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.:o
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.

:cheers:
Tom

[GOLDCYLON]

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.

[Jeffvette]

Unfortunately, there is no calibration.

[GOLDCYLON]

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]

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]

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?

[Jeffvette]

VSS is only used to calculate speed, nothing else.

[Paul Workman]

Tiz a std 3.45 gear. That wouldn't affect the tac, as others have indicated.

[DaveK]

ahhh okay - I thought the vss signal was used on both, my mistake

[Tom]

I have seen this on a 90 ZR1, the problem was in the instrument cluster which had to be pulled and repaired. I would ask a local Chevy dealer where they send their warranty electrical repairs.

[Jarek5_7]

mine tach in 1991 read twice as real rpm ;(

[WVZR-1]

Tach calibration is done on the "main board" in the cluster. I've had conversations with a fellow who likely can accomplish it but he's not into the C4 clusters presently. He's pretty close to me and has offered to look at mine and with snapshots I've provided he's agreed with me which capacitors/resistors are responsible for the calibration. I just haven't had the opportunity to let him have at it. He does early stuff for many of the "resellers". The resellers that sell other than his product send theirs to him to correct when the one they sold fails.

The cluster can be sent to several cluster re-builders for the calibration and they usually come with a 1 yr warranty. There's not as many authorized service centers as there used to be but there are "responsible" people out there. A build should be something $235 to maybe as high as $350.

The error is sometimes conversational. When I bought my car I was told "stock everything". Seat of the pants it sure felt like it was "stock" but the tach reflected something that would have led a person to believe it had a gear! I checked it with my Tech1 and confirmed the error.

If you've an "error" now it will likely get substantially worse.

The tach can also be calibrated with a "box" similar to a Dakota digital. A plus to this would be you could occasionally re-calibrate it yourself. I've had conversations with a builder and it seems quite easily accomplished.

[gbrtng]

Check with these guys

http://www.instrumentclusters.com/

to see if they can "calibrate" a tachometer. They repaired a 91 cluster for me and
their work is good.

The tachometer is just an analog gauge which gets its input from the ECM - it's kind of like a voltmeter.

And the tach reading has NOTHING to do with the rear gears.

[WVZR-1]

Check with these guys

http://www.instrumentclusters.com/

to see if they can "calibrate" a tachometer. They repaired a 91 cluster for me and
their work is good.

The tachometer is just an analog gauge which gets its input from the ECM - it's kind of like a voltmeter.

And the tach reading has NOTHING to do with the rear gears.

If your reference is to my post here's an example:

2500 RPM / 3.45 gear / 1-1 (4th) / 25.7 tire = maybe 110 MPH,

2500 RPM / 3.90 gear/ 1-1 (4th) 25.7 tire = maybe 98 MPH

If your car (stock) is at 2500 RPM in 4th gear and the speedo is something substantially less than 110 MPH with no speedo error there can be only a couple explanations, it's had a gear installed or it's got a tach error. Mine were numbers similar to these and I confirmed the tach error with a Tech1 reading both speed and RPM from ALDL connector. I didn't SAY the rear gear effected the tach's accuracy.

The Tech1 confirmed the "tach error" and also the fact the car I bought was "stock" at least in respect to the rear gear ratio.

[gbrtng]

If your reference is to my post here's an example:

2500 RPM / 3.45 gear / 1-1 (4th) / 25.7 tire = maybe 110 MPH,

2500 RPM / 3.90 gear/ 1-1 (4th) 25.7 tire = maybe 98 MPH

If your car (stock) is at 2500 RPM in 4th gear and the speedo is something substantially less than 110 MPH with no speedo error there can be only a couple explanations, it's had a gear installed or it's got a tach error. Mine were numbers similar to these and I confirmed the tach error with a Tech1 reading both speed and RPM from ALDL connector. I didn't SAY the rear gear effected the tach's accuracy.

The Tech1 confirmed the "tach error" and also the fact the car I bought was "stock" at least in respect to the rear gear ratio.

My post was not in response to yours. Yours hit the list while I was typing.

[XfireZ51]

The issue with our tach, like all the other electronic tachs since at least 84 is that the damn thing is slow to react. Get a shift light.

[bobbyhi]

The issue with our tach, like all the other electronic tachs since at least 84 is that the damn thing is slow to react. Get a shift light.



On my '90 the tach is also off by about 10%. I was thinking of just putting on a tach with the shift light and not worry about fixing the console tach.

[Jarek5_7]

guys do you know how the console tach is connected?

I dont think it reads the impulses from ecm because ECM reads good rpms. I checked it today with aldl cavble and datamaster. adn the tach shows almost exacly the twice rpm as shown on datamaster.

so I thought maybe tach is connected to coil or coils and there is somewhere a short and tach reads impulses from two coils instead of one or something like that.

[WVZR-1]

Rest assured that the error you're seeing is from "failed/failing" resistors/capacitors on the instrument cluster "main-board". Your confirmation is the "correct" output from the ALDL with your software.

Your location certainly doesn't offer a very economical solution to your situation. If you've an acquaintance here in the US you could arrange the repair from a facility here.

The error you see I'm sure has increased over time. Minimal error when you first noticed or suspected it and it WILL become worse over time.

[Jarek5_7]

I think I`ll get aftermarket tach and connect it on the electronic ingnition diagnostic terminal F, book says its instrument cluster tach input so then I will see if its the problem with instrument cluster or some wiring.