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MG MGB Technical - Relays

Relays For Dummies---That Be Me. I have read several post about relays, but without laymans terms, I'm still not quite sure what I'm doing. Rayex Relay LD1A-12F 40amp/14VDC with 85/86/87&30 (also marked 87a)pins. Please identify pins by + from battery or power source /// - to earth.(negative earch car) Now we have two left, which goes to where concerning switch and what you are to power
-up? Once I understand the relay, my plan is to relay/fuse lights,heater,stereo, on 65-B.
Thanks so much,
Steve C.
STEVEN CROWDER

Here's a basic relay schematic:
http://www.classictruckshop.com/clubs/earlyburbs/projects/bosch/foglite.jpg
Kemper

Thanks Kemper, that pretty much clears it up for me.
STEVEN CROWDER

A single fuse like that is OK for fog/driving lights, but for headlights you will need two relays, and I would strongly recommend one fuse per filament or you could lose the lot at a critical moment. If you use a main feeder fuse as well then it should be at least four times the rating of the filament fuse, which is fusible link territory.

BTW, you don't need a relay with the 87a connection for the circuits you mention, that is a 'normally closed' contact that opens when the relay operates. Most relays used for lighting circuits will only have the other four terminals.

The heater fan is already fused, as should be the stereo. Neither should need relays, unless the stereo is a monster. Parking lights shouldn't need relays either, although on a 65 they aren't fused. The best way to do that is with in-line fuses at the bullet connectors where the two red wires from the main harness join the single red in the rear harness in the mass of connectors by the fusebox. Determine which of the two reds from the main harness is the feed (disconnect them and measure the voltage on them) then use that in the 4-way with the two in-lines plugged into it, one plugged into the rear harness and the other into the other red wire from the main harness which is for the rears. That is how they were fused in 68 and 69 anyway, i.e. one fuse per end, before the 4-fuse box in 1970 which fused each *side*, instead of each end.
Paul Hunt 2010

Paul,
Will do and thanks. One thing you mention is the heater motor needs no relay and mine has none. Wouldn't it benefit from one?
Steve C.
STEVEN CROWDER

Steve
Have you looked at this info?
http://www.danielsternlighting.com/tech/relays/relays.html

Kent
DK McNeill

Thanks Kent, very good information for my arsonal. Starting improvements this weekend.
Steve C.
Steve Crowder

Steve - the only time the heater fan might benefit from a relay is if there are bad connections in the 12v and earth circuit feeding it, and personally I would rather correct those than fit a relay. If the bad connection are in the brown, white or green circuits then *everything* will benefit from correcting them, whereas only the heater fan would benefit from a relay. Unlike uprated headlights the heater fan doesn't draw enough current to warrant a relay.

A as comparison something that really does benefit from a relay is the GT heated rear screen, which takes a helluva lot more current than does the heater motor. Mine improved from 7v at the element to 10v with a relay in the engine compartment, I've known people fit the relay by the battery and get even more voltage. This makes a significant improvement to the performance of the screen heater, as well as taking a big load off the green circuit which helps things like the indicators. The factory did actually fit a relay in 4-cylinder RB cars up to 76, but penny-pinched it out again after that, and the V8 never had one. Although the V8 cooling fans do have a relay, it takes the power for them from the green circuit, which is mad! These also take a lot more current than the heater fan.
Paul Hunt 2010

This thread was discussed between 12/07/2010 and 14/07/2010

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