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MG MGB Technical - Early B (Electric Tach) Harness Install Issue

This is a tough question, but I am wondering if one of you might have some insight into this situation.

I recently installed a Moss harness (356-370) into my 1965 (with electronic tach). The harness was for a very early car, up to #48XXX and although my car is #56XXX this harness style (the cut of the harness and the wire routing) matched my old harness perfectly. I had previously ordered the next harness up (356-390) (#48XXX to 138XXX), but the routing of the wires did not match, plus it included a lot of extra wires (such as for backup lights, etc.) that I did not require.

The install was successful except for one issue with the electric tach hookup. On the wiring diagram for the mechanical tach cars absolutely no wires (besides the ignition lamp wires and the panel light for the gauge) are routed to the tach. Makes senses, since its mechanical. On the diagram for the 'late' early cars (with electric tach), there are two white wires going into the tach- one from the (on a positive ground car) SW side of the coil running through the harness and looping behind the tach and plugged into the ignition switch, making a complete circuit. A second white wire runs from the 3 way connector of white wires by the washer bottle and actually plugs into the tach itself, presumably to give to the tach power.

The harness I installed has such a wire to plug into the tach itself, but seems to be missing the wire that loops behind the tach. I am wondering if someone built a better mousetrap here and all I need to do is loop this one white wire behind the tach and then plug it into the back of the tach- since the wire originates from the connectors, it receives power from the coil wire, and since the other two white wires from the ignition switch also route to this connector, it would seem I have a complete circuit. A quick check with the multi-meter confirms this- the coil wire indeed runs to the end of this white wire (or at least powers it).

This is a tough question because the harness basically follows the schematic of a mechanical tach car, but includes this white wire to power the back of the tach, a la an electric tach car, but does not include the white ‘loop’ wire. As I said, I am thinking that the white power wire can also be used as the 'loop wire' for the tach to read the impulse signals from the coil. As an alternative, maybe I can re-use my old 'loop' wire and run it to the 3-way connector near the washer bottle- in theory this should complete the circuit and allow the tach to read the impulse signals from the coil to the ignition switch.

It's a confusing scenario, so I can explain more as we go. Any comments are welcome. I am hoping the board can pull through on this one.

Matthew Mugherini

The tach will only work if the wire from the ignition switch to the coil goes through the pickup. Looping the power wire through the pickup will not work.

Positive ground cars *should* have a continuous loop of white coming out of the harness and going back in, this is the tach pickup loop which should be fed through the external tach pickup in a particular way. The white that terminates in a spade supplies the electronics.

On later +ve ground cars the white power feed (unfused ignition) was replaced by a green (fused ignition) feed also terminating in a spade.

About the time the change to negative ground was made the continuous loop became two separate whites, one terminated in a male bullet and one a female. the tach was changes to put the pickup inside the case, permanently wired, and brought out to two matching bullets on the back of the case.

If you only have two whites, and one definitely goes to the coil and the other the ignition switch, then these must be the pickup wires, and if they are separate wires at the tach I'd expect to find a green in the vicinity to power the electronics. What type of connectors are on the ends of the two whites?

If they hadn't fused the tach electronics then they *could* have connected the wire from the ignition switch to both the pickup and the electronics, using the other white from the other side of the pickup to the coil. If you *don't* have a green then you could do this anyway, although personally I'd be happy having the electronics run off a green fused supply and not the white.
Paul Hunt 2

There are no 'extra' green wires available to run the power to the tach. It would seem that the makers of the harness included the white wire (terminating in a spade) but not the 'looped' wire from the coil through the loom to the ignition switch.

There are only two wires routing to the ignition switch, not 3. As I understadn it, in theory anyway, the last (3rd) wire should be the 'looped' wire, terminated in a spade conection to the ignition switch.

I wish I could link to the stock schematics from an internet source, it may prove to be helpful. The harness itself follows the stock schematic for a mech. tach car- except for the presence of this white wire with spade connector to pwoer the tach. The routing of the white wires on an electric tach car is a little bit different, but obviously incldues both of these white wires.

I figured that I could not use the included white wire, but I thought I'd ask. I guess the real solution is to reuse the old 'looped' wire, but since it is not an integral part of the harness, I wonder if I can just connect it to the 3-way connector of white wires near the washer bottle. 1 such wire is for the fuel pump, another is only used in the event the car has overdriveand the 3rd wire runs from the ignition coil. Like I said, a test with teh multimeter confirms that the ignition coil wire junctions at this conenctor (which is also confirmed by the schematic). If I could connect the 'looped' wire to this connector, it would of course be a lot easier.
Matthew Mugherini

The 4-way bullet connector by the fusebox probably does have only 3 wires - one coming out of the main harness from the ignition switch, one going into the rear harness for the fuel pump, and a 3rd going back into the main harness. This 3rd wire *should* go to the fusebox only, and there should be an additional white wire in the main harness, not visible at this breakout point, running from the tach to the coil directly. If this 3rd wire goes to both the fusebox and the coil, then the manufacturer has mixed up the very early electronic tach harness with the 73 and later tach arrangment. If so, perhaps it has a black/white wire running from the coil -ve (one of two) to the tach?

Personally I'd send the harness back and get a proper one from elsewhere, but if you are happy to stick with it then you need to disconnect the existing white on the coil and tape it back out of harms way, and run a new one from the coil +ve, through the tach pickup, to connect with the single white already at the tach (you could go all the way back to the ignition switch connector but there seems little point). Anything else will give either no tach reading or incorrect readings. However having a loose white going all that way without any additional protection is not good, it could chafe or get caught and burn the new harness. If it has the additional black/white above then you could use that.
Paul Hunt 2

This thread was discussed between 06/11/2006 and 07/11/2006

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