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Testing the tach?

Old 12-18-2018, 12:56 PM
  #1  
DALLAS916
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Default Testing the tach?

I have my Rally Pac clock repaired and ready to reinstall, but I still don't know if the tach will work properly.

When the clock points fused it shorted it, but also affected other components like the tach and ammeter. The tach, before In pulled the RP out and dismantled it was acting like it was "sticky". It stopped moving past 2500-3000 rpms, then didn't want to move at all with the engine running. Having the assembly out, it does allow the needle to "bounce" freely if I move the whole assembly. I have not taken the lens off!

So, now, HOW DO I TEST THE TACH BEFORE I REINSTALL IT?

And, my ammeter was not working when the shorted clock was in the system. How do I test the ammeter BEFORE I reinstall the RP or button up the instrument cluster??
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Old 12-26-2018, 02:29 AM
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08'MustangDude
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You can't bench test one without the proper equipment. They read A/C pulses from the
source, so you need something that will generate adjustable A/C pulses, sorry...

Like SO:
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Old 12-26-2018, 09:59 PM
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Originally Posted by 08'MustangDude View Post
You can't bench test one without the proper equipment. They read A/C pulses from the
source, so you need something that will generate adjustable A/C pulses, sorry.........
If the tach reads at all, and your interest is in checking the accuracy of it's calibration, one way to do that is the figure out what the tach should be reading at, say, 60mph REAL SPEED. To do this, you must take tire rolling diameter and rear gear ratio into consideration. The trick is to drive some well-marked road (with mileage markers), and adjust the car's speed until it is covering each mile in a minute, then note what the tach indicates. This is done in HIGH gear only, not Overdrive, unless you throw in that factor also.

Example: Tire diameter is such that each tire makes 500 revolutions in every 5280 feet, or 1 mile. If rear gear ratio is 3.00 to 1, the engine and driveshaft will be turning 3.00 X 500 = 1500 revolutions every mile. Thus, at 60 mph, the tach should read 1500 rpm.

Gear ratio is easy; tire rolling distance must be measured beforehand, a chalk mark on sidewall in line with a mark on pavement, car pushed straight ahead until sidewall mark again lines up with pavement, measure distance between marks on pavement. Let's say you get 75 inches, that's about 6.3 feet. Tire revolutions per mile are then 5280 / 6.3 = ~ 840, that being closer for a car tire than example thrown out above. 3.00 X 840 = 2520 rpm at 60 mph.
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Old 12-27-2018, 05:31 AM
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08'MustangDude
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Tach has nothing to do with drive-train, it's engine crank RPMs. When the engine is
IDLE, how does any of that come into play? He can hook up a decent scan tool, any
tuner, to get the RPM from the ECU. All that is nonsense... Compare the TACH bought
with the ECU.
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Old 12-27-2018, 10:42 AM
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Urambo Tauro
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I don't know if this helps, since the original question seems to be more about bench-testing the tach, but another way to check tachometer calibration is to compare its readings against a tach-equipped timing light. I don't know what year the car is, but this being the Classic Mustangs subforum, it's probably easier to hook up a timing light than it is to find an ECU to communicate with.
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Old 12-27-2018, 09:54 PM
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Originally Posted by 08'MustangDude View Post
Tach has nothing to do with drive-train, it's engine crank RPMs. When the engine is
IDLE, how does any of that come into play? He can hook up a decent scan tool, any
tuner, to get the RPM from the ECU. All that is nonsense... Compare the TACH bought
with the ECU.
Sorry. I was explaining the speedometer, not the tach. Shame on me. However, it pays to know that the newer EFI with OBD-II will NOT WORK one of the pulse-operated tachs which still work with OBD-I.
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Old 04-02-2019, 12:12 PM
  #7  
DALLAS916
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So sorry....I should have qualified this request. This is a 1966 Mustang GT with the original Rally Pac. I think I might have installed a Pertronix in the distributor to replace the mechanical points when I did the restoration (2004-2008). Not sure that matters if I am searching for the right signals. I see what you are saying, though.

But, if I hooked up the wires to an underhood source the same as they are under the dash and instrument cluster ( 3 wire connections), would I get the same pulse readings? What is the source of the pulse?

Last edited by DALLAS916; 04-02-2019 at 12:15 PM.
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Old 04-03-2019, 03:10 PM
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fastbackford351
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I asked Freddy from Mustang Masters this very question several years ago and he told me that if the needle bounced freely then it was probably good to go.

Ford wired their tachometers completely different from GM or aftermarket setups. Instead of hooking up across the circuit, FoMoCo wired their tachometers as part of the circuit so slapping it together under the hood for a quick test is a non-starter. DO NOT try to hook it up like a regular non-FoMoCo tach or you will toast it.

Last edited by fastbackford351; 04-03-2019 at 03:15 PM.
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Old 04-04-2019, 10:54 AM
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DALLAS916
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Thank you so much. That's exactly what I needed to know....before I fried "something". I find these circuits very delicate, but not just because some of the harnesses are original. It doesn't take much to get them mixed up and burn something up.

I'll do a little "bounce" test of the tach while it's out.
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Old 04-18-2019, 09:18 PM
  #10  
DALLAS916
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Default Success....so far...

Originally Posted by fastbackford351 View Post
I asked Freddy from Mustang Masters this very question several years ago and he told me that if the needle bounced freely then it was probably good to go.

Ford wired their tachometers completely different from GM or aftermarket setups. Instead of hooking up across the circuit, FoMoCo wired their tachometers as part of the circuit so slapping it together under the hood for a quick test is a non-starter. DO NOT try to hook it up like a regular non-FoMoCo tach or you will toast it.
Freddy was right. The tach is fine. We hooked up all the Rally Pac connections and it all works fine again. BUT...I can't install it yet because the ammeter is dead.

I could just leave it alone, but we already have the instrument cluster pulled forward to reconnect the RP wiring. The question now is, if I'm going to replace it should I stay original like the car has been restored....or should I get one of those VOLTAGE gauges that looks exactly like the original ammeter? I think the voltmeter needle drops to the left when the key is off. That might look strange to a purist, I think.

What do you think?
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