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MG MGB Technical - Ignition relay gets warm

1977 MGB. Restoration project so I don't know that much about it.

When the white and brown wire are connected to the ignition relay it warms and the voltage drops to 2.5 volts. If only one or the other is connected no problem.

I was wiring a radiator fan positive lead to the starter. I got an arc when the wrench touched the frame while tightening the solenoid nut... too lazy to disconnect the battery, DCO.

I have removed the fan wiring but the problem continues. I tried a new relay, same issue.

I'll clean all the starter connections ASAP but I'd appreciate any sugggestions.

Thanks.
Brian Denis

I've localized the problem to the green wire 1-to-3 connector behind the glovebox. Fiddling with the wires gets 12 volts to that point rather than 2-3.

So I'll be cleaning those connectors.... again... and looking for a short.

Again, any ideas are appreciated.
Brian Denis

Brian. Is the relay connected up properly? The brown wire is always hot and comes from the lower fuse, front side. The white wire comes from the ignition switch and should only have power flowing with the ignition switch turned on. The back wire is the ground and when the white wire circuit is engergised, allows the relay to function. The brown wire is the power input and the white/brown wire is the power output (when the relay is engergised) providing power to the third fuse down, front side. (Front is power in, back is power through the fuses.)

If you have the wiring diagram in your workshop manual copied, then expanded 300%, it will make it easier to read.

My copy of the expanded diagram shows the white wire going to the terminal marked W1 and the black wire going to the terminal marked W2. The brown wire goes to a terminal marked C1 and the white/brown wire goes to the terminal marked C2.

If you have the original Lucas flasher, there is a single copper colored terminal at the side nearest the engine and there are three silver colored terminals, at right angles to the copper one. The instruction sheet notes that the W1 terminal (white wire) goes to the terminal marked 86--the silver terminal furthest forwards. The black wire goes to terminal 86, the furthest rearwards silver terminal. The brown wire goes to the sliver terminal closest to the fender, marked 30/51. The remaining wire, brown/white, goes to the copper colored terminal marked 87.

In addition to the power wire (brown/white) going from the ignition relay to the front of the fuse box, there are two additional brown/white wires carrying power from the front of the fuse box (unfused power). One goes to the fan thermocouple while the other goes over to the distributor and the resistor for the Lucas 45DE4 distributor. If anything was grounded out in those two circuits, it would cause an excessive drain on the ignition relay.

Hope this helps. Les
Les Bengtson

Relays will usually get warm in use. However there are two types of the shiny rectangular can Lucas 6RA relays - one for intermittent use and one for continuous. The intermittent use relay is used for the starter, takes a higher current to give a lower contact resistance, and will burn out if used continuously. Unfortunately I don't have a record of which reference number is correct for which function. The black cylindrical relay is for the ignition, but again there may be different relays for different applications on other vehicles.

It could also get warm if something is drawing too much current i.e. a short, which will also cause a voltage drop on the 'other' side of the fault to the voltage supply. Whilst this would normally be expected to blow the fuse if the short were on a green circuit, you can also have bad connections i.e. in the fusebox which can limit the current to below fusing level, but still be drawing too much, *and* causing a volt-drop into the bargain. Measuring the voltage on the brown - ignition relay - white/brown - fuse - green etc. circuits will indicate where the bad connection is, by showing 12v on the supply side and a much lower voltage on the load side. Once you have found and fixed the bad connections you may then find the green circuit fuse blows, so you will then be off hunting for shorts! Temporarily replacing the fuse with a high-wattage 12v bulb, like an old headlamp bulb helps in this. While the short is present the bulb will glow at full brilliance, dimply or not at all when it is not. When the bulb is in the fusebox some higher current circuits will not work as they should.
Paul Hunt

PS. A high contact resistance in the relay will also cause it to get warm and give a volt-drop.
Paul Hunt

"If you have the wiring diagram in your workshop manual copied, then expanded 300%, it will make it easier to read"

No need to do that, go to http://www.advanceautowire.com/mgb.pdf and download a set of redrawn schematics in color. These are a lot easier to read than the factory diagrams. If you wish, you can take the file to Kinkos and have them printed as larg as you like.
Dan Masters

Dan. Have you checked these against the factory diagrams? There is one "Late US Market MGB", from Haynes. The Bentley manual has two wiring diagrams for the 1975 model, two wiring diagrams for the 1976 model, a 1977 supplement to the 1976 wiring diagrams and a 1978 and later diagram. My daughter's 77 does not work with either 76 wiring diagram, even using the 77 supplement. The 78 and later wiring diagram seems to be the one which works best on her car.

The wiring diagram on the website would be, if it is correct, an excellent thing to have expanded and laminated. But, until I can verify exactly which cars it works for, I am not going to have this done, nor am I going to recommend it. Les
Les Bengtson

I'm not sure I understand what you're asking me, Les. Each diagram clearly identifies the source of the information, ie, Bentley diagram 10, Haynes late UK market, etc. Are you asking me if I have verified that Bentley diagram 10 accurately reflects the wiring configuration of a US market 70/71 MGB? The answer to that is no, of course not. That would take many, many more hours and a physical walk down of each of the many MGB variations produced over the course of the 18 years of MGB production to be sure.

Are you asking me if these diagrams accurately reflect the data supplied in the source material? In that case, the answer is yes, as much as I possibly can. I can't guarantee 100.00% accuracy, but I did spend an enormous amount of time making sure they were as accurate as I could make them. If there are any errors, I will correct them as I learn of them.

You recommended that Brian take the diagram from his workshop manual (which is most likely either the Bentley or the Haynes) and have it enlarged, even though it might not, as you found out with your daughter's 77, match his car, yet you warn against using the diagrams from the web site for fear that a particular diagram might not match a particular car. With your daughter's car, you learned from experience which of the various diagrams available most closely matched her car, which is what I assume most people have done with the Bentley or the Haynes manuals. Could you not find an official factory manual to match a 77 model? I'm assuming the Bentley diagrams are the real factory diagrams, is this not so? If not, where does one find the real factory diagrams?

The differences between my diagrams and the source diagrams are:

1. I have corrected the obvious mistakes, including the one diagram where a power lead was shown connected directly to ground,

2. I have redrawn them to make them easier to read (at least I believe they're easier to read - you may be of a different opinion),

3. I have drawn them in the proper colors for the wiring,

4. I have drawn them on 11 x 17 paper so they can be easily read without squinting, important for old folks like me.

As for accurately matching any particular car, these are as good as there are. I am working with some knowledgeable folks to try to arrive at a better match, but until then, these will still be a major improvement over what is available now. If you prefer to work your way through the various factory diagrams, more power to you, but I think you are doing MGB owners a disservice to caution against using my diagrams for the reason you gave. If you think I plain screwed up and got the diagrams wrong, that would be a valid reason for warning against using them, but if you've confirmed them to be wrong, let me know and I'll correct them. Whether or not any one uses these makes no difference to me, I'll not make a nickel from them. I just got tired of squinting through the originals and redrew them for my own use. I assumed others were tired also, so I made these a gift to the MGB community. Like any gift, they may be accepted or rejected.
Dan Masters

Dan. Did not even look to see whom the author of these items was. I like the way the various items have been identified by a written description rather than a number which requires you to constantly refer back to a seperate sheet to decode.

However, as I said, I only find a single wiring diagram for the "Late US Market MGB". So, what is at question here is "which late model does this diagram represent"? There seem to be five wiring diagrams and a supplement associated with the model years 1975-80. I know, from personal observation, that there are some significant differences between the various diagrams. Thus, for this diagram to be of use to me, I need to know what year(s) it purports to represent.

Les
Les Bengtson

Les,

If you (or someone else) can scan those 5 diagrams and send them to me, I'll convert them and add them to the file.
Dan Masters

Dan -

Very nicely done. I appreciate that you took the time to do that work - I for one, will use it as a reference, or at least an additional source of info! I MUCH prefer looking at it on a nice big monitor rather than having to put on my reading glasses to decipher all the tiny stuff in the manuals.

Roger
'69 Roadster
Roger Hotelling

Dan,
And a big thanks from me, I am sorting out some electrical problems at the moment and found it impossible to follow the Haynes diagram. Your diagrams make it look positively simple by comparison,also in the Haynes book the key to the diagram is not the one opposite which is a real pain.

Thanks for sharing this upgraded information.

Kevin Jackson

75GT conversion Rover 3.5
K.R Jackson

Dan. As soon as my wife returns for her annual tour of active duty, another couple of weeks, I will get her to have the pages scanned and will send them to you. As the others have noted, your drawings are much easier to use than the factory ones. Hence, if you are willing to do the heavy work, I am more than willing to make minor assistance. Many thanks. Les
Les Bengtson

This thread was discussed between 15/05/2005 and 18/05/2005

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