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MG MGB Technical - Relays, round, black

Information required on the two round black relays located on the right hand side facing forward.

The relay forward of the fuse box is thought to be for the ignition circuits, whereas the relay aft of the fuse box is a mystery.

The car is a 1980 Fed LE.

Another mystery is that a silver starter relay fits into this wiring loom near the rear round relay, and there appears to be room in the circuit between the fuse box and the forward relay for another oblong relay type metal box which has the stamping

"Short S Top
12V 25Amp"

The S is an elongated S more like a winding path and has a ZEE shape streak of lightning coming off each side of the pathway, a bit like a bolt of lightning.

The silver starter relay is only a mystery because the parts catalog shows it as for the earlier models, whereas the other item is not shown anywhere.

Also failed to find the overdrive relay and wonder if this is missing, or intergrated into the O/D unit somwhere internally.

All this stuff was in a box, and not tagged by the guy who stripped the car down.
Jack

Jack, I think the forward relay may be for your electric fans. My son has a 79 MGB and the forward relay on his can controls the fan. Not sure about the others though.

Ken
Ken Waringa

Thanks Ken, however when we played around with the wiring yesterday, the forward or front relay clicked when the ignition key was turned to the START position.

The moment the ignition key was relaxed, clicking ceased.

Jack

Jack,

Refer to the excellent article about relays in the electrical help section of this site, its at the top of the MGB Technical page. What color wires do have at each of the terminals on the relays, i.e brown to #30, black to #85, red/white to #86, and white/brown to #87 equals the starter relay. This assumes no previous owner has modified the wire harness (which is a very big assumption).

Doug
D. Cook

The one marked Short Stop is a circuit breaker, and in your case it is rated for 25 amps. It is a self re-setting breaker, not a fuse. I don't have an 80 model car so I don't know what is to protect, but I am quite familiar with that particular device you describe.
Mark T

D. Cook refers to a section I cannot find.

May I request further directions, as to where it is on the site.

Have looked in the archive also, nothing found.

Mark T, thanks for that, you are the only person to give me a clue as to what it is, however any thoughts on what this device actually does.

Jack

The foward one is the ignition relay and the rear one the starter relay.
Frank

Jack,

The link to electrical help is in the small print just as you enter the MGB Technical list of threads on this BBS. Here is the short-cut: http://www.mgcars.org.uk/electrical/relays.html.

As Frank says, in the best of all possible worlds the ignition relay is forward and the starter aft. The circuit breaker works like a fuse to protect the wiring circuit its attached to, but I don't think it was standard equipment on the rubber bumper models, at least I haven't found one in my harness...yet.

The wire colors are an important part of the description because they tell us what part is being powered by the relay. The circuit breaker may have only two wires. If yours has brown going in and grey going out it protects the anti-run-on circuit, if it has brown and white inbound and green outbound it protects the cooling fan circuit.

Doug
D. Cook

Without the wiring diagram or some actual digging, I would not know what the breaker does (I have a 73B). There is an easy way to find out though- Unplug it. Start the car and then unplug the breaker. If the engine shuts down, then it protects the ignition circuit. If it stays running, plug it back in and turn on the headlights. Then go back and... well, you get the picture.

Mark

Mark T

I am fairly certain that the silver circuit breaker is for the electric fans. Mine went bad, intermetently, an caused me a lot of diagnostic time until I figured it out. I replaced it with a modern spade type fuse and have had no problems since.
Rich

Rich, the only possible concern I see with using a fuse is that it is non-resetting. I don't have electric fans on my 73, but if a fuse blew because of a road repairable short, you might be hurtin if you didn't have a spare one in the glove box.
Mark T

I'm glad I own a 67 :) No stinkin emissions equipment. Even a dummy like me can figure out what's wrong.
Mike MaGee

Mark T.

You are correct. But to show you what a "belt and suspenders" type of guy I am I not only carry spare fuses but I wired in a seperate switch to turn on the fans if the thermostatic switch fails. That way if the fans are not coming on automatically I can try turning them on manually. If that fails then I know the fuse is out. Has not happened yet, since I installed the switch, but I am prepared.
Rich

I too am working on the two round relays under my hood (78B)(ignition relay and starter solenoid relay).

Upon inspection, they are identical...same numbers on top and same connectors on bottom. Should they be?

Also, I would love to look at the existing wires to determine what is going where (by color), but the wires from both relays have been replaced as were the starter solenoid-to-alternator wires. PO had bypassed some stuff and made the problems worse.

I am in the process of replacing the wires and am finding the electrical help of this site VERY helpful. I am still disecting all this stuff and am getting a bit loopy as you will see by my next paragraph...

Come on, sing with me...The Positive Battery Cable is connected to the...Starter solenoid...The starter solenoid is connected to the...Starter solenoid relay...the starter solenoid relay is connected to the...???

Happy Days

Todd
Todd C

Todd,

Nice song! The relays are alike, and you can get replacements at your local Radio Shaq for about $4.

The wiring diagrams at the link above are in color and label the connections by number to the various switches, relays, and stabilizers. That link is one of the most helpfull benefits of this BBS.

Doug
D. Cook

Thanks Doug,

Honestly, I am still trying to decipher the diagrams accessed through the link above. There are 4 (I believe..cause I am at work...) wires connected to the starter solenoid relay (which would make it a super ISO relay), Brown (from battery which actually comes hot from the starter motor), Black (to ground)...and thats as far as I got. I will keep working on it as the weather is getting nice here.

Todd


Todd C

Rich,

Rock on, man, for you ARE seriously prepared! As for me, I'll keep the ol plastic fan blade (and I don't keep a spare!)

Mark
73 "Little Purple Car"
Mark T

Many thanks, that electrical link is excellent.

Further question concerns the wiring under the dash.

Just below the glove box, well slightly towards the LH side of it are two wires with spade connectors, one wire is green the other white/red.

Have seen a picture somewhere of a tubular device connecting these two wires in line, a bit cigar shaped like those things seen in radio circuits, but do not have such a device in the 'bits to go on' tray.

Any thoughts on what this might be.
Jack

Do you have both your turn signal and hazard flashers present and installed. They both live here as of course does the windshield wiper motor. You really need a wiring diagram for your car to see what goes where.
Frank

Yes Frank, those relays are in the system.

These wires clearly have something linking them, and I can see the device in a picture I obtained ages ago, however it is not clear or close enough to identify its nature, except that it is cigar shaped, and colored orangey or reddish, but that could be the color distortion of the camera w/ took the pic.
Jack

Jack,

The cigar-shaped part is a diode that allows current to flow in only one direction. The red and white circuit is one of those circuits that is used for double duty. It operates the strater relay and the dash lights. In start mode power is supplied to the starter relay to activate the relay switch via the red and white wires from the ignition switch, then in run mode power is supplied to the wires via the green wire to operate the dash lights, thus the dash lights work while starting and while running. The diode keeps the starter from cranking while the car is running.

Doug
D. Cook

On late model cars some fuses were replaced by circuit breakers as the fuses holders were tending to melt under high currents.

The diode lights the handbrake/brake failure warning light while cranking as a test to show it is working. However if the handbrake is on while cranking the light is on anyway! This is the only function you lose with the diode removed, they do occasionally go short-circuit, and when they do so the car keeps cranking until you drop the handbrake! Which probably explains why yours is missing.
Paul Hunt

Excellent explanations, thanks guys.

Only question now is how do I obtain a replacement.

Is there a name for this diode, Moss part # etc.

Also can Paul Hunt help with a rating or any figures which might enable a circuit breakerto be purchased.
Jack

The description in Haynes is 'Diode for brake warning circuit'. I can't find it listed in any of the usual sources although the MGOC lists a 'blocking diode' AAU5034 which in generic terms is what it is. I'm fairly sure that when I rewired a UK 1980 which has the same diode a new one came with the harness. Any old 100v 1amp diode will do, but you would need to mount it rigidly to a couple of spade connectors such that there is no physical stress on the wires of the diode itself.

From what others have said 25amp would seem to be the rating of the original breaker, which does fit between the rated and fusing currents of the standard fuse - 17amp and 35amp. Looks like Moss lists these at http://mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28984&SortOrder=28 and it is for the fans as previously mentioned.

Paul Hunt

Many thanks for those great responses.
Jack

This thread was discussed between 29/03/2004 and 05/04/2004

MG MGB Technical index

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