Positive thinking required
Q I took on an abandoned 1957 MGA project and plan to convert it to a single 12v battery and negative earth. Having read your positive-tonegative earth conversion article (April 2022), I’m clear on all items except the tachometer.
The car is already fitted with a late MGB engine with alternator, and many items are new or of MGB origin.
The tachometer is not connected and is positive earth. Is it as simple as changing the pulse loop wire lead from the coil from coil terminal SW to CB?
Paul Moss
Aldershot
Matt says
AThe tachometer is an RVI type. The supply from the ignition switch to the coil passes directly through the instrument, which detects high-voltage impulses in the wire. Conversion is worth investigating. You will have to swap the connections on the coil over in any case.
The wire that passes through the tachometer can be left as is, we think. You will, however, have to reverse the power supply connections of the tachometer. Before you do this, check with a multimeter that the instrument is not earthed directly to its casing. Put the multimeter on its diode/ continuity setting, then put one probe on the original positive instrument supply terminal and one (firmly) on the casing. If it beeps, or shows a reading other than ‘1’ then the two are connected; the easiest thing in this case is to fit an MGB negative earth unit. If they’re not, swap the wires over behind the dash. We think the instrument light can remain unaltered – but do check that it doesn’t become the source of a short. If so, replace it with a plastic bulb holder with two wires from a scrap instrument.