MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Help needed with relays

My car, a 1970 BGT has two plastic box relays bolted to the inner o/s wing (obviously fitted by a PO)which during my extensive refurb were disconnected and although I tagged the various wires the guy that I have checking and replacing any 40yr old defective wiring cannot seem to find where the wire that runs from terminal 85 goes to?? every thing such as starter,headlights,horns all work fine. the relay that has the wire missing, (it was attached before dismantelling because my markings are still there),looks very much like a type A2 the wire in question was on terminal 85 but we can't find where it was supposed to go to.if any of you are familiar with a type A2 relay,and that looks to be like the one I have then perhaps you know the answer and could solve my problem. The reason I have a guy in to do it is because I am colour blind and my electrical knowledge is scant to say the least.
p j mayo

PJ, Assuming its a 4 pin relay, the standard wiring is for 85 and 86 to be the primary circuit. ie the circuit that takes the low current, (which activates the relay) -"making" the high current circuit across 30 and 87. Therefore 85 is either a + feed from the dash switch or connected to earth! What is 87 connected to? And connect 85 to the opposite! When you turn the dash switch you should hear the click of the relay operating.

As to what it does, what is connected to 87? If you find this is +ve, what is connected to 30? (one should be permanently + and the other to the "accessory") The permanent + could/should have a fuse in circuit.

Might it have been for spot lights? Standard 1970 did not AFAIK have relays for horn/headlights or anything else for that matter other that the starter relay, etc as standard. Not sure about a heated rear window as I have a roadster!HTH
Michael Beswick

As emailed:

With one exception the only relays the MGB had were the starter relay from 1970 on and the ignition relay from 1978 on. The exception was an additional one for GT heated rear windows on rubber bumper cars up to 77. As such any other relays will be a PO addition and so impossible to guess what they might be for. You have two choices - try and trace the wiring to switches or components, but if taped into the harness that makes it difficult, and they could also be no longer used with the 'other' ends from the relays chopped off and hidden. The other option is to disconnect them and see what no longer works!

I don't recognise the style of the green relay, but it looks to be a basic 'normally open' type i.e. the same as the standard relays. Wired as you describe the implication is that it gets 12v from the green/black on 1 to operate the relay to earth from the black on 2, then 12v from brown on 1 is extended through to the blue on 3 via the inline fuse. Green and black wires were used at various times for the wipers, fuel gauge sender, heated rear window and the electric cooling fan. Blue was used for headlights between the main lighting switch and the dip-switch. But PO wiring could have used any old colours that were to hand so you can't go by those. However if your car is a 78 or later UK spec then it's possible the PO added a relay to the cooling fan circuit, especially if they changed the sensor switch and/or the fan types.

The black one is a modern one with ISO terminal numbering and is of the 'changeover' type i.e. one terminal is switched between two others as the relay is operated and released, 87 being the normally open contact, 87a the normally closed, and 3 the common. The relay winding is on terminals 85 and 86, so if one of those has no wire then that relay isn't doing anything anyway. Green ( in light and 'normal' shades) and brown wires were originally used at various times for the dynamo, heater fan, reversing lights, wipers, alternator and indicator and hazard flashers. White (normally unfused ignition) on 30 would be switched between Blue on 87a with the relay released and the braided brown on 87 with the relay operated. I can't even guess what this might have been used for.

As it's now apparent it is a 70 BGT it could be the first relay is for the heated rear window, wisely taking the load off the ignition switch. As such the blue from the relay should be going to the original wire that runs the length of the car to the rear hatch. The schematics show this is *not* part of the rear harness, i.e. it was a dealer-added optional extra.
Paul Hunt 2010

Thanks fo your input Mike as far as the relays are concerned I did try turning the key and listening and feeling for them to click but here was nothing. I know that the 1970 only came with the normal rectangular metal starter relay and I do have that,and it working perfectly, so as mentioned before these were probably a PO fitment.There is no heated rear glass fitted but the wiring is in place so there may have been one at some stage before and as I plan to fit one at a later date I will leave that in place.I will come back with an update when my sparks comes and takes another look togeather with the info an tips kindly put forward. Also thanks to Paul Hunt for your email,really appreciate you taking the time to help another member.Cheers Peter


p j mayo

I'm just wondering if a PO installed a ballasted ignition & used the relay to short the resister out while cranking. Since everything works without this relay, it's a possibility. Barrie E
B Egerton

Further to my problem with the relays I wonder if this next picture gives any better clues to our electronic detectives within the membership. The black relay is the one in question that has a missing wire (terminal 85) seen in the picture with two white dots an as you can see is also connected to what is possibly something to do with ballasted ignition?? Paul you mentioned that if 85 or 86 is missing then the relay is not doing anything anyway, but the point I am trying to make is that both were originally wired,its just that I can't remember where the wire from 85 went to, if that can be solved then both relays will be back to how they were before I decided to try and be an electrician.Regards Peter


p j mayo

Hi Peter,

Barrie E seems to have picked it, a relay to short out the PO fitted ballast resistor(s).

From the picture it appears that the white wire goes to ground at the resistor mounting screw, so terminal 85 should go to the starter solenoid coil active, so that the relay is energized, when cranking.

Herb
Herb Adler

That makes things much clearer, and does indeed seem to be bypassing a couple of resistors. However for that there should be one side of the relay winding connected to the white/red at the starter relay and the other side of the relay winding to earth, one side of the resistors connected to the white at the fusebox and one relay contact, and the other side of the resistors should be connected to the coil +ve and the other relay contact.

But the coil has a red wire that goes direct to the white connection on the fusebox, so those resistors aren't in that circuit. You need to measure the primary (between the spades) of the coil to determine whether it needs a ballast or not. 3 ohm (conventional) or 2.4 ohm (Lucas sport which this coil may be) must not have a ballast. 1.5 ohm and 1.2 ohm must have one, and the ballast needs to be about the same resistance as the coil. For those two resistors to be a coil ballast each one would have to be about 0.75 ohms, which doesn't seem very likely. Also the white from the relay, plus another white, seems to be going to the resistor casing, which may be an earth if the resistors are mounted on a metal plate. From other information from Peter that white is connected to terminal 30 on the relay which is one side of the contacts, so the relay would appear to be feeding that earth out to the brownish braided wire when the relay is operated (87) and blue when it is released (87a). None of that makes sense with a ballast resistance bypass. Maybe you have got the wires on the wrong spades Peter, i.e. there should be two on the winding spades 85 and 86, and two on 30 and 87, the fact that it is a relay with a normally closed contact simply being that was all that was available at the time.

Did I say it makes things clearer? Until you trace where *all* the wires from the relay are going you are going to be none the wiser.
Paul Hunt 2010

Futher to my relay saga (if only Abingdon's run had been as long)I think that my picture may not have been as good as I'd hoped judging by Herb's veiw of it.There is no white wire going to ground,it just looks like it does.The wiring is as follows,white wire from loom to terminal 30,( I dont think that's the original colour,)looks like a new splice. light green/dark green from loom to terminal 86. short braided wire from bottom left resistor to terminal 87. Blue wire from bottom right resistor to terminal 87a and the tops of the two resistors are connected by a Blue loop,and (altogeather now) terminal 85 missing.From a colour blind non electrical person, Paul I really had to laugh when you said, ^That makes things much clearer^ And as far as the coil is concerned it is a Lucas DLB 105 which has a resistance of 3ohm and I believe is a sports coil. Here's hoping that this latest missive!! (indication of my age) gives some one out there the information to untangle this sorry mess.regards Peter
p j mayo

DLB105 *is* supposed to be a Sport coil, but as such should measure closer to to 2.4 ohms. Doesn't need a ballast in either case.

Cropping and expanding your photo the white from the relay definitely looks as if it goes to the casing of one of the resistors, even if another white from there goes into the harness. As it looks as if this second white would reach the relay perfectly well, why is it going to the resistor case unless it is picking up an earth, or something?

But in any case, there is no white wire in that position of any factory harness, so relay and wiring is non-standard, and this keeps going back to the same point which is that until you can tell us where the other ends of all these wires go to no one can tell you what the relay was for. And without a wire on 85 it is all doing nothing anyway, so you might just as well chop them off, tape them up, and forget all about them :o).

Oh, and it could be a PO dim-dip headlight system which some manufacturers used in the 80s, that used resistors in series with the headlights.


Paul Hunt 2010

This thread was discussed between 23/06/2010 and 28/06/2010

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.