Electrically operated tach/hour meter

Mechanos · Nov 2, 2012

If I understand correctly, the electrically operated tachometers from a signal wire tapped off the alternator. I was presume that this tap is off the AC side prior to rectifier and that the varied AC frequency or voltage is what the tach uses to determine engine speed.

The hour meter on my 955 recently quit working, but I do have an aftermarket electrical hour meter that runs on either AC or DC. I'm not fond of the idea of the hour meter ticking away whenever the key is on and the engine isn't running. Would the hourmeter work if attached to the tach lead on the alternator? The alternator that is in my 955 has the tach lead.

greg_g · Nov 2, 2012

I'm not sure that's correct. On most of the tractors I've owned, the tach was driven by a flywheel sensor and the hour meter was triggered by oil pressure.

//greg//

Oldtractorfixer · Nov 2, 2012

I doubt the hr meter will work from the tach lead. It has impulses the hr meter wants constant voltage.

greg_g · Nov 2, 2012

Nope. Hour meters wired that way run as soon as you turn the keyswitch to ON. Not so say there aren't some entry level machines that are wired like that, but not a John Deere. The hour meter clearly needs 12v to run, but also needs a trigger to actually turn. And that trigger is very often oil pressure. Some of the smarter hour meters combine tach speed and oil pressure. This type tics off hours relative to PTO speed. At 540 rpm, the hour meter runs 1:1. At less than PTO speed, the hour meter tallies proportionately slower.

//greg//

Mechanos · Nov 2, 2012

Ok. The electric hour meter than I have just has two terminals. You either connect 12v positve and negative or two leads from an AC source, and it will tick off the hours. I guess I could get creative and use an oil pressure sender (or other sender) and a relay and build my own trigger. Or buy a new tach/hour meter assembly from JD for a gazillion dollars. I do like the mechanical tach/hour meter that was originally used.

Oldtractorfixer · Nov 2, 2012

So, if you don't want to fix the original, just wire your new meter from a switched 12volt through a normally open oil pressure switch and you got it. Before I did that I would at least do some diagnoses on the original and make sure it didn't stop due to a bad connection or something simple.

LD1 · Nov 2, 2012

I would first like to ask why you dont want to tie it in to just a switched 12v powersource??

Sure, it ticks hours up when just the key is on, but how often do you do that?? IF its an aftermarket meter, I assume it is strictly for YOUR benefit as to know when to change oil and such right??? cause if you are worried about re-sale, forget it. With an afterarket hour meter, especially, several years down the road, there isnt much difference between 1000 and 1100.

I'd say save yourself the headache, and just wire it to 12v switched power.

Mechanos · Nov 2, 2012

Oldtractorfixer said:
So, if you don't want to fix the original, just wire your new meter from a switched 12volt through a normally open oil pressure switch and you got it. Before I did that I would at least do some diagnoses on the original and make sure it didn't stop due to a bad connection or something simple.

The original is cable driven mechanical, not electrical. I would like to replace it, but don't like the $110 price tag.

Mechanos · Nov 2, 2012

LD1 said:
I would first like to ask why you dont want to tie it in to just a switched 12v powersource??

Sure, it ticks hours up when just the key is on, but how often do you do that?? IF its an aftermarket meter, I assume it is strictly for YOUR benefit as to know when to change oil and such right??? cause if you are worried about re-sale, forget it. With an afterarket hour meter, especially, several years down the road, there isnt much difference between 1000 and 1100.

I'd say save yourself the headache, and just wire it to 12v switched power.

On an 20+ year old tractor, especially one put together from several donor tractors and a repurposed industrial engine, any hour meter is strictly for my benefit and means nothing towards resale. :laughing: