How can I test my tach?

I am having issues with the Tachometer on the XJ. I though it was just needing to be lubed, so I followed RickCoMatic's method of lubing the Tach but it is still slow but not nearly as slow as before. I don't know if its because it is 45 degrees outside of because it is bad.

Is there a way to test the tach by applying voltage to the input line? If so what voltage should be applied and I assume it would need to be varied to make the tach move. Or do I need a pulse generator? (which wouldn't be a problem either) I have a variable power supply and am pretty good with electronics ~ Navy ET for 6 years and now work for Uncle Sam repairing Navy Radars.

 

i did test one once back in the days of dwell meters V8's and Holley 4 barrels
about all i remember is it needed a signal generator with a square wave output

 

Joe:

Bet the farm on it being affected by the COLD.

I'd have to ride my 900 home -- at night -- in the late Fall, when I'd have frost on the seat. No Tach.

The Speedo would work just fine. The Tach wouldn't even bother coming off the pin. Next day ... 1:30PM fire-up for the ride in -- in the Mid-50's ... it's working perfect!

The damn thing's just like New England, Small Town, Volunteer Fireman! When it's really, really cold outside ... and, the Fire Alarm sounds in the middle of a cold, cold night ... he sits-up and sniffs, reaches over and feels the wall ... and, if the wall is cold and he can't smell smoke ... he don't even get-out from under the covers.

 

RickCoMatic, I'm really hoping it is the cold but it would be nice to test it and know for sure. I may have to bring home an Oscilloscope from work and see what the input to the tach is. Then I can bring it inside, produce a signal to make the tach using test equipment and also vary the signal to make the tach vary.

 

3 wires
B= ground
Br=hot +12v
Gy=neg side of coil 2-3....switched ground.....+12 to 0

it's a little late now but WD-40 isn't a very good lubricant

 

Hold the phone there shippy, do not apply DC voltage to the tach. You are correct about using a pulse generator. I'm going to use a little assuming here so bear with. Back in the 80s (ahh, time of the Police, no bills nor responsibilities), TTL was still used to build circuits so I am assuming that our beasts are no different. TTL uses a logic high (+2.7 to 5VDC) to a logic low (0.8VDC or lower) so set your generator accordingly. Most modern generators have a TTL port and you simply select the waveform you care to use and set the pulse width. To be safe (this assumes that there might be CMOS logic chips, but I'm doubtful), keep the voltages between 0 and 3 volts for your waveform, this will avoid frying the chip if it is CMOS. You should be able to pulse the tach (it is electronic right?) and vary the speed. Shoot any more questions or observations you have out, we'll get this nailed.

 

i'am looking at a diagram from a 82 and the trigger voltage comes from the neg side of the 12v 2-3 coil, the 83 might be different
is there something i'am missing?

 

I found the wiring diagram in the crappy haynes manual and Polock is mostly correct (could be totally correct but I'm not sure)

B= ground
Br=hot +12v
Gy = from TCI which looks to be controlling the tach and the ignition coils. Like Robert says, I believe it is a pulsed signal. The amplitude, width and pulse rate of that pulse it not known. I'm hoping a simple test with an O-scope will let me know something. It would be nice if someone had the Yamaha service manual that would give the exact specs.

I do agree with Polock about WD-40 not being a good lube for this application. I used light oil that is meant for lubing throttle and clutch cables.

 

my yamaha manual doesn't have anything like that in it
the amplitude should be about 12v, pulse width would be the dwell but they don't tell you that either, the rate is the rpm
but remember that coil fires two cylinders
"(could be totally correct but I'm not sure)"....me either, sometimes i just get lucky

 

Start at 30msec @0.5 V and see where it goes.