Jinma 284 Tach died

[piper184]

2006 Jinma 284 with Y385 engine. All working fine on a rather warm day. Shut down for a cold drink and re-fuel. When started back up tach was dead. No flaky behavior prior to this, working one minute and dead the next.

All other instruments working. Looking at a wiring diagram it appears that the entire cluster gets power from the same source and since that is working, it has to be something else.Disconnected the sensor at the plug and it was in rough shape. Testing the sensor for continuity it was open. Testing the tachometer end at the plug showed about 38 ohms of resistance. Put the ohm meter on AC voltage and spun the engine with the starter and decompression, no change in reading of 0.0 volts.

Removed the sensor and it showed that it had been scraped on the end by what appeared to be ring gear teeth. Not bad, but a minor scratch. This was the factory installed sensor from 850 hours ago, I have never touched it. No debris on the sensor and it had a slight magnetic field to it. I gently filed out the scratches and polished the end and cleaned again. Still no continuity and reinstalled on the tractor. Turned in until it touched, then backed out 2/3 of a turn. Again spinning the engine showed no voltage on the AC scale of the multimeter.

For ~ $40 I have already ordered a new sensor. Even if that is not the problem, it won't hurt to have a spare on hand at that price.

My question is, how did that scratch get in the sensor? Dumb me didn't take a picture before I filed it out. Is it possible that the engine crankshaft has developed enough end play to have damaged the sensor after so many trouble free hours? I can see no signs of other wear or any oil leaking from the rear main seal.

Should I be able to try prying on the damper pulley to check for end play, just in case something more serious is wrong?

Is it possible that the sensor scratch was done at the factory and actually has no role in the sensor going bad?

Any and all ideas are welcome. TIA

 

[Bob Rooks]

The sensor is actually a reluctor. Think of it as a tooth counter. It produces a square wave chopped AC signal. It's the same as a WSS (wheel speed sensor) found in cars and trucks. There is actually no continuity. I had mine out for cleaning and checking awhile back and found a few light scratches, but it still works fine. Most of the ones I've seen are the Hall Effect type with a single pole at the tip. It's possible that you have it backed out too far, 1/2 turn is recommended as tightening the jam nut brings it out a little more. IMO, the magnet has passed the threshold of being effective, so is likely the culprit. That said, my integrated hour meter failed at 650 hours but I wasn't going to spend $200+ for a new unit so I installed an aftermarket meter.

 

[piper184]

Yeah, I figured it was a reluctor. They have a coil inside so I thought there should be some resistance. I even have a listing where someone had measured it and got a little resistance, not just open.
The information I found said either 1/2 turn or 3/4 turn back out after contact. I split the difference. I guess now that I have order confirmation, I can go ahead and risk closing the gap a little. Also, probably wouldn't hurt to cut off the plug and strip the wires back for a little better test of continuity. With the dodgy wire and connectors, I might just upgrade both sides of the plug to a better connector. I will have to see what I have on hand.

 

[Bob Rooks]

Good stuff here.

https://www.fwmurphy.com/uploaded/documents/pdfs/00-02-0181.pdf

 

[piper184]

Thanks for the link Bob. Good info indeed. I find it interesting that Jinma chose to mount the sensor 90 degrees to the ring gear instead of in line with the teeth as shown by Murphy. The Jinma method would seem less effective and is prone to issues caused by any run out at the ring gear. I'm glad they mentioned putting a load on the circuit to test for voltage. Duh, if I had thought it through I would have known that. I bet my el-chepo multi-meter doesn't have sufficient internal resistance to work the way I was using it. Priority this week is haying so it may be a few days before I can update this post.

 

[Bob Rooks]

I wish I'd known about the loading a long time ago too. Wish I still had my Simpson 260.

 

[piper184]

Good news, bad news, more good news, more bad news.

Sensor arrived a day earlier than expected and I had time to fiddle with it this afternoon.

I ordered an 18mm sensor when I should have ordered the 15.5 (16) mm one. I didn't see that option when I ordered the first one. Went at the search from a different direction and DANG, there it was. Dumb ass parts guy in the mirror....

Checked the new sensor and like the old, no continuity at the terminals, But I had a drill motor laying on the bench and happened to notice it had a series of nice little grooves cut in the side of the chuck for easy grip. Looked like teeth to me. Flipped the multi-meter to A/C volts and spun up the chuck. As I moved the sensor tip close to the chuck I could see voltage on the meter. Checked the old sensor same way, Nothing. Pretty much confirms I need a new sensor.

Went back online to order the correct part and got all the way through checkout and my browser froze. I had just updated it earlier today. Backed out and tried again, same thing. Called the nice guy at the parts counter and explained the situation. He checked and there were no orders processed but he took care of it over the phone and I should have the correct parts in a few days.

Sheesh!

 

[Eagle1]

Good to know, easy way to test. Now to remember that when needed.....

 

[Bob Rooks]

I don't see any bad news here, just one instance of not having enough coffee.
Using the drill chuck was a brilliant observation. It's all a win-win. Bravo!

 

[piper184]

Got the correct part in today's mail. Tested with VOM on A/C volts and could see voltage using the drill chuck as a trigger. Installed on tractor and the tach now works!

With the engine idling it was showing about 27 volts on the leads so it is a significant little generator.

Now all I have to do is figure out how to use the mis-ordered part to make a tachometer for my lathe or Shop Smith..... Never waste a good part.