Tachometer Problem

[Gregm]

I own a John Deere tractor with a tachometer problem. It does not start to work until some component inside warms up. If I start the tractor in the shed it does not work. If I leave the tractor outside in the hot sun, it works when I start the tractor. The battery connection is fine. The wiring to the magnetic, pick-up sensor is clean and connections are good. I have replaced the magnetic, pick-up sensor. Not starting the tractor, just turning the key on, the needle on the tachometer moves to zero. The tachometer is analog. The wiring to the tachometer is good. When the tachometer is not working, while the hour meter shows hours the hours do not increase. The hour meter is part of the tachometer. The tachometer has a switch on the back for 3 and 4 cylinder I have insured it is in the 3 cylinder position. The needle is not touching anything. The 2 other gauges, lights, turn signals and warning lights in the cluster work fine.

Any suggestion which component would act this way? Thank you.

 

[Resqueline]

It could be as simple as a bad solder joint, and of course anything else. The big question however is if it's possible to open up the tacho?
A picture or two of the guts would go a long way in taking a guess at what it might be.

 

[Gregm]

Tachometer Picture

Not the best picture, plus I had to crop the instrument cluster to only include the tachometer part to make it smaller. Hopefully I attached it correctly. I have opened it up a couple times and have not seen a bad joints. Granted I do not have any experience in this area.

Thank you for your help.

 

[LTX71CM]

The black cylinder top right between two resistors looks interesting. Can you get a close up of it or perhaps a description of what's on top of it?

 

[Gregm]

Reply to LTX71CM

What you are referencing is or was the switch for 3 or 4 cylinder engine. It just had a little slide on the top which I broke off, trying to figure out what was wrong. So I soldered the two poles together. Granted it is not the best job in the world but it does seem to do the job. The tachometer is currently in the tractor. I have purchased a new tip for my solder iron and smaller solder size and the next time I take it out of the tractor and apart I plan to clean that up. With an ohm meter I tested that my solder joint is holding and is not touching the bottom pole.

Thanks for looking at it.

 

[Resqueline]

Not easy.. So the needle moves to zero wether it's working or not? How does the mechanism look behind the four nuts? Is the hour meter electronic (LCD) or mechanical?

 

[Gregm]

Reply To Resqueline

Resqueline, yes the needle moves to zero no matter if it is working or not. The tachometer is an air-core meter. If the tractor is off, I can tap on the glass and the needle will move off zero and return to zero when the key is in the run position and the tractor has not been started. Once the tractor is started the needle will not move off zero when I tap the glass. I have removed the 4 nuts and cleaned up the contacts. The face plate has little rivets holding it to the cluster. I have not removed them. The hour meter is LCD. While I can not see in there very well, it would appear that the hour meter has its own circuit board. The manual says the hour meter only moves when the tractor is running. Thank you.

 

[Gregm]

Another Picture

Not shown in the picture, there is 2 large capacitors, which are located between the backside of the face and cluster. Could a going bad capacitor cause my problem? If yes, is there a way to test capitors with a ohm/voltage meter?

 

[Resqueline]

Bad caps can indeed cause problems like that. If the meter doesn't have cap test then you could try setting it to the highest ohm range and see if it moves from low to high.
One of the big cap's could in theory be a backup "battery" for the hour meter so check for voltage first.
The needle always moving to 0 indicates that the large IC driving the meter is ok, but I'm afraid I don't recognize the numbering system on that IC.
The other small IC (U3) is probably an amplifier for the magnetic pick-up sensor. The numbering on that one starts with LM - but then I can't see the rest.
If you can measure some voltages around the circuit as it heats up we might be able to locate the offender. Using freezing spray (when it works) might also be very helpful.

 

[Gregm]

Reply to Resqueline

Resqueline, my ohm meter is digital will that work or will I need an analog meter? The big IC is CS-8190. I found the following on a web-site. The CS8190 is specifically designed for use with air−core meter movements. The IC provides all the functions necessary for an analog tachometer or speedometer. The CS8190 takes a speed sensor input and generates sine and cosine related output signals to differentially drive an air−core meter.

I have need of the tractor this week, so it will be a week before I can take the cluster apart.

Thank you for all your help.

 

[Resqueline]

It'll work as well, but you won't be able to tell the capacitance or ESR anyway, only if there is some capacitance or not (catastrophic failure).
The circuit is simple and looking at the data sheet I have doubts if any of those cap's would fail in the manner that the symptoms would require.

The CS8190 is an IC that has eluded my eye until now but it looks like a simple-to-use and robust circuit. Still it could be that IC being faulty.
However, measure the Sine coil of the gauge first (when cold) to see if it has a break.
Then check that the voltage across the protective zener (or Vcc at pin 8) is between 8.5 & 16V.
Check that the reference voltage at pin 14 (Vreg) is 7V.
With the engine running check if pin 2 (SQout) has a DC voltage of around 3.5V and also some AC voltage.
Check if F/V out (pin 15) varies (with RPM) between 2 & 6V.

We'll wait for you to get back, and in the meantime you could also try to get hold of some freeze/cold spray if you can as it would be a very useful tool in this case.

 

[Gregm]

Reply To Resqueline

Resqueline, the tractor is the size of a mid-size car, located in a shed and I do not have an oscilloscope. So I cannot perform the tests you suggested on the IC. I did remove the instrument cluster and open it up. Please know that I have been working on this for about a year. I took it apart tested the two big caps and the test should they were ok. I put it all back to together and problem still exists. I opened up the cluster again. I pushed down on the top of the variable resistor as it did not look like it was making contact. Pushed apart the pins of the little blue capacitor on the tachometer circuit board, this is the only part that I see, that has long pins. Put it back together and it WORKS! It has been work for 4 days now. My fingers are crossed that it will keep working. Thank you for all your help! I need the tachometer to insure that I am running the PTO attachment at the right speed.

 

[(*steve*)]

If something had bent up the wiper on that trimmer, it almost certainly would have affected something.

If pushing it back seems to fix it, I would contemplate replacing the trimmer. The reason is that it would be extremely difficult to bend it back so that it exerts pressure on the track.

If it's working for now, then you could leave it. If it ever starts playing up again, replace this component first

 

[Resqueline]

Glad it's working now no matter what it was, and like steve says. The measurements I suggested doing would only require a multimeter however, like you indicated you had.

 

[Gregm]

I wanted to update my post that it has been 2 months with 20 hours put on my tractor. The tachometer has been working at 100%. The fix was for me to re-solder the 3 connections for the variable resistor. My guess why the tachometer would work after awhile was that the inside of the instrument cluster would warm up just enough to expand the variable resistor wire to make contact.
Thanks again for every ones help.

 

[shrtrnd]

I figure Resqueline is keeping you on the right track, but I've got some input.
I had the same problem with my van, took me a long time to find it. Culprit was a frayed/detached wire at the connector. When it was cold, dead. When the sun came up and warmed things up, it worked. I speculated, as unusual as it sounds, that when the wiring warmed-up, the insulation became flexible enough to allow the wire a better connection (or who knows, less resistance with warm, over cold wire).
My point is, check the actual wires going into any connectors, for poor connection to the terminals.
It's probably in your circuit like Resqueline is working, but one other thing to check.