How to calibrate the tac??

[Paul Workman]

I have verified that my tac registers "a bit" high. (Reading the FSM, I see the tac registers rpm via monitoring pulses - but that alone doesn't tell me if it is a digital on an analog tac (but, I digress..).)

Now knowing the error amount I can allow for it. But, if I want to, how do I calibrate the tac?

TIA,

P.

[tomtom72]

I don't know the answer.
But I've seen alot of threads on Tach subjects over in the C4 Tech section over at you know where......land of mojo & savewave.....I've never read any of them as I'm not fluent in electronics.


Tom

[GOLDCYLON]

Originally Posted by Paul Workman

I have verified that my tac registers "a bit" high. (Reading the FSM, I see the tac registers rpm via monitoring pulses - but that alone doesn't tell me if it is a digital on an analog tac (but, I digress..).)

Now knowing the error amount I can allow for it. But, if I want to, how do I calibrate the tac?

TIA,

P.

Paul what does you Tach read at idle? The 90s had a PROM upgrade due to surge issues and such but Id like to know what you standard idle number is? D

[Paul Workman]

Daryll, at idle it reads just below 1000, I'm guessing between "750 and 800". Mine is a 90, and I don't know if it has a modified chip or not, but idle speed varies, depending on engine temp, etc.

I compared the tac reading to the scanner reading at a 2k and 3k (on the tac) and it would seem there is roughly a 15% error at 3k. I don't know if it is purely proportional or if there is also a fixed error integration at work as well. That's about as far as I got before reading up on it in the FSM (which I didn't find anything useful as far as calibrating goes).

However, with the switch on but not running, the tac indicates "-500" rpm, for whatever that is worth???.

P.

Unfortunately, there is no calibration.

[GOLDCYLON]

Originally Posted by Paul Workman

Daryll, at idle it reads just below 1000, I'm guessing between "750 and 800". Mine is a 90, and I don't know if it has a modified chip or not, but idle speed varies, depending on engine temp, etc.

I compared the tac reading to the scanner reading at a 2k and 3k (on the tac) and it would seem there is roughly a 15% error at 3k. I don't know if it is purely proportional or if there is also a fixed error integration at work as well. That's about as far as I got before reading up on it in the FSM (which I didn't find anything useful as far as calibrating goes).

However, with the switch on but not running, the tac indicates "-500" rpm, for whatever that is worth???.

P.

Paul well that Idle reading is normal. At least thats what my 91 reads and a lot of other folks are in the 750-800 idle range. The chip "prom" ugrade in the 90s was done as a recall to lessen the instances of high idle that would occur for longer than 5 seconds after you would come to a stop. I will need to look at that minus -500 issue on mine to see if mine does the same.

[Paul Workman]

Thanks for the info, guys. Well, it looks cool to run the tac up past 8k and it's still pulling hard!

P.

[DaveK]

Are you running a different (non standard) rear-end ratio? When I first got mine with 4.10s the gauges were about that far out and I used the gears from Marc Haibeck to correct it.

[GOLDCYLON]

Originally Posted by DaveK

Are you running a different (non standard) rear-end ratio? When I first got mine with 4.10s the gauges were about that far out and I used the gears from Marc Haibeck to correct it.

Thats a very good guess and a strong possibility.

[Aurora40]

Originally Posted by DaveK

Are you running a different (non standard) rear-end ratio? When I first got mine with 4.10s the gauges were about that far out and I used the gears from Marc Haibeck to correct it.

Doesn't that just correct the VSS signal? I.e the speedo? I would hope RPM information does not come from the transmission.

It sounds to me like your tach is just one of the ones that isn't that accurate. It sucks, but I get the impression there isn't much to do about it unless you want to try a new gauge cluster?