Moparts

Test your Tach (you may be surprised)

Posted By: Rapid340

Test your Tach (you may be surprised) - 06/02/13 01:14 AM

I tested a Mallory and Autometer tach and found them both to be off by what I consider to be quite a bit (more than 5%) at different points from 1K to 7.5K RPM.

Since most of us dont have a frequency generator, I sketched up a simple schematic (parts that can be purchased from digikey for under $20) that can be used instead. All you really need is an accurate frequency meter. Many digital volt meters are accurate to within .5% in this range (you can do a rough check by measuring the line frequency of your house AC, it should be 60Hz).

Build the circuit powering it with the battery of your car and connect the tach green wire to the output (LM555 pin 3 thru a 100 ohm resistor). The tach wire should go to this circuit only. You will need to disconnect it from the coil or ignition box to do this measurement.

Vary the 25 turn pot until your tach reads the desired RPM and then record the frequency (Hz) that produces that tach reading.

Actual RPM = DVM frequency (Hz) * 15

This allows you to check the upper RPMs without the engine running (The key or power switch will need to be on so the tach is powered up). Dont use this circuit with a voltage that exceeds 16V. A battery charger for a 16V system would exceed 16V.

Example: A 400Hz signal should produce a 6000 RPM Tach reading on the nose. One of my tachs read 5700 RPM with a 400Hz input. The Autometer Tach was way off around idle RPM range.

Note: Use a little 9V battery instead of your car battery in case you make a mistake. (or use a fused car battery power source)


Attached picture 7727767-DSCF4139.JPG
Posted By: nss guy

Re: Test your Tach (you may be surprised) - 06/02/13 01:46 AM

I check the electronic tachs against my cable drive tach

Attached picture 7727803-interior.JPG
Posted By: CHAPPER

Re: Test your Tach (you may be surprised) - 06/02/13 01:47 AM

I wonder how many cars will burn down from people like me trying this..??
Posted By: Runner

Re: Test your Tach (you may be surprised) - 06/02/13 01:53 AM

my mallory 685 ignition boxs has a tach tester built in both of the autometer tachs i have a spot on but i had a sun pro that was way off
Posted By: VernMotor

Re: Test your Tach (you may be surprised) - 06/02/13 02:09 AM

I wish you was a little closer I let you test my tach..
Posted By: Rapid340

Re: Test your Tach (you may be surprised) - 06/02/13 02:16 AM

Quote:

I wonder how many cars will burn down from people like me trying this..??



That is a good point. I will add in a note for a fuse. I tested mine on a bench with a benchtop power supply.

Actually a little 9V battery would be preferred.

Be careful out there.

Posted By: Rapid340

Re: Test your Tach (you may be surprised) - 06/02/13 02:56 AM

The reason I started investigating this in the first place is because I wanted to get an idea what the converter slip was. When I went through the calculations, based on my tach reading and trap speed, I was coming up with zero slip. We all know that is not possible with a conventional torque converter.

Posted By: Cab_Burge

Re: Test your Tach (you may be surprised) - 06/02/13 04:07 AM

Quote:

The reason I started investigating this in the first place is because I wanted to get an idea what the converter slip was. When I went through the calculations, based on my tach reading and trap speed, I was coming up with zero slip. We all know that is not possible with a conventional torque converter.



I saw this very thing years ago, I built two Identicle NHRA legal 426 Street hemi motors, one would rev to 7000 RPM in 1st,2nd and 3rd gear, that car ahd a earlt Sun tach, the other one was a stck car and it would stop pulling at 6000 RPM according to the very expensive auto Meter race tach I ended up putting Jones Motorola mecahnical cable driven tach in it and when the Autometer showed 6000 RPM in third gear the mechanical was reading 7000 RPM I saw it once more with a cheap Auto Pro or what ever the cheaper line of Auto Meter was in the mid 1970s, it was on a stickk shift 340 car with a big solid roller cam, it would quit pullling around 6800 to 6900 RPM, he put a Stewart warner mechanical tach drive tach in it, the car had 5.57 gears in it awith a 3.09 1st gear tranny, he look down in third gear just before shifting to see what both tachs read and the electric read 7000 RPM and the mechanical read 8200 RPM, It spun a rod bearing shortly after that while idling in nuetral after a run Check your tachs, all of you
Posted By: 72Swinger

Re: Test your Tach (you may be surprised) - 06/02/13 04:43 AM

I know my tach doesnt match my rev limiter. Im guessing its off about 100-150 rpm. I do wonder where my idle REALLY is.
Posted By: Tig

Re: Test your Tach (you may be surprised) - 06/02/13 11:06 AM

I'm pretty sure if anyone has a digital type ign system you can monitor and/or download engine rpm. I'd guess this to be more accurate than the tach. Mine is out -200 rpm at the bottom of the range and and -210 at the top a fairly linear error.
My indicated 1100rpm idle is really a rowdy 1300.
If anyone has access to a waveform generator remember to switch to square wave 12v o/p and common connected the -ve to the psu.
Posted By: F1Scamp

Re: Test your Tach (you may be surprised) - 06/02/13 02:52 PM

Sorry to hijack Jeff. 11.000? When the hell did you run that? Congrats man.

You going to Martin?
Posted By: MR_P_BODY

Re: Test your Tach (you may be surprised) - 06/02/13 03:30 PM

When doing this test will the tach jump up to the RPM
when you have it hooked up to the tester.... also
do you have any other values for different rpm's
as in 1500 rpm and maybe 4000 rpm to see if its linear
thanks
Posted By: OUTLAWD

Re: Test your Tach (you may be surprised) - 06/02/13 04:24 PM

Hmm...interesting....I still might even have a 555 chip laying around from other projects
Posted By: Twostick

Re: Test your Tach (you may be surprised) - 06/02/13 04:44 PM

Quote:

I check the electronic tachs against my cable drive tach



That would work assuming the mechanical tach is known to be accurate ie been calibrated.

Kevin
Posted By: RodStRace

Re: Test your Tach (you may be surprised) - 06/02/13 05:13 PM

Great info!
I know fuel pressure gauges are all over too.
Posted By: roadhazard

Re: Test your Tach (you may be surprised) - 06/02/13 05:25 PM

Quote:

Great info!
I know fuel pressure gauges are all over too.




Why are we paying so much for these guages when their accuracy is questionable

So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make
Posted By: Cab_Burge

Re: Test your Tach (you may be surprised) - 06/02/13 05:37 PM

Quote:

Quote:

Great info!
I know fuel pressure gauges are all over too.




Why are we paying so much for these guages when their accuracy is questionable

So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make

I think the main message is to not take the instrument readings as the absolute truth, some gauges are not ACCURATE I've seen a bunch of supposedily high quality aftermarket mechanical temp. gauges be off 40 degrees, reading 140 F when the actual water temps are right at 100 F according to a certified claibrated thermometer All instruments are a measuring device to provide us information, accurate information When in doubt, check it out I learned on that first electric tach to move the lever when the motor quit pulling,no matter what the tach read, not to keep it in gear with the right fooot down waiting for the tach to read 7000 RPM
Posted By: MR_P_BODY

Re: Test your Tach (you may be surprised) - 06/02/13 06:32 PM

Quote:

Quote:

Great info!
I know fuel pressure gauges are all over too.




Why are we paying so much for these guages when their accuracy is questionable

So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make



Accuracy sure is nice ... specially if you are trying
to figure your conv slippage rate... when I was working
and setting up test cars we had to test and calibrate
all the gauges and transducers before we put them
on the car and sign off on the calibration... we
had the calibration equipment right in the lab so
it was pretty easy... before that we had to take
them to another lab just to get calibrated and that
could take a week or better depending on where we
were in line of other customers
Posted By: Rapid340

Re: Test your Tach (you may be surprised) - 06/02/13 11:09 PM

Hey Brian, Where've you been?

https://board.moparts.org/ubbthreads/show...e=3#Post7676286
Posted By: Rapid340

Re: Test your Tach (you may be surprised) - 06/02/13 11:21 PM

Quote:

So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make



There may some kind of calibration adjustment on some, I haven't got that far yet, I was mainly wanting to know what RPM I was going thru the lights.
Posted By: bigtimeauto

Re: Test your Tach (you may be surprised) - 06/02/13 11:36 PM


i use this when setting up cars.

Posted By: Bignick

Re: Test your Tach (you may be surprised) - 06/03/13 05:09 AM

Bigtime what is that?
Posted By: Moparmal

Re: Test your Tach (you may be surprised) - 06/03/13 10:47 AM

I run an MSD +6013 spark controller with my Gen3 6.1 Hemi - I set the rev limiter at 6200.

My Stewart Warner reads 5200 when the limiter cuts in!

I'm pretty disappoined because its a darn nice tach....just useless!

Posted By: bigtimeauto

Re: Test your Tach (you may be surprised) - 06/03/13 11:57 AM

Quote:

Bigtime what is that?



It's a MSD ignition tester. Has a digital screen to tell you your rpm and a dummy load for your coil. So it will fire your ignition and you can use it to check your rpm limits and or your rpm triggered stuff like shift lights or automatic shifters.
© 2024 Moparts Forums