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komatsu water temp sending unit

berky

Well-Known Member
Joined
Jan 10, 2017
Messages
83
Location
Indiana
Recently picked up a nice WB140-2n at local auction for use on my farm. Going through fixing a lot of little things. Water temp gauge was not working so I first pulling sending unit. Ohmed it out and there was nothing. Aha, I found it. Ordered one from Komatsu hooked it up and nothing. Then checked the resistance, NOTHING just like the one I thought was bad. Even warmed the sensor to see if the reading would change, still no resistance. Meter works if I short across sensor line and ground so thinking that is not a issue. Now I am wondering if these Komatsu's use some kind of non standard circuit. Anyone with any knowledge out there?l
 

Delmer

Senior Member
Joined
Jan 3, 2013
Messages
8,864
Location
WI
I think you mean no continuity, right? or no reading on an ohm meter that means infinite ohms. If you had no resistance (extremely low ohms) then it would work the same as grounding the sensor line, which works.

I'd try a junkyard sensor (whatever's handy that won't be missed:D) and see if the gauge registers cold and with the sensor in boiling water/hot oil. It sure seems the gauge and system is normal, and your new sender is bad.
 

Shblack

Well-Known Member
Joined
Feb 22, 2015
Messages
59
Location
Tennessee
Seems like I am traveling your path. Temperature gage doesn’t register. Checked sender, reads open circuit. Wire to sender has 12V on it when open circuit. When I short the wire to ground 428mA flows, but gage doesn’t move. However the front and side dash over temperature lights come on when shorted. Gage has three terminals, black (ground) red (measures 12V, it’s daisy chained to other things) and a gray wire that measures 7.4V, voltage doesn’t change, sender shorted. I am guessing this is the sender signal, but there must be something else in between the sender and gage, which is activating the lights. We’re you ever able to fix your problem?
 

berky

Well-Known Member
Joined
Jan 10, 2017
Messages
83
Location
Indiana
There are two water temp sending units. One for the over temp light and one for the gauge.
 

Shblack

Well-Known Member
Joined
Feb 22, 2015
Messages
59
Location
Tennessee
Yep - figured that out, what I was measuring that was open circuit was the switch. Sender for the gage is up on top, when I short the wire the to this sensor the temperature gage goes full scale. Sender resistance measures around 7K ohms, but is erratic (resistance wanders around). Unfortunately, the Komatsu parts book shows a plug in this location (no sender), so I have to look elsewhere.
 

berky

Well-Known Member
Joined
Jan 10, 2017
Messages
83
Location
Indiana
If it is like mine the gauge setup is not like usual temp gauges on almost anything else. When at operating temp my temp gauge is right at the limit of full travel. I originally thought the gauge might be bad but it met specs if I remember correctly. I even took sender back out set it up in a water bath at 175 and the gauge was at full swing.
I didn’t like not having any area of movement to indicate that it might be overheating. I ended up putting a low ohm pot at the gauge and dialed in a little resistance till gauge was about 3/4 swing at operating temp
 

Shblack

Well-Known Member
Joined
Feb 22, 2015
Messages
59
Location
Tennessee
If it is like mine the gauge setup is not like usual temp gauges on almost anything else. When at operating temp my temp gauge is right at the limit of full travel. I originally thought the gauge might be bad but it met specs if I remember correctly. I even took sender back out set it up in a water bath at 175 and the gauge was at full swing.
I didn’t like not having any area of movement to indicate that it might be overheating. I ended up putting a low ohm pot at the gauge and dialed in a little resistance till gauge was about 3/4 swing at operating temp

I went digging through my junk today looking for a pot, unfortunately my shop flooded last summer and a lot of parts were ruined. Would you happen to have the part number for the water sensor? I haven’t found anything on line that I have high confidence in.

one more electrical gremlin after this - need to figure out why the AC clutch isn’t getting power.
 

Shblack

Well-Known Member
Joined
Feb 22, 2015
Messages
59
Location
Tennessee
Got the sensor on order, thank you for the number! BTW - got to thinking yesterday, by eliminating the safety module we also eliminate two parts that really need to be there - the diodes across the coils of the starter relay and start solenoid. Without these diodes, a negative going high voltage spike occurs when the contacts (or start switch) opens, leading to arcing and premature contact failure. These diodes shunt the back-EMF from the relay coils when the contacts open. A 1N5400 diode should work, banded end goes to the 12V side, they are widely available.
 
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