Hi all,

Trying to figure out this tacho issue. When I start my car, idle is strong and tacho will read about 800. When revving, I can rev past 800 and the tacho will read up until 1500, but then ceases to read any more and bounces there. The engine, however, is clearing revving to rpms higher than that.

What I have done to solve this issue:
-Replaced coils with msd blaster 2s
-Replaced ignitors
-Cleaned all fusible links


I am not sure what else could be the issue. I took the entire gauge cluster out and am in the process of testing all of the tacho circuit components for continuity. If anyone has a better understanding of what might be the issue, please let me know. I did clean out the tacho so I know that it is not any gunk stuck there.

 

Bad tach gauge or cluster board.

 

I see. So I would have to source another cluster from a different car.

 

Yes. You could try just replacing the tach portion. Each gauge can be unbolted. You could also just try an aftermarket tach if you or a friend has one to validate it's the cluster/gauge. You could also look for bad solder joints.

Did you save your old coils. The factory ones don't go bad that often and are better than aftermarket.

 

I would also recommend sourcing a spare cluster and replacing your current tachometer with the spare. You could always verify its operating condition by simply plugging in the replacement cluster and checking all functions before disassembling either cluster. If the tach works, you know everything in your car is fine. If it doesn't, you have issues to sort out. Honestly though, it sounds like a bad tachometer. I have seen them go bad a handful of times of the last 28 years that I have owned and worked on these cars.

 

I would verify that the tach signal from the trailing ignitor is good by using an external tach like the one in your DVM or an old school tach gauge. Don't blame the cluster or tach until you do this. If the signal is good then move up the line to the feed into the tach and do the same. If its still good, then its a board or tach issue in the cluster.

 

Updating this thread:

The tacho issues are related to the module itself. A fellow 7 owner supplied me with his S3 tacho. The modules worked when swapped.

Not sure what causes this issue, but I have seen this issue pop up on a facebook group again, so it isn't isolated to my car. In all fairness, these cars weren't designed to last this long so some sort of electrical failure is bound to happen.

 

Glad to hear you were able to determine the cause. I have replaced more than one tachometer over the years in RX-7's. These cars last a long time if maintained properly. I think Mazda did a great job in designing them. Simple but effective.