by removing the cover and examining which voltage regulator is in the alternator can you be certain if it is needed - but it is simpler to run a small wire to the solenoid than to figure this out.

So, the brown/yellow wire on the harness goes to one of the small terminals, the thin brown wire (that you will run to the solenoid) goes to the other. Which goes to which? The easiest way to tell is to pick one configuration and hook it up that way. If the dash lamp glows when you turn the key to the ignition position, you got it right. If not, reverse the two small wires and that should correct the problem. There won't be any damage by interchanging them (but the alternator may not work until you correct the connection).

Of course, the large spades go to the thick brown wire. The factory used two connectors to reduce voltage drop.

Did that help or did I misunderstand the problem?



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

GMs MG Avatar
GMs MG Gold Member Gary Mackin
Sheridan, Arkansas, USA   USA
1976 MG MGB "Blanche"
Terry,

Isn't the indicator light circuit (brown/yellow) always on the small connector nearest the outside edge of the alternator?

In reply to # 3608999 by ingoldsb Hold it! spinning smiley sticking its tongue out First, I want to be sure I understand the problem.

The small-large-small configuration is intended to be connected as follows:

small1 - thin brown wire. This was used to supply a voltage reference to the alternator. Some alternators used it, some did not (even if they had the male spade connector for it)

large - power output via thick brown wire

small2 - brown/yellow wire. This connection went to the dash lamp and supplied initial power to the field coil of the alternator

------
I think you want to use a large-large-small harness with a large-large-small alternator - if so, that will work fine.

On the other hand, if you want to use a small-large-small alternator with a large-large-small harness, here is the information you require.

The large-large-small configuration eliminated the thin brown wire, so if you want to use a small-large-small alternator with a large-large-small harness, you will need to run a wire (ideally, brown) to the starter solenoid. This will serve as a voltage reference for the alternator. Strictly speaking, you might not need the wire. Only by removing the cover and examining which voltage regulator is in the alternator can you be certain if it is needed - but it is simpler to run a small wire to the solenoid than to figure this out.

So, the brown/yellow wire on the harness goes to one of the small terminals, the thin brown wire (that you will run to the solenoid) goes to the other. Which goes to which? The easiest way to tell is to pick one configuration and hook it up that way. If the dash lamp glows when you turn the key to the ignition position, you got it right. If not, reverse the two small wires and that should correct the problem. There won't be any damage by interchanging them (but the alternator may not work until you correct the connection).

Of course, the large spades go to the thick brown wire. The factory used two connectors to reduce voltage drop.

Did that help or did I misunderstand the problem?



Gary

GMsMG



Edited 1 time(s). Last edit at 2017-10-12 07:27 PM by GMs MG.

jdevol Avatar
jdevol Silver Member J Devol
Swansea, MA, USA   USA
I looked at a harness with a black small large small configuration and the wires are the same color and size as the Large large small white connector. I looked at another harness with a white connector and it too has small brown, thick brown, and small yellow brown wires - large, large, small.

On the black connector the small brown wire goes to a small terminal and on white connectors it goes to a large terminal. I suppose its going to work. Going to swap and hope the harness does not melt...:winking smiley

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

To add your reply, or post your own questions

Members Sign In   or   Create an Account

Registration is FREE and takes less than a minute!


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster