Tachometer Needle Moves By Itself

jd5210 · Nov 13, 2011

I am having a problem with the tachometer on my tractor. The tach is electronic and gets its signal from a magnetic pick-up. The problem is when the tractor has not been used for a day; the temperature is less than 50 degrees outside. Without starting the tractor just turning the key to the run position, the tach needle begins to move slowing and will continue to move sometimes as high as 2500 rpm before falling back to 0. The colder it is outside the higher it moves. This will occur even when the only wires connected to the tach are power and ground. If additional information is needed let me. I am guessing some component is leaking voltage, my question is what will do this only when cold? Thanks

Joe G · Nov 13, 2011

have you checked for condensation? you may have a bit of moisture (water) in/on it.

jd5210 · Nov 13, 2011

Thanks for the reply. There is no condenstation as the tractor is in an enclosed building, plus I have opened it up and have seen no signs of condenstaiton.

user_88 · Nov 13, 2011

Ground connections are sometimes a concern on vehicles. Have you checked the ground wire ... where it attaches to the frame or chassis?
A fine grit emery paper and some electrical contact cleaner spray should insure that you have a proper ground contact to the chassis.

If the ground is not the problem, then it is probably something internal to the tachometer ... You might try installing another identical unit to see if the problem still occurs.

alec_t · Nov 13, 2011

The tach may have a fair bit of electronics inside it, unless all the smarts are in a main ECU. Moisture and poor earthing, as mentioned already, are the prime candidates for electronics problems. It's also quite common for farm livestock to damage any exposed wiring, so check that all wiring and terminations are sound.

jd5210 · Nov 13, 2011

I have no farm livestock. It is in an enclosed building. The chassis grounds are clean and good. No moisture in the building. The problem is inside the tach. The cluster cost $455 nor do I have access to another unit. Any idea what component, diode, cap., resistor, ECU might cause the problem I described? While not the best I have attached a few pictures. The tach is in the center. Thanks.

alec_t · Nov 13, 2011

C12 (leaky?), the big resistor(dry solder joints?) (can't see a number) and the contacts under the nuts/washers on the mounting bolts might be worth a check.

jd5210 · Nov 13, 2011

Alec, I do not see any signs that C12 is leaky, but how would you suggest that I test it? I have cleaned the contacts (nuts, washers and circuit board, along with insuring the nuts are on securely). I will open it up again and re-solder the big resistor. Thanks

duffy · Nov 13, 2011

jd5210 said:
how would you suggest that I test it?

Don't test it - replace it. Costs fifty cents.

Another suspect part is that trimmer, on the bottom just to the right of the "6n8j400" yellow cap. That kind of open-frame trimmer is notorious for getting oxides under the wiper arm.

jd5210 · Nov 13, 2011

Is the trimmer a 10K or ? I had my ohm meter set at 20K. From my picture the top left (TL) to the top right (TR) the reading was 8.68. TL to center (C) reading was 2.12 and RL to C reading was 8.35.

From the reading I done, I need a preset (open style) variable resistor. I looked at Radio Shack (www.radioshack.com/product/index.jsp?productId=2062301) and this one looks a little too big. Any suggestions where I can get just 1?

Thanks

duffy · Nov 13, 2011

Yes, 10K, you might even see a "103" stamped on the top of it? That's 10k - "10" plus 3 zero's. The resistance is off a little because these were low-tolerance parts.

That Radio Shack one's actually smaller than the one you are replacing. Bend the leads out a little and I think it just might fit.

Or, one of these -
http://www.mouser.com/ProductDetail...EpiMZZMsl2q/phOiXBPBIGmvZi3q8knwFz%2bok5%2bA=
http://search.digikey.com/us/en/products/3319P-1-103/3319P-103-ND/612868
http://search.digikey.com/us/en/products/262UR103B/262UR103B-ND/863
http://search.digikey.com/us/en/products/3309P-1-103/3309P-103-ND/84801
- may match up perfectly. Check the diagrams under "Datasheets" to see the lead spacing.

I have not seen that exact same open-frame style trimmer sold for a while now.

jd5210 · Nov 13, 2011

Duffy, thanks!

What is stamped on the top is J223, what if anything does that mean?

duffy · Nov 13, 2011

Hmm... that would mean it's a 22k trimmer, also a "standard" value. Pull it out of the circuit and measure it again, by itself - could be something in parallel making it read different on the meter.

Not so many to choose from with this guy -
https://www.mouser.com/ProductDetail/Bourns/3319P-1-223/?qs=sGAEpiMZZMsl2q%2fphOiXBB19Va%2fojNfOyauttSi0dWk%3d

pfofit · Nov 13, 2011

Can't see the other side of the board, but there are several questionable solder joints that need to be touched up. May be bad or just hasn't wicked though the vias. At least one of the crappy joints at J3 appears to sneak over to the center / tach circuit

jd5210 · Nov 13, 2011

Duffy, when I measured the trimmer it was in the circuit. Guess I am not going to find that at Radio Shack.

pfofit, thanks for the close examination, I will touch them up.

debe · Nov 13, 2011

The trim pot is unlikely to be the problem, it will be for calibration of the tacho. If you must take carefull note of where its set & just shift it slightly one way & then back. Dry joints realy need to be adressed.

Tilamsik · Nov 18, 2011

I wonder if that small black or burnt component between the 2 resistor left side of C2. You might try to check it.

duffy · Nov 18, 2011

^ Yeah, what is that thing? Looks like someone took the body off a slide-switch and soldered the lower two contacts together.

davenn · Nov 18, 2011

duffy said:
^ Yeah, what is that thing? Looks like someone took the body off a slide-switch and soldered the lower two contacts together.

my thoughts exactly when I first saw it, I wonder if it was the manufacturers easy way to produce a solder blob selectable mode. eg 4 cly / 8cyl or something

Dave

Sceadwian · Nov 18, 2011

jd5210 said:
Without starting the tractor just turning the key to the run position, the tach needle begins to move slowing and will continue to move sometimes as high as 2500 rpm before falling back to 0.

I think my first question would be if the engine is running are their any abnormalities? If not then why are you trying to fix something that is not a functional issue when the engine is actually running, which is the only time it matters for a tach... Perhaps it was noted after they designed and built the tach assembly but since it doesn't manifest when running then it's not a problem..

By all means if you're seeing irregularities when the engine is running I'd be wondering what's going on to, but if you don't, then you have to ask yourself if you really have a problem in the first place, or just a odd effect of the electrical system which is irrelevant under running conditions.

If the RPM registers properly without any significant glitches after starting then you're only perceiving a problem, one doesn't necessarily exist.

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