FOAK: 1980 240DL tach. problem

Hoping to acquire some of the vast experience present on this newsgroup.

Here's the problem:
I gotta have a tachometer. Don't like to drive strictly by ear. So, I went
to my local salvage yard and found a '79 244, 4 spd. with a tachometer.
Carefully pulled said instrument cluster and brought home new car object and
carefully removed tach from salvage unit and cautiously installed into my
cluster. Installed my cluster back into my vehicle. Tach doesn't work.
All of my idiot lights work and both the gas and water temp. gauges work
marvy. The date of manuf. on the back of my cluster is two months later
than the donor cluster. Salvage tach unit has three connections on the
back. One is positive and another is negative and the third is marked 1.
There wasn't a lead connected to the 1 terminal when I pulled it and I don't
have any "leftover" wiring in the dash of my vehicle. Both circuit boards
on the backs of the clusters matched exactly (Nothing on one that wasn't on
the other).
I might add that the speedometer in my vehicle was causing me to want to
kill many of my fellow motorists. They seemed to be driving just under (5-7
mph under) the posted speed. Since I drive the same route to work very day
and previous to owning this Volvo, I only infrequently thought of
dismembering others, I suspected something amiss. So, I checked my
speedometer against some milepost markers and found that * I * should be the
dismembered motorist. My speedometer was reading 6-7 mph slower than actual
mph. Someone in my vehicle's past replaced the speedo. and wasn't cognizant
of the different ratios printed on the face of the gauge cluster. This
wouldn't make a difference in the tach's operation, but it could be a useful
detail. Or not.
I would greatly appreciate any insights others on this group might have....
Take care,
Basaltrock

 

I believe on the early style cluster the 1 terminal on the back of the
tach took a single black wire directly from the - side of the coil. IIRC
the + side of the coil was brown from the ballast resisitor and the
negative side was black, later changed to white with a red trace. The
wire may have come loose when you snapped the cluster out. The positive
and negative feeds come from the round plug.

Bob