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MG MGB Technical - Wiring Question

I'm in the process of installing a complete new harness in my 77B. Mine is a hodgepodge of old and new, with a 72 engine and Weber Carb put in by a PO. Most every connection has worked out fine but there are a couple under the hood that I want to be sure of:

1) The car has a 25D4 dizzy (model # 41288) There is only one spade to connect a wire to, while the harness has three wires with a three-pin connector. (White, White/Black, and White/Blue) The PO had modified the harness and had only the White/Black wire connected to the distributor. Can I simply remove the 3-pin connector, add a spade connector to teh W/B wire and ignore the other two - or is it more complicated than that?

2) The emissions control systems are all gone, as is the anti run-on valve. Can I just leave those wires hanging since I have nothing to connect them to?

Finally, before I reconnect teh battery - are there some simple checks I can perform to be sure things are right? (I know to check with a voltmeter for 12v before connecting the ground strap) My meltdown likely came from the ignition circuit - since all White wires under the dash were melted and burned. How can I be sure I won't burn out my new harness the first time I try to fire her up?

Thanks!

PS: Thanks to all who recommeded BritishWiring.com. Excellent service and product! The job has been considerably easier than I expected.
Scott

Scott. I believe the basic 18V engine, used from Aut 71 to Oct 80 is almost exactly the same throughout the years. I do believe the 77 on cars used a different front plate for the motor mount attachement. As far as I know, as long as the front plate has been changed out, the use of an earlier engine should not be a problem, nor should it affect the wiring.

As to the dizzy, this does not matter much. Your car had the Lucas 45DE4, or Opus system, originally. That was not a very good system and was commonly replaced with the later 45DM4, or CEI system, or the older 45D4 and 25D4 points type distributors. To operate your distributor, you need a single wire going from the negative terminal of the coil to the distributor. There should be an second wire attached to the negative terminal going to the tach. This is the White with Black stripe wire.

My wiring diagram (Bentley manual, NA 10, 1978 and later--seems to fit the wiring on my daughter's 77 better than the NA 8 wiring diagram, 1976 with a 1977 supplement), shows two White with Light Green wires going to the positive terminal of the coil. One comes off the second, or uppermost, starter spade terminal while the second connects to the white wire circuit. This is for the 6V coils as originally equipped. Some cars, expecially when retrofitted with points type distributors, have also had a 12V coil fitted and the wiring altered. Thus, I would check to see what type of coil you have before making final determination of how it should be hooked up.

Loose hanging wires. Never leave them hanging loose, particularly if they have current to them. Tape up each of the ends, then tape or cable time them out of the way. makes for a neater installation and you do not have hot wires grounding out the system and causing fires.

Initial hook up of the battery. You might try a test light between the detached ground cable and the negative battery terminal with the alternator un-plugged and all the fuses removed. I do not believe you should seen any current flow under that circumstance. Then, you can have an assistant install the fuses and plug in the alternator to see what happens with the light. You have some "always hot" circuits off the bottom fuse in the fuse box. These are off the brown wire circuits which come from the main terminal of the starter to the fuse box and from the main terminal of the starter to the alternator. There is also an always hot brown wire going from the input (front) side of the fuse box, bottom contact, to the ignition relay. There is also a brown wire circuit from the main starter terminal to the starter relay. Big thing to remember is that these are always hot circuits. Other circuits are only hot when the fuses are installed and/or the ignition switch is turned on.

Paul Hunt will probably have better start up procedures than I have. This is an area I seldom encounter (complete wiring harness change out) and I tend to get a big copy of the wiring diagram and work things out as I go. Les
Les Bengtson

The white and white/blue are power feeds for the 45DE4 'Opus' distributor. As these are supplied with unfused 12v from the ignition they must be securely insulated and taped up as Les says, as should all unused connectors. I assume this is a 77 harness. The 72 engine used a 12v coil but the 77 a 6v. You could use either, as the white/light green supplies a ballasted 12v to the 6v coil but the white you mention should supply an unballasted 12v. The 6v coil with ballast, with the additional white/light-green connection to the special contact on the solenoid if you have one, will give more reliable starting. However if your solenoid doesn't have that contact then you will better off with the 12v coil using the white wire.

If you don't use the anti-runon valve then you will almost certainly, with otherwise standard 77 electrics, not be able to turn the engine off with the key. The fix for this is either to locate where the white from the ignition warning light joins the main white, usually near the ignition switch connector, and move it to the white/brown on the output of the ignition relay, or alternatively power the coil from the white at the relay rather than the white/brown.

I've used a couple of headlight bulbs in parallel when first powering up a new loom, that has enabled me to check the unfused circuits, one at a time, with no risk of loom damage is something *past* a switch should be shorting out.
Paul Hunt

This thread was discussed on 02/05/2003

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