TC45D tach calibration

[Professor Marvel]

Back again looking for help.
My tack says 0 when key is off. When I put key to position where I let glow plug heat it says 5. This leads me to believe that my tach is 500 high when I am running. My service manual explains how to ohm it out to see if the tach is ok but it does not tell me how to calibrate it. What should I do.

 

[jinman]

Instruments are notorious for being inaccurate at the lower portion of the scale and often at the top. The ideal situation is for them to be accurate and linear at the working range. Does your green 540 light come on when the needle is pointing to about 2600 rpm? I agree that the needle pointing to 500 with just power on is really much more inaccurate than expected. If it were 100 or less, you could ignore it, but 500 is just crazy. When your engine is started and comes to idle speed at warmup, where does the needle point? If it is around 1000 or slightly above, that's a good "ballpark" check. Idle speed is supposed to be adjusted for 1025 rpm (warm). If your tach shows that, it is probably okay at higher rpm. If your tach reads 1500 rpm at idle, then the tachometer is surely off and probably needs replacement to cure the problem. As far as I know, there are no adjustments. You can adjust where the 540 light comes on, but not the tach needle itself.

The other solution would be to buy a photo-tach. These come with reflective tape you can put on the crankshaft or the crankshaft pulley and do a direct reading of rpm. That's really the only way I know to get a truly accurate reading. Even with the photo-tach, you won't be able to make any adjustments to your tach to fix the problem.

 

[Professor Marvel]

Thanks Jim
I went out to check the 540 light and the PTO would not even engage. I checked under the tractor and a pin is missing between the pto lever and the linkage so of course the light will not come on. I have had my land plane on for a while and not used the pto. I will get another pin. It is raining and I did not want to roll in the mud and I could not get my scale close enough to measure it so I will get a few pins that are assorted near the size.
Really you have given me enough information to make it work for me. I did not think about the light as a tool. The main reason I am worried about the tach is for pto use. If I experiment with the light I will know pretty much what the RPMs are in general and when the pto is engaged I will use the light.
When it idles it says about 1800 and I know that is not right but it does sound a little high. It is still consistent with being 500-550 out and just idling high Once I figure it out I will reset the idle stop so it is really about the RPM you suggest.

 

[JC-jetro]

The only way to check the rpm out is against a calibrated test device. You want to use a non-contact Tachometer and measure rotational speed of crank shaft pulley exterior to the tractor. Cheaper ones require a reflective tape to be put on the rotating shaft (obviously with the engine off) and the reader to shine a light and read the reflection pulses back to be converted to RPM. I much rather use the strobe light tach that I have. They are absolutely safest to use but pricey as well.


JC,


Electro-Meters:Monarch Nova Strobes

 

[Professor Marvel]

Thanks JC
Honestly it has been 25 yrs since I have messed with a timing light and frankly I forget what to do. I will drag out my old light, I think I can find it, and figure it out.

 

[jinman]

Thanks Jim
I went out to check the 540 light and the PTO would not even engage.

PM, the lights on your instrument panel for 540 (green) and 600 (red) do not depend on the PTO being engaged. They are strictly a function of the RPM sensed within the tach and may be off the same as the tach is. I have an idea your tach is off by 500 rpm no matter where it reads. That means the light will not come on until the tach reads about 3000-3100 rpm. The engine will be racing pretty good at that point, but of course it will be 500 less than the reading so it should not be in any danger. I thought my lights had quit working once, but they are kinda weak and the sun was shining directly onto the inst. panel. I shaded it with my hand and could just see the green 540 come on and go off around 2600 rpm.

 

[JC-jetro]

Thanks JC
Honestly it has been 25 yrs since I have messed with a timing light and frankly I forget what to do. I will drag out my old light, I think I can find it, and figure it out.

This is a bit different than a timing light. I can't remember the last time I used my timing lights in the last many years. On engine timing light you get a spark signal with a magnetic pick up off of a spark plug. By manipulating rotating the distributor CW or CCW then you can figure out right timing, advance or ****** looking at the notch on the crankshaft pulley.

With stobe light tach , you shine a light on a rotating piece of equipment, let say fan pulley or propeller. you have adjustment knob on the strobe light and can change the number of times the bulbs turns on/off per second (frequency). if frequency matches the speed of rotation then object appears to be motionless, you then read off the rpm on the display. You can pretty much light the rotating pto shaft, increase or decrease light pulse and the rotating device slows down to stop and then you read the rpm. very fast , accurate and safe. Obviously pto just appears to be stopped under light, once light is removed you can see the rotation. with this device . It enables you to check any rpm without shutting a device or getting too dangerously close to read the reflective tape.



JC,

 

[jinman]

This is a bit different than a timing light.

Yep! All you said JC and a gasoline engine timing light has no use whatsoever on a diesel tractor. There's nothing to pulse the light. Timing lights normally used an inductive probe on #1 spark plug wire to trigger the flash. On diesels, there is a pulse train out of the alternator or a mechanically driven cable driven off the flywheel (like the old Ford Proofmeters). In PM's case, his tach is reading 500 rpm with no pulse train from his alternator.

Evidently he has a leaky diode somewhere that is allowing current flow through his meter movement that drives the tach needle. To fix the problem he will have to buy a new tachometer insert for his instrument panel. I think they are around $130. It's expensive, but a lot cheaper than buying the whole instrument panel around $400.

PM: The output from your alternator is a single pink wire that is separate from the main connector. I'd disconnect that wire and then turn on the key to be sure there isn't leakage within the alternator causing a DC voltage level shift to the tach. I'd hate for you to buy a tach and find out it was a faulty alternator the whole time. If your tach still jumps up to 500 rpm when the alternator output is disconnected and the key turned to ON, then you can be sure it is the tach at fault.

 

[Professor Marvel]

Thanks guys
Great info as usual.
Jim I will do as you suggest and see if the problem is the tach.
JC and Jim Can you suggest a decent strobe tach? I now want to determine what speed my motor is running at.

 

[jinman]

Amazon has all types and price ranges of non-contact tachometers. I don't have a particular strobe type to suggest.