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MG MGB Technical - Tach reads fuel pump pulses

I am in the final throws of a Devin MG built on an MGA chassis, with late MGB drivetrain. The harness I am using is for a 63-64 MGB,with MGB generator, regulator, and fuse box, all wired for positive ground.

The positive ground MGB electronic tach I am using is reading the impulses from the fuel pump, causing tach to read 1000 rpms +- too high. If I disconnect the wire to the pump, tach reads fine, or if I wire into a green (unswitched) power line, it also reads fine. Any connedtioon into the white wire circuit causes the problem, it is currently wired directly to the white side of the fuse box, going directly into the rear harness. Tried both a facet style pump and an SU pump, same problem.

My guess is that the early MGB wire loom was designed for a cable driven tach, so there was no provision to isolate the impulses caused by the fuel pump throughout the white wire circuit?

How can I fix this and still keep the fuel pump switched by the ingnition key?

Joe B

Joe Buchmiller

Run a wire from the switched pole of the ignition to the tach white input lead, run a second wire from the tach white output lead to the ignition coil. remove and insulate the existing white wire at the coil.

An alternate is to run a wire from the switch pole of the ignition to the white fuel pump lead where it is spliced in front of the bulkhead, disconnect and insulate the existing lead from the harness.
John H

Joe - Go to Advance Auto Wire at: http://www.advanceautowire.com/ Click on Stock Schematics then on MGB, then scroll down to the diagram for the 64 - 67 MGB. You will need to set the magnification to 300 to see the real detail for the wiring of the white wire, but basically it goes from Terminal 2 of the ignition switch to a 6 pin bullet connector, from the connector to the power terminal of the tach (plus 4 other places, one of them being the fuel pump). there is another white wire that goes from terminal 2 of the ignition switch and loops through a pickup block on the back of the tach and on to the negative post of the coil (this is for the early tachs used in the 64 through 67 MGBs, a later tach design has a bullet connector that the above wire plus into for one side and another bullet connector that connects to a wire going to the coil, the loop being internal in the tach.). There is no problem with the tach reading pules from the fuel pump with either of these configurations. I would suspect that you have a high resistance connection somewhere somewhere between the battery and the 6 pin connector where al of these wires converge, or you have a bad ground to the tach for it to be sensitive to the fuel pump pulses. Good luck - Dave
David DuBois

Thanks John and David
I ran the wire directly from the tach loop to the coil instead of thru the loom and that cleared it up...
Joe B
Joe Buchmiller

This thread was discussed between 15/08/2006 and 23/08/2006

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