How do you bench test a tach?

barring calling the mfr & asking them for a procedure I would jump the black to batt neg post, the red to batt positive post, the sense wire to coil primary negative post & start the eng & see if the RPM reads close to what you think the eng is turning

I'll try that. if it matters, it is the stock tach in a 69 cuda

no difference. its dead right now I am assuming?

A 120 VAC 60 HZ battery charger puts out an imperfect DC voltage, it is actually a pulsating DC voltage that corresponds to:

1800 rpm for an 8 cylinder tach,
2400 for a 6 cylinder
3600 for a 4 cylinder

To test a tach using the battery charger, hook all wires but the black wire to the charger's positive lead and the remaining black wire to ground/negative lead.

What Kunkel said. The actual bench test procedure was using a filament transformer (12V AC to power the lights inside vacuum tubes) with a diode with no filtering to give you 60 pulses per second. The numbers John posted are the equivilent speeds on a four cycle engine in America. You will have to convert if you are using 50 cycle current in Europe.
Craig

John is the man!

So this will work on any battery charger regardless of amp setting?

got around to pulling the dash out on my 69 cuda today. my tach has 1 blue wire that comes out of the harness and 1 gray that goes to the negative on the coil. I hooked the positive from the battery charger where the blue wire goes and when I touched the negative from the charger where the gray goes, sparks flew. it only touched for a split second so I hope no damage was done to the tach.

I haven't tested a tach using a battery charger myself but I have repaired Mopar tach's and tested them with other methods.

So here is what I would do:
-Connect a +12 volt source to the long stud (the normal +12v input to the tach). This powers the tach circuitry.
-Connect your 12 volt source ground lead to the tach metal case somewhere.
-Connect battery charger + lead to the tach input terminal where the gray coil lead normally goes.
-Connect charger - lead to the tach case somewhere.

If I had a Mopar tach handy I would have tried this before posting. Sorry.

EDIT:
Re-reading John's post I believe he is saying to connect the charger's + lead to both tach terminals (where your blue and gray wires connected). This will power the tach AND supply an input signal by going to both terminals. Connect the charger black negative wire to tach case.

Let us know.

I hooked it up like you said and got nothing.

As I mentioned.....I have never tried testing one using a battery charger so I can't really speak with authority to that.

I'm not sure if this is a known working tach that you want to check for accuracy or if you are simply wanting to know it will function.

To check for functionality, connect the tach to a car that has a coil.
I have done this with Mopar tach's (and aftermarket) many times.

Lift the hood and place a towel or similar on the inner fender area and put your cluster/tach on it. Connect an alligator lead from the battery (+) terminal to the tach's long stud (blue wire terminal). Connect battery (-) terminal to the tach's metal case, ensuring a good ground connection.
Connect an alligator lead from the coil (-) terminal to the tach short stud (gray wire). Start the engine and rev it, look for tach deflection.



This absolutely does work so providing all leads are good and connected properly and you get no tach response.....then the tach is bad.

This may not work if the car has MSD ignition.
It DOES work if the car has Mopar electronic ignition.

EDIT: After er-reading the thread I realize you just want to see if the tach will function.

I will add this about your tach since it sounds like it may be bad.

My experiences repairing these tach circuit boards pretty much requires replacing every component on the board to make them accurate again.

The last one I fixed had a shorted transistor making it completely inoperative. I bought transistors via ebay from a German seller because they are nearly obsolete. Then it worked, but barely. Found a bad zener diode that had drifted in its turn on voltage, replaced that. Then it worked better but was way inaccurate. I started checking resistors and almost every one was way out of tolerance so I replaced ALL resistors with modern metal film resistors which are very accurate to rated values. NOW the tach worked and was within 100 RPM throughout its range (I only checked it to 5K).

So unless you have someone who can do this type of repair, I suggest you buy a replacement board from RT Engineering and install that. They sell 2 different priced boards for your tach, the more expensive has an onboard jumper you relocate to a test position and then tweak an adjustment for 4K RPM on your tach meter. I tested this board using a pulse generator and O-scope and it is very accurate.

BUT first you may want to check your tach meter movement to ensure it is not open. This can be done with a 1.5v battery applied to the meter movement leads internally (got this info from RT-Eng web site).

I can give more info if anyone desires.

No, I do not think so. The charger puts out a very noisy and ripply brand of DC. It is not clean enough to power the tach but just right for simulating the open and closure of points or the pulses from a solid state spark controller. That is all the charger is supposed to run with the black to ground as normal and the positive to the gray lead. A clean 12 Volts sourced from either a battery as in a car or from a bench power supply is needed to power the tach itself.
Almost all battery chargers have simple half wave rectifiers with a limited amount of filtering. The 60 pulses per second of positive half of the AC waveform is what appears to the tach as a running engine. This design of chargers had no filtering because it was not needed, the battery was the filter. It also had a secondary effect of vibrating the plates in the battery to possibly knock sulfated material off to rejuvenate it. I am not sure I buy that and with AGM technology, it wouldn't work anyway.
Craig
Craig