trulytex

Does anyone know if you can calibrate the Tach on a 1990 Vette?

Mine started out reading about 300% too high and it seems to be getting worse. I was told that the tach filter replacement is for a tach that is erratic and that mine probably needs to be recalibrated. When idling at around 700 RPM's my tach reads near 3000 RPM's. It pegs out past 6000 when I reach 60 MPH. My Vette is an automatic & I dont rawhide it so I know that it's not crucial, but I hate something not working properly. Any thoughts on doing this DIY? Or should I remove it and send it off?

Waynester5.7

Be very interested in peoples replies too....
I have a '90 auto with exactly the same symptoms..my tacho goes off the clock.
At 750/800 rpm the needle shows nearly 2000rpm..??

Anyone help me and my buddy upstairs...

adi4

Mine reads too high as well

eptaylor

Still most likely the problem is the filter. Erratic is most common but your symptoms can also be from bad or loose connection.

adi4

Mine was erratic but I am running mine off of my MSD 6AL, no longer erratic just reads high ......

trulytex

I was told by someone who knows Corvettes that an erratic Tach is the sign of a bad tach filter. And a Tachometer that reads way too high is a sign that it needs to be recalibrated. Not really sure, that's what I heard. Does anyone know for sure? And does anyone recalibrated a Vette Tach?

Banshee13

I have a 91 and was wondering where the tach filter is located? No one seems to know.

adi4

Its located between the dist cap and the loom and looks like this .......

golfpro77

You can get the actual rpm on your electronic heater control :

hold down the up & down fan buttons until -00 shows
use the up or down arrows to scroll to -06
then push the fan auto button
your actual rpm will be displayed (divide the number shown by 25)

this is how it shows in the GM manual for my 90

Once you have the actual RPM you will need to "reattached" the needle on the tach display to the actual reading... you'll need to use a fork to pry it off then just push it back on to the proper setting.... did mine while I was rebuilding it last year.

As far as I know I do not think there is a procedure to calibrate the tach without actually moving the needle on the tach

trulytex

How difficult was it to remove the needle from the stem and to replace it? Did this actually work, is your tach reading true now?

Do you have to remove the guage cluster from the dash to get under the plastic protective window?

This is very interesting. I would like to try it in the tomorrow. I can also hook up my Actron computer readr to the car and it will tell me the RPM's.

golfpro77

I did this when I was replacing the black back ground to a night light background... not difficult to remove the needle BUT use a fork to pry it off to make it easier and not to break it...

It will take a few of times until you have the needle where you want it so it will read true. Yes you need to remove the cluster from the dash .. not really difficult... then disassemble the guage cluster (clear window to access the needle) again not difficult ... the hardest thing is getting the needle to the exact position you want as it will rotate if you do not put it on straight down ie do not twist it on.

It might take a couple of hours start to finish you will have to put the needle on... then install the cluster to see if is reading what you want it to. I would do this with out the window installed in case you have to adjust the needle again... I had to 3 times until I got it where I wanted it and it read correctly.

Once you get it right..reinstall the window and cluster to the dash and your good to go!

VetNutJim

My '91 Tach reads 400~500 RPM too high.

Hmmmm.... calibration by needle adjustment....hmmmm, I never thought of that.

I reckon one should check to be sure the innacuracy is consitent or linear throughout the normal tach range.
Otherwise it could be made accurate at say the low end and be off even further in the mid or high numbers.

I'm not certain mine is a linear error but I'm going to check and if it is....
I'm raiding the silverware drawer for a couple of forks, LOL.

golfpro77

if you use one right after eating macaroni & cheese it's easier to slide under the needle....

trulytex

Well, I tried the "reattatch the needle" trick on my tach and it still doesn't read true and now the needle bounces some, so I guess that it's time to go after the tach filter. Getting to the needle was actually quite easy. I didn't need to remove the cluster, just 4 screws hold on the black bezel around the Instrument Cluster and then 4 screws (with nut heads) to remove the clear panel - took maybe 10 minutes. I've got it to read true at around 65MPH but then it's way too low when going slower and way too high when going faster, but at least it's not going to the limit at regular driving speeds.

Can you put some kind of homemade resistor in line between the old tach filter and the distributor without removing the old tach filter? I have still not been able to see the Tach Filter that is on the car. Has anybody replaced one of these?

johnnyevans

It's not the filter. There is a component out of kilter in the assembly. I think it is the air core assembly itself. I replaced many components on the board without any change.
I finally put a 93 dash in mine. Plug and play, except for one change. Put the 90 LCD in place of the 93. No other changes are required. No problems since then.
johnny

trulytex

The "assembly", is this Instrument Cluster? What is the air core assembly?

johnnyevans

The air core assembly is the tach drive itself. It works the same way the the autometer gauges do.
The assembly is the circuit board that drives the tach, and lcd display. THe gauges are driven by the sensors from the engine, oil temp, water temp, oil pressure, the volts drives from the voltage into the board.

I've replaced most of the components on the ckt board. I have not replaced the air core motor for the tach needle. If I could have found the air core motor I would have replaced it to see.
There is one chip that had no #s on it that controls the tach, I have replaced with the 93 part, it made no difference. 1 is a voltage regulator and 1 drives the lcd board.

Originally my tach was only a couple hundred rpms off, but it got worse as time goes on.

I replaced the tach filter on the 90 and it made no difference.

I would think any year would replace the cluster, just remember the LCD is different when the gas level indicator goes above the speedo, I think it changed in 93. With the LCD change, any year should work.
johnny

trulytex

Thanks Johnny, I'll be on the lookout for a new Instrument Cluster. Will that do it? It looks like you may have saved me a major headache in replacing the tach filter.

johnnyevans

Brad
I would stay away from the 90-91s, 92-93s are the same color, I think 94 is when they went to white instead of orange lettering.
You don't have to change the LCD until the gas gauge goes on the bottom, 93.
It's plug and play.
The data input changes with the gas gauge move and you'll have a scrambled LCD on 93 and I believe up

You should be able to pick up a cluster for around $100 or so. Don't get ripped.
johnny

1992B4C

What about the odometer reading? Is that data stored in the LCD? If so, then I guess changing the LCD takes care of the odometer.

Could that part have been replaced? I have a 94 cluster (with white lettering) that I want to put in my 90 to fix the tach issues. Maybe I can just replace the air core ***'y. If not, could the face plate from the 90 be used to replace the 94 white lettered face plate?