Tachometer Calibration ?

[sca037]

I checked the Factory Service Manuals, but could not discern how to adjust the Tachometer signal by way of the pot(s) on the top of the gauge cluster.

Long story short, I had a gauge cluster fire a couple years back-
(see post #15 here; http://www.nsxprime.com/forum/showthread.php/170618-oem-head-unit-capacitors?highlight=gauge+fire )

I was able to procure a working used cluster and transfer my odometer to it, then send it to Briank to rebuild it with new capacitors.
It works GREAT, except that for some reason the Tachometer has read a little high ever since.

Does anyone have any information on how to properly adjust the (2) pots above the Tachometer on the gauge cluster, so I can lower the displayed revs?


Thanks,
Brian

 

[sca037]

Anyone have an idea how the Tach adjustment works?

The speedometer procedure is in the WIKI, but nothing on the Tachometer........

 

[Old Guy]

I have never had my cluster out of the car and the manual does not make any reference to or show potentiometers for calibration, so I will have to take your word that they exist. The service manual says that the tach is calibrated to display a 100 RPM increase for every 300 pulse increase from the ignitor (which is what you would expect for a 6 cylinder engine). If the cluster is out of the car, I suggest that the most effective way to calibrate the tach may be to program something like an Arduino as a mono stable multivibrator with an output frequency 50 hz (3000 per minute) which would correspond to an RPM of 1000. Hook the output of the Arduino (which will be a 0 - 5v pulse train) up to the tach input and adjust your pots until you read 1000 RPM. Reprogram it to a frequency of 100 hz and confirm that the tach reads 2000 RPM. You may need to fiddle a bit with the duration of the up state on the multivibrator to match the pulse duration that the tach gets from the ignitor. You may also need to add an external driver chip to the Arduino output as digital outputs from the Arduino cannot source a lot of current.

In the absence of a service manual procedure for testing and calibrating the tach, that is about the best that I can come up with.

 

[greenberet]

Kaz, one of the engineers who was on the design team of the original NSX, has touched on the subject a few times, for example:

Here: http://www.nsxcb.co.uk/showthread.php?11827-Speedo-problem

Here: http://www.nsxcb.co.uk/entry.php?1769-Carpet-Interior-Panel-Health-Check-etc-12

Unfortunately, it doesn't sound like an easy fix if you don't have the right jig. Maybe you could send your instrument cluster to T3Tec in Japan.

My tachometer may be reading a bit high close to the rev limiter and my speedometer seems to be reading a bit low at low speeds. Both classic indications of an instrument cluster in need of a refresh, it seems.

 

[sca037]

Thanks for the information (and links) guys, it's much appreciated!

The capacitors in this cluster have been replaced, so no more worries about another fire at least.....

Maybe I'll just live with the slightly higher RPM reading rather than open a can of worms ;^D



Brian

 

[Old Guy]

Capacitor replacement will cause the error. Electrolytic capacitors typically have pretty loose tolerances. +/- 20 % is not uncommon. The circuits in analog tachs are pretty simple frequency to voltage conversion devices and altering the value of the capacitance changes the tuning of the circuit and alters the calibration. The loose tolerance is probably why Honda includes adjustment pots to calibrate the tach for the uncertainty in the actual capacitance values. Anybody who gets their instrument cluster 'recapacitored' is going to have to deal with the error issue on the tach and the speedo.


A more robust way to do it would be to have a digital pulse counter to voltage out device driving the tach / speedo which would be immune to the uncertainty in capacitor values. That kind of now easily available technology post dated the arrival of the NSX so we are stuck with the old school analog conversion circuits.

 

[nomac]

Does anyone have a list of the capacitors for the tacho and speedo circuit boards? I see a list needed for the Climate Control was published by Russ on Prime DYI.
Seems like we should all be replace the caps in all our CB for better reliability and reduces the chance of a fire. Comments?
It would be nice to have perfectly accurate instruments but first I would like mine to work!

 

[osus2k]

I'm resurrecting this old post ad I have a tachometer problem, first time today I got the car to temperature and decided to run it through the gears, I noticed that the tach seems to be working fine but the range of the tachometer was about 800 at idle and "redlined"at 5500(basically when I felt the fuel cut off)it was a smooth progression and consistent which makes me think it may just be a calibration issue, this car has low miles but I suppose it could be the capacitors next step, I will send it out for repair or has anyone successfully tried a do it yourself calibration.

 

[Jinks]

I think you can adjust the rpm via the dash pots located on the top of the cluster once pulled out. I know you can adjust it for the speedometer just a few mph and there is a dashpot located above the rpm gauge so I think it can be adjusted just not 100% sure. Maybe some wizard can chime in here?

 

[Honcho]

Resurrecting this thread. I replaced all of my gauge cluster capacitors and the gauges are just a little off. The speedo is reading about 2 mph slow and the tach is accurate at lower rpm, but it's about 500 rpm off up high (reading ~7,500 when at 8,000). Does anyone know which direction to turn the pot adjusters? I know to use a non-conductive screwdriver. @Heineken? @drew?

 

[Wild Turkey]

I just had @Briank do my cluster. I think he calibrates the speedo when he does them. I wonder if he calibrates the tach as well? I'll find out soon when the snow melts. @osus2k 5500 RPM cut-out seems like more than a tach calibration issue though.
But seriously @Honcho, you can read your speedo to 2 MPH while you're driving?

 

[Honcho]

I just had @Briank do my cluster. I think he calibrates the speedo when he does them. I wonder if he calibrates the tach as well. I'll find out soon I guess. @osus2k 5500 RPM cut-out seemed like more than a tach calibration issue though.
But seriously, you can read your speedo to 2 MPH while you're driving?

I have the phone on GPS to compare.

 

[sca037]

I've just lived with the tach on the purple boostmonster reading a bit high, no big deal really.

On the 5500rpm cutout, I bet it's time to replace the speed sensor.....which fixed that exact problem on the same car a few years back.

 

[Heineken]

Sorry for the late reply - Easter holidays with the family.

The calibration process - using a calibrator or signal generator is described in one of my posts at NSXCB:

Gauge Cluster Capacitor Replacement

Failing capacitors at the gauge cluster of the NSX is a serious topic. First signs are a non-accurate speed indication or a permanently lit brake light warning even though bulbs and wiring are OK. The leaking acid near the warning indicator circuit can cause a short and even potentially lead to...

www.nsxcb.co.uk

Two pots for each dial and some back-and-forth to get good readings. Without at least a signal generator that can be a fiddly task.
The original post about the tester contains info about the frequencies and signals required for calibration:

Gauge Cluster Calibrator

Even though working-at-home, home-schooling and home-kindergarten doesn't leave much spare time it's important to have a bit of a hobby for a change. Since I'm working through the ECUs related to capacitor replacement it's now the gauge cluster's turn. Replacing its capacitors pro-actively is...

www.nsxcb.co.uk

 

[Honcho]

Sorry for the late reply - Easter holidays with the family.

The calibration process - using a calibrator or signal generator is described in one of my posts at NSXCB:

Gauge Cluster Capacitor Replacement

Failing capacitors at the gauge cluster of the NSX is a serious topic. First signs are a non-accurate speed indication or a permanently lit brake light warning even though bulbs and wiring are OK. The leaking acid near the warning indicator circuit can cause a short and even potentially lead to...

www.nsxcb.co.uk

Two pots for each dial and some back-and-forth to get goods readings. Without at least a signal generator that can be a fiddly task.
The original post about the tester contains info about the frequencies and signals required for calibration:

Gauge Cluster Calibrator

Even though working-at-home, home-schooling and home-kindergarten doesn't leave much spare time it's important to have a bit of a hobby for a change. Since I'm working through the ECUs related to capacitor replacement it's now the gauge cluster's turn. Replacing its capacitors pro-actively is...

www.nsxcb.co.uk

Thanks Lars! My low range on the tacho is accurate- it's the high range that is about 500 rpm too low. Would a signal generator like this work as a basic calibrator to test the high pot?

https://www.amazon.com/1HZ-500KHZ-F...s=signal+generator&qid=1681261446&sr=8-3&th=1

 

[Heineken]

Thanks Lars! My low range on the tacho is accurate- it's the high range that is about 500 rpm too low. Would a signal generator like this work as a basic calibrator to test the high pot?

https://www.amazon.com/1HZ-500KHZ-Functional-Generator-Seesii-Frequency/dp/B08NSR5GYR/ref=sr_1_3?keywords=signal+generator&qid=1681261446&sr=8-3&th=1

Happy to be of help.

While the speed gauge can be driven with 5V, the RPM gauge requires a 12 V rectangle signal going from 0-12 V:

Looking at the spec of the signal generator, it maxes out at 10 V (more like 7 V in real life) and won't be enough, I'm afraid. To make that signal generator work it needs further amplification e.g via: https://a.co/d/eRfyVFI and an extra 12 V source. To assemble all this correctly I would highly recommend a scope.

The frequencies required to drive the RPM gauge can be taken from the face plate of the first gen cluster tester (right hand size):

PS: I still have PCBs for my cluster tester, if you're interested.

 

[Heineken]

Just curious but have you been successful in calibration the gauges?

 

[Honcho]

Just curious but have you been successful in calibration the gauges?

Ha! No. My wife just had ACL replacement surgery so I have been devoting 100% of my time helping her recover and, of course, taking care of all the stuff she normally does. No time for the NSX right now. However, I'll PM you about your kit- I'd like to be able to set it perfectly.

 

[Heineken]

Oh, hopefully she'll get well soon. Please take your time!