My '84 has no amp, the tach works, my '83 has an amp, which I replaced, now that works also. There are two different gauges, with two different hook ups. I removed the cluster from my '82 300 DT, (amp) on the rear of the tach the plug in is a two prong affair with the #230/107/10 with 6000Imp underneath. I have another tach from an unknown car, it has a three prong plug, the middle prong is just plated through, leading me to believe it is a ground, and the #902/74/1 with 14000 Hz underneath. Both of these gauges are clock/tach. I have not pulled the cluster from my '84. I also have a cluster from a 126, the year printed on the back is '83, it has a two prong plug with the #902/29/5 with 6000Imp underneath.

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83 SD

84 CD