Yanmar Tach help needed

[sharkwatcher]

I have a Yanmar 2gm diesel W/ magnetic pickup tachometer. Here is the problem: At idle the tach reads 500 rpms, at full throttle in neutral it only reads 1200 rpms. I have verified the rpms at full throttle is 3200 rpms using a laser tach. I have cleaned all the contacts and pulled the mag pickup and cleaned it also with no improvement. I have verified the tach and pickup are proper models for this engine. I am faced with a couple of obvious choices: 1.Replace the pickup $144. 2.Replace the tach $166. 3.Pull new tach wires and use new connectors. 4. Install an alternator triggered electronic tach.

Any other ideas or suggestions?
Any help would be greatly appreciated.

[sailormed]

5. Install a fixed laser tach with remote display.

[alctel]

Is the tach display adjustable? On mine I had to set it to the correct revolutions per second or whatever via dip switches

[sharkwatcher]

No, there are no adjustments on the tach. It is model specific. Probably why it is so expensive to replace.

[kenbo]

Other things to look at:

Is the clearance properly set between the pickup and the flywheel?

Do you know for certain the number of teeth on the flywheel, that's critical for the gross adjustment of the tach. There are number of teeth ranges on the tach and you have to be in right range.

After the gross adjustment gets the reading in the ballpark, there's a pot for fine adjustment.

All else fails get a Tiny Tach https://www.tinytach.com/

Ken

[kenbo]

I've never seen a Yanmar tach that isn't adjustable. There are two adjustments, the gross adjustment where you set the flywheel teeth range to match your flywheel. I believe 2GM's have 97 teeth. And then a fine adjustment made with a pot.

Are your sure you have the clearance set correctly between the pickup and the flywheel teeth? You screw the pickup all the way until it just touches and then back it off 3/4 of a turn to give 1.3mm gap.

All else fails get a Tiny Tach, works off pulses from your fuel injection.

[sharkwatcher]

You are correct about the tooth count being 97 on this model, but there is no adjustment on the back of the tach that I've found and nothing in the manual about adjustments to it. The mag pickup was screwed in and torqued tight. I searched thru the manual and found nothing about adjusting the pickup, only that it's clearance is supposed to be 1.3mm but no way to verify clearance or adjust it.. The manual was the first thing I consulted.

[kenbo]

Sharkwatcher,

In my previous post I told you how to make that gap adjustment. Your magnetic pickup has a 3/4"x16 thread, so 16 revolutions of the pickup = 25.4mm. 0.8 revolution = 1.3mm or a tad more than 3/4 of a revolution back out AFTER the pickup just touches the ring gear. You will have to run the locknut all the way up the pickup to be able to screw the pickup in enough to touch the ring gear.

Typically, on the back of the tach there's a 3 position switch that represents all the different flywheel teeth Yanmar makes. The ranges are 90-120, 120-150 and 150-180. Sound like you could be in the 150-180 range based on your readings.

Ken

[sharkwatcher]

Kenbo, thanks for your help, but, firstly, there is no lock nut on the pickup. It is screwed all the way in. There is no evidence looking at it that it is making any contact with the flywheel teeth. Unless I'm going to put shims under it, then retighten it, it doesn't seem to have any adjustment. Secondly there are no adjustments anywhere on the back of the tach to adjust range that I can find.

[DrivinSteve]

Did the combination of tach and pick-up ever work correctly?
If so, what has changed?
First, check connections and grounds.
If you have a Nippon Seiki tach, it should be marked on the front "P=97" corresponding to 97 teeth. No calibration option for this Yanmar OEM tach.
Before I took anything apart or started buying parts, first I would check to see if the wiring was compromised or affected by some proximity to other newly routed wiring. Anything new installed? Any fine iron filings/dust near the ring gear teeth/pickup face? Next, I would put in a temp jumper harness.
Only then, failing access to more sophisticated diagnostic tools, would I start replacing parts, starting with the one that looks like it is most "corroded" and then cheapest.
Good luck and please do keep us posted on your progress.

[alctel]

Originally Posted by sharkwatcher

Kenbo, thanks for your help, but, firstly, there is no lock nut on the pickup. It is screwed all the way in. There is no evidence looking at it that it is making any contact with the flywheel teeth. Unless I'm going to put shims under it, then retighten it, it doesn't seem to have any adjustment. Secondly there are no adjustments anywhere on the back of the tach to adjust range that I can find.

Do you have a model number for the tach?

[sharkwatcher]

DrivinSteve, This problem came with the boat when I bought it 3 years ago. I can't honestly say if it ever worked properly or when it ceased doing so. You are absolutely correct about the type and model tach. I have thoroughly inspected everything looking for a simple solution such as corrosion, loose wires or damaged wires. Neither the pickup or the tach looks in anyway worn or damaged, that I can see. The only thing that I can add is that the problem is consistent and has persisted for the past 3 years without change.

alctel, the model # for the tach is 128170-91100
The model # for the pickup is 128170-91160

[sharkwatcher]

Probably the easiest next step would be for me to run new wires from the pickup to the tach (2 wires), eliminating any possible wiring issues. If that doesn't correct the problem, rather than replacing the tach and pickup with Yanmar replacements ($310), an alternator triggered electronic tach might be the final solution. I'm more interested in having things work properly and getting accurate rpm readings, that replacing with original oem parts.

[bill good]

Have twin 1GMS & after spending too much on re pitching props to get the correct RPM, found the tachos were both reading the same & both were out of calculation by the same amount!! The dials were marked as P97. Suspect they were always out from day one. Some multimeters can read frequency & AC voltage. That is your best way if you can do so or you have to take it an instrument repair shop. Then you can decide what you need to buy. Wires & plug problems normally result in intermittent readings.The adjustment is inside & needs to be done professionally.
You could just note the difference between the optical results & " carry on".

[sharkwatcher]

Thanks Bill, "carrying on" is what I've been doing so far. it's not something that has to be fixed, but it just really bothers me when things don't work right. I just have to fix it!