MONTE CARLO Another Tach Question

88AZSS

Master Mechanic
Mar 13, 2018
266
93
Mesa, AZ
So my tachometer has never been right since I’ve owned the car. It floats around 1K RPM when the car is off. When it’s running it’ll sit at that 1K until they RPMs get there then go to about 2500 and then fizzle off.

I read through this fix from another forum:

In that write up, the author says the correct resistance value between the pins should be 200K ohms. So I bought some 100K resistors and put two together to get the 200K. Well before I started cutting into my stock tach, I decided to test the values just to be sure.
7C9656E3-0505-4A93-A0D2-CF24F843C02D.jpeg

The two resistors read at 199K ohms, which is a minimal variance.
But when I tested the stock tach before I did anything to it, it read at 200K ohms on its own.
5E354760-8D98-45FA-B62F-147F33F71735.jpeg

So now I’m not sure what the issue is. I don’t want to cut this tach up just to add the resistors if it’s already reading what it’s meant to anyways. There is a tach filter in line to the distributor, would that be my next point to check? Is there a way to bypass that filter if it is the problem?
 

fleming442

Rocket Powered Basset Hound
Dec 26, 2013
11,157
113
The tach filter remains a mystery to me. I believe Mike's Montes or Dixie have new ones, but no one has tested a new one to find out what exactly it is. My suspicion is a capacitor that acts as a passive high pass filter like on a tweeter. That being said, you should be able to jump around it to test, but I don't want you to blow up your tach, either.

With the resisitors, the 200 reading is for the gauge itself or are you supposed to replace certain resistors?
 

88AZSS

Master Mechanic
Thread starter
Mar 13, 2018
266
93
Mesa, AZ
The tach filter remains a mystery to me. I believe Mike's Montes or Dixie have new ones, but no one has tested a new one to find out what exactly it is. My suspicion is a capacitor that acts as a passive high pass filter like on a tweeter. That being said, you should be able to jump around it to test, but I don't want you to blow up your tach, either.

With the resisitors, the 200 reading is for the gauge itself or are you supposed to replace certain resistors?
According to that write up, and others like it, a certain capacitor goes bad over time. The fix is to cut the lines to that and place these new resistors in line instead. I've heard good things about this working from more than one source. I just don't know that this is my problem at the moment. I will try to bypass the tach filter soon and see if that fixes anything. Im open to other things to check as well if anyone else has some input.
 
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