BJ8 Tachometer Reads High
By Frank Filangeri
I Have repaired many Healey electronic tachs with the same problem. See
the post below and it describes the problem and the fix.
"The problem with these was that as they aged they would begin to read
high (as much as 3500 rpm!! on some I saw) and get progressively worse.
The problem turned out to be a capacitor that was becoming electrically
leaky with age and not doing its job blocking the DC voltage component of
the signal being sent to the meter movement hence the high initial
"offset". If you can open your tach, look for a capacitor marked ".2 uf"
(uf =micro farad). It should be a tubular device about 3/8" diameter X
5/8" long probably reddish or black in color. I will most likely have
one lead out of each end and mounted to the circuit board on end
(standing up). You can get an adequate replacement at Radio Shack in a
.22 uf value. Should be rated for at least 25volts DC. Higher is
better. The ones I have had in the past are green in color, have both
leads out of the same end and are shaped sort of like an overgrown tooth.
Cost all of about $2.00 for a two pack. Unsolder or clip out the old
one and solder in the new. No polarity to worry about, either direction
will do.
If this turns out to be the problem and the tach then appears to work it
must then be calibrated. There is an adjustable pot on the back of the
circuit board with a small scewdriver slot facing the inside of the back
of the tach case. I drill a 1/2" hole in the tach case at that location
so I can access the adjustment with the unit fully assembled. Reconnect
in the car then just compare it to a know good tach/dwell meter and
slowly adjust the pot to read the same at 1000, 2000 and 3000rpm. Should
be good everywhere after that. Tape over the hole when done to keep out
dust and moisture."
Good luck and let me know how you make out.
Frank
