Mechanical Tach Problem Debugging

[Steve Huneck]

The Smiths mechanical tachometer in my BJ7 is having some problems. The problem started with tach reading around 1000 rpm for the first 15 minutes of driving, and then it would function correctly. The problem slowly got worse and some days it will be off, all day, by about 1000 - 1500 rpm. I drive the car quite a bit so I have a pretty good idea where the rpm should be different speeds. It almost always read too low, but on some occasions it read 4500 when it should have been around 3200. I installed the heater just before this started, so on the chance I might have kinked the cable I replaced it, but no change.
Here's the question; because this is a mechanical tach the problem has to be the gauge. It would be impossible for the mechanism below the distributor to turn the cable faster than the engine is running.
Any comments, opinions or advice are appreciated.

Steve

 

[Genos2]

Being mechanical, it is driven by a cable from the drive beneath the distributor.The cable, similar to a speedo cable,can fray, round off& bind. I would unhook it from the distributor end, but it in a drill motor& spin it slowly& see how it reads, some cables must be turned in reverse. Sometimes lubing the cable helps. Genos2

 

[Steve Huneck]

Thanks for the great tip. I hooked up a drill to the cable and the tach gauge seems fine, it does run in reverse. The problem must be in the tach drive mechanism. Thanks again, Steve