Tachometer is out of Calibration

   / Tachometer is out of Calibration #1  
H

HBN

New member
Joined
Feb 2, 2013
Messages
4
Location
twinsburg OH
Tractor
John Deer 2520
i have a 12 yr old JD 2520 tractor. The cable had broken. I found that the tachometer shaft was very difficult to turn. I freed it up with WD 40. In the process I rotated the shaft backwards and forwards slightly several times. It now turns freely. However, when the engine runs, the tach indicates a much higher speed than it should. Can this be corrected? I heard that it may be due to oil in the tach's input shaft bearing.
 
   / Tachometer is out of Calibration #2  
If the tachometer was hard to turn in the past friction may have caused it to read low. Now you say it's reading higher than it should. I don't mean to offend but how do you know it's reading high? Have you borrowed or used another tachometer to learn the actual rpm? Perhaps a non contact tachometer would help.
 
   / Tachometer is out of Calibration
  • Thread Starter
#3  
Roadworthy said:
If the tachometer was hard to turn in the past friction may have caused it to read low. Now you say it's reading higher than it should. I don't mean to offend but how do you know it's reading high? Have you borrowed or used another tachometer to learn the actual rpm? Perhaps a non contact tachometer would help.
Click to expand...
This morning (0F this morning) the idle speed was indicated as 2500rpm versus 400 or so that it should be. It was somewhat lower the other day when it was warmer. So, I suspect it's related to the WD-40 I used to free up the tach shaft. The bottom line is that I will probably have to replace the panel. I can get the panel out of the tractor easily enough (without removing the steering wheel). However, do the harnesses for the panel have connectors in that area so it is easy to disconnect the panel from the tractor.
 
   / Tachometer is out of Calibration #4  
Yep, 2500 does sound a bit high. Since that was the indicated rpm what was the ACTUAL rpm? Is the panel available for a tractor that old? At what cost? Is the panel actually at fault or does the fault lie elsewhere? You may have a problem but a bit of troubleshooting is in order.
 
   / Tachometer is out of Calibration #5  
New cable about $40. Might that be what needs replacing?

Harness will have connectors at the panel.
 
   / Tachometer is out of Calibration
  • Thread Starter
#6  
beenthere said:
New cable about $40. Might that be what needs replacing?

Harness will have connectors at the panel.
Click to expand...
The cable was replaced with a new cable. The wire bundles coming out of the panel are hard wired to the panel. I don't know where the connectors are located for the cables. Are they in the same basis area? One would think so; but I would like to know before I order a new panel.
 
   / Tachometer is out of Calibration #7  
   / Tachometer is out of Calibration #9  
Before electronics, old speedos and tachs used "flying magnets" under a drum to move the needles. Some had magnets on the drum and the cable spun a yoke. The drum would have clock spring dampeners and/or friction disks. If you got oil in the friction disks, the gauge would generally read LOW, not high. I suspect your gauge may be dried out from not being used or a clock spring may have broken.

It's hard for me to believe JD would still be using mechanical gauges on a tractor only 12 years old though. Electronic gauge drivers/stepper motors have been around much longer than that.
 
   / Tachometer is out of Calibration
  • Thread Starter
#10  
The tachometer in the tractor apparently uses an aluminum can over a magnetic rotor. To free the tachometer rotor shaft we used WD-40. In the process of cleaning the shaft the fluid apparently got in that assembly causing increased drag between the rotor and can causing the tachometer needle to read high. After spraying CRC brake cleaner into the rotor assembly and thus removing the WD-40, the tachometer works as it should.
 
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