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MG MGB Technical - 67 Neg Grd Tachometer Test New Harness

67 BGT with a new harness purchased from British wiring that is stock except Negative Ground and Alternator. I know the tach wiring needs to be reversed as the tach still has the old white wire (cut) and looped on the tach post as the car was previously changed to negative ground.

My new harness has two white wires with female spade connectors ready for the do I determine which wire to which terminal? The ohm meter shows them common to each other so does it matter? Why the loop in the old harness when it was previously converted? Since the harness was ordered for negative ground, has the tach conversion issue changed?

I keep a fire extinguisher nearby when making morning toast to illuminate my electrical skill set. So I am very good at following instructions and studying the diagram once it is explained to me. Cheers and thanks!
R Stegs

The loop is the pickup, and is original. Which way the juice is going in the wire determines the magnetic field polarity the pickup sees - that's why the sense of the loop is changed when you change car polarity. Put a male connector on each end of the loop and hook it up to the two females. If it doesn't work, reverse the connections.

Fletcher R Millmore

Thanks FRM. Well I discovered the new harness actually has three white wires... two white wires common to each other the third is the white wire from the coil. I now believe the two common whites are one for the tach and the other for an overdrive switch if applicable.

So if I read the STOCK pos earth wiring diagram correctly... I have one fused circuit white wire to the non pickup side of the tach and one white coil wire looped through the pickup and ignition switch...

....then the change in earth requires the white coil wire to pass through the pickup opposite of stock.... not the swapping of the wires on the tach itself. Do I have the proper understanding of the circuit and the needed change.....that white wire from the coil needs to pass through the tach pickup looped opposite of stock?

Cheers and Thanks

R Stegs

The W connected to the coil is from the tach loop. Other W are straight of the key, no fuse. One of these is the other side of the loop. Key > loop > coil.
At that location, the other W is probably for the IGN warning light, other side of that is NY.

Fletcher R Millmore

There shouldn't be any fused white wires. Mk 2 cars should have a fused green wire to the tach which powers its electronics, before that it was an unfused white from the ignition switch. Two other white wires at the tach should be another one from the switch and one to the coil (probably a continuous loop in Mk1s with a tach). On a Mk2 other whites from the ignition switch are one to the warning light, and another to a double-bullet connector by the fusebox with one white for the fuel pump and another to the fusebox, daisy-chained from there to the OD switch.

When changing polarity you have to change the 12v and earth polarity inside the tach as well as reverse the direction of the white wire through the tach pickup. If changing the harness on a car that has already been converted i.e. the tach electronics have already been converted, it's simply a matter of trying the wire from the coil and a wire from the switch connected first one way then the other to the two short bulleted wires in the tach pickup, if that is what you have. Reversing these wires won't cause a problem, the tach simply won't work when the wrong way and should when it is the right way. Only if you have the continuous loop of white from the harness would you need to try routing it first one way then the other through the external pickup.
Paul Hunt 2010

Ralph -
This is a little confusing, since the original harness before anyone screwed with it had the tach loop as a continuous W passing through the pickup block, so you had no ambiguity.

As Paul said, there should be one W with a spade connection to power the tach (I missed that, forgot the early tach power was unfused).

Also as Paul said, W wires are NOT fused, so be careful with them! I have taken to fusing the N feed to the IGN switch. Lots of people, and I think Paul is one, don't seem to like extra fuses, but all modern cars have everything fused with no problems, and I HATE burnt wires!

You have the tach with loop cut off, from the earlier conversion, and you seem to have a harness with two stub W leads, one going to coil. Later cars used two stub leads with bullets, which plugged into the tach (or short pigtails sticking out of the tach) for the pickup. Your harness should have bullets on these two W leads, and you should put mating bullet connectors/sleeves on the pickup loop stubs. Your tach must have had the internal work done, IF it worked before the harness change. If you don't know if it worked, then you gottaproblem.

The warning light W & NY should come out of the harness together; some of these had connectors, and sometimes the lamp socket was part of the harness - I cannot recall what this car had originally.

Mine (66)is so butchered I couldn't use it as a sample! It has a later ('77) version tach stuffed in the early housing, and a rat's nest of blue and yellow wires, lamp cord, cheap toggle switches, etc. My warning light isn't there at all, and the car has a Fiesta?? alternator installed just like the rest of the wiring.

Fletcher R Millmore

"Lots of people, and I think Paul is one, don't seem to like extra fuses"

Not willy-nilly, but I have fused the fuel pump and OD on both my cars. Both of mine and two friends have evidence of the pump having shorted in the past damaging the harnesses.

All the cars I have looked at have had the ignition warning light bulb-holder as part of the harness, same with the main beam and gauge illumination bulbs. The ignition light is a one-off in that both wires are required and both must be isolated from any earth, like the body of the case they are pushed into. The others, on earlier cars at least, used the gauge body for their earth (the earth wire being under a thumbwheel securing the gauge in the dash), and inadvertantly using one of these for the ignition warning light could cause damage to wiring or other components.
Paul Hunt 2010

Paul -
"....not willy-nilly"
Hard to figure where that starts/stops! Somebody on here fried a tach and all the W leads a few months ago, An IGN feed fuse would have prevented that. I agree 100% on the need for a pump fuse, lots of harnesses cooked by the pump feed, very vulnerable in its trip aft. Same for the OD down tunnel.

I am starting to love lots of fuses, since not only do they prevent damage, you only lose small bits of circuit, and they make diagnosis really easy.

On the various bulb holders, agreed they are most commonly part of the OE harness, but I have some with bulleted pigtails, no idea what I got them off. And I don't know if repro harnesses have them included - Ralph?

Fletcher R Millmore

Thanks guys. The brand new harness is of good quality and seems complete EXCEPT the gauge lighting..... the harness only comes with the new plastic holders for the high beam indicator in the speedo housing and the ignition red light in the tach housing.
It does NOT include the fuel gauge light, the tach light, the speedo light nor the oil/temp gauge light. Basically all the red/white bulb holder for these gauges.....seems stupid to pay lots of hard earned dollars for a complete new harness from BRITISH WIRING only to discover that I have to fabricate those lights with separate new parts or try and cobble up the 40 year old stuff.
The harness does have the correct red/white feed for the lights but why they leave a large majority of the dash work to the purchaser is beyond me when it is sold as complete. I am lucky enough to have a couple of the original bulb holders but the idea of fabricating into a new "complete" harness with old stuff or buying additional components does not make me feel like I got quality and value as expected.
I have learned that my MG British car expectations can not be the same as my old Chevy when dealing with things like the wiring... helps keeps me sane!
R Stegs

Ralph -
The IGN and headlamp high beam indicators are the tricky ones. Some cars of the time had a separate subharness RW daisy chain for instrument lights; I don't recall if this was one of them, and as stated, mine is too chopped up to be of any use. Several guys on here have cars of this 65-67 sort, David Steverson is into his harness right now, and Bruce Cunha ought to know.

"In some cases, this (wiring harness) excludes wiring such as that for dash lights, dimmer switches, etc." Moss catalogue

You might also call British Wiring and ask them about it; entirely possible they forgot to put the RW daisy chain in place when they built it, if it is in fact an integral piece of the harness. Always good to begin complaints with those responsible!

Fletcher R Millmore

Thanks FRM,
Oh be certain that indeed I called British Wiring long before expressing any public unhappiness about the incomplete presentation of a new harness. I was promised a call back with resolution of some sort the next day... never heard anything days later so I left a couple more messages hoping to learn why the red white lamp circuit is not built in the new harness. Still waiting to speak with them as they were going to check with England I was told. Must be technically challenging to manufacture that aspect of the harness with any uniform application for those ealy years. Buyers just need to plan for a bit of additional wiring and fabrication. Ralph
R Stegs

Can't see why they would leave it out, all cars have them after all, not like some of the detailed differences in later models. Don't blame England per se, it all depends where your supplier got them from, I'm sure crap is obtainable here as it is anywhere. The 1980 model I bought had them (albeit with different bulb holders).
Paul Hunt 2010

This thread was discussed between 15/03/2010 and 19/03/2010

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