QuestionHi Barrie,
I've a '60 MGA that has an early B 1800 engine with electronic tach which ceased functioning. I'd like to know how to diagnose the problem, although electronics are definately not my forte'...
Any help would be most appreciated. Thank you.
Thanks also for your reply to my earlier question regarding my
MGB's oil burning. You're right. New rings are needed.
Answer
Rev counter wiring
Hi John. The MGA was originally wired for positive earth. I assume that the tachometer has the words POSITIVE EARTH written on the front of the dial in small letters? If not, you may have reversed the wiring and blown the amplifier inside.
Similarly, if you have rewired the MGA for negative earth, then you must use the appropriate tacho.
You say the unit ceased functioning. Did it EVER work when fitted to the MGA, or are you saying that it worked on an early MGB, but not when fitted to your MGA?
Assuming that it worked in an MGB but not in your MGA, Then you have probably wired it wrongly.
1) The casing must be earthed. Check that the internal illumination works, because the bulb uses the same earth.
2) There must be a power supply to the lucar terminal. This is normally taken from a green wire, such as the power supply to the heater switch.
3) The power to the coil must be routed via a dedicated wire from the WHITE side of fuse A3-A4 via a loop of wire through the horseshoe on the back of the tacho and then directly to the SW terminal on the coil. The tacho counts the pulses through this wire. Please see the attached diagram. Do not make the loop too tight.
Hope this helps
