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Old 11-24-2016, 10:07 PM   #1
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Tach Adjustment

The tach on my port Lehman, driven off the alternator, reads about 150 rpm on the high side. I tried using a screwdriver in the adjusting slot but it is already at the stops to decrease the needle. I can make it read higher but not lower. I can always keep my cheat sheet but I would like to make the tach accurate as the one on the other engine is also off, but by a different number. I usually set the port engine to the known rpm and sync the other one by sound.

Any ideas on how to fix this?
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Old 11-24-2016, 10:19 PM   #2
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First I would check the engine rpm with an optical tachometer to be sure of the "real" rpm. I have no doubt about your musical ear but 100 rpm by difference just by comparing the sound of the other engine looks highly hypothetical to me.
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Old 11-24-2016, 10:21 PM   #3
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I did use an optical tach to verify the actual rpm
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Old 11-25-2016, 05:52 AM   #4
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Quote:
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First I would check the engine rpm with an optical tachometer to be sure of the "real" rpm. I have no doubt about your musical ear but 100 rpm by difference just by comparing the sound of the other engine looks highly hypothetical to me.
While I have tin ears, many synch their engines not by listening for RPM, but by the thrumming of harmonics caused by the drive train.

Many captains seem to be able to get it to 50 RPM or less, even closer.

I have the same tach issues, but as a single...they are upper and lower......I am comparing it to the photo tach only and wondering if there is something else to adjust the tach if maxed out or replace.
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Old 11-25-2016, 06:22 AM   #5
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While I have tin ears, many synch their engines not by listening for RPM, but by the thrumming of harmonics caused by the drive train.

Many captains seem to be able to get it to 50 RPM or less, even closer.

I have the same tach issues, but as a single...they are upper and lower......I am comparing it to the photo tach only and wondering if there is something else to adjust the tach if maxed out or replace.
Yes synching by the harmonics is pretty easy to do.

I would check the ground connections, making sure they are good or NEW and see if that changes the tach reading allowing adjustment.
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Old 11-25-2016, 06:30 AM   #6
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You could send the tach out to a specialist. Google tachometer repair. There are lots of companies out there.
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Old 11-25-2016, 06:49 AM   #7
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Is your alternator belt a V or ribbed? How does the alternator belt fit the pulley? A V-belt settles at a different radius from the axle depending on the width of the belt and the width of the pulley's groove. As this radius changes due to belt wear or just differences between manufacturers, the ratio of crank shaft revolutions to alternator speed will change and throw off your tachometer. Your harmonic balancer (pulley on the crankshaft) is probably a larger diameter than the alternator pulley so while this effective belt diameter changes with belt thickness as well, it is at a different %.
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Old 11-25-2016, 07:42 AM   #8
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Good call Gdavid.
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Old 11-25-2016, 01:21 PM   #9
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The screw is not to adjust the RPM display, it is to set the meter at zero with the engine stopped.


Your tack probably has a switch to match it to the number of cylinders it is connected to. Work the switch back and forth rapidly a few tomes to clean off any possible corrosion, then set it to match the engine. If that doesn't help, send it to a tach repair shop.


If you think it has something to do with the pulleys or belts on the engine, swap the tachs and see what happens.
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Old 11-25-2016, 01:37 PM   #10
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Here is an adjustment paragraph for teleflex tachs that includes pole setting AND potentiometer adjustment for accurate RPM.

My VDO tachs have the same adjustments.



tor.If the number of alternator poles is not known, measure actual RPM with a Master Tachometer. Rotate tach selector switch to the letter that puts the tach closest to actual RPM. Final calibration adjustment requires a Remote Master Tachometer. Connect the master tach and start the engine. At about 1,500-2,000 RPM, use a jeweller's screw driver to carefully adjust the potentiometer in the "CAL" hole (about 10 o'clock on the rear of the tach), bringing the tachometer into agreement with the Master Tach. Only a minimal amount of turning is required.*Overtorquing*of adjustment will damage gauge mechanism. Turning clockwise increases tach reading, counter clockwise reduces reading. If the tach cannot be brought into calibration, the black Range Selector Switch is probably on the wrong setting.

http://www.jamestowndistributors.com...t.do?docId=837
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Old 11-25-2016, 03:18 PM   #11
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Take bracket off the back and see if it is covering a port where there are dip switches. May need to change range, then dial it in with the cal pot. If no joy, you may want to shop for another tach.
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Old 11-25-2016, 05:42 PM   #12
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Some older tachs just have the 2 ports for small screwdrivers.

Mine just have the 2 screwdriver ports and I remember setting a few 2000 vintage Sea Ray tachs that didn't have the dip switches either.

Guess it may depend on age and/or make.... just keeping someone from going crazy looking for the dip switches.
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Old 11-25-2016, 06:38 PM   #13
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The tach could be correct for what it is measuring.
Use your photo tach to measure crankshaft speed and alternator speed.
Alt speed / Crank speed gives you a number like 2.763. This is the speed ratio between the crank and alternator. Do this at 1000 rpm, whatever midscale is on the tach, and a higher speed like 2500. Compare ratios for both engines to make sure they are the same. I would guess the port side is numerically higher meaning faster turning alternator for a given engine RPM.
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Old 11-26-2016, 01:37 AM   #14
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I suggest you use a photo tach to measure alternator rpm. Then you will know if the dash tach is correct or not.
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Old 11-26-2016, 05:53 AM   #15
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I suggest you use a photo tach to measure alternator rpm. Then you will know if the dash tach is correct or not.
Post #3 .....OP did use one.
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Old 11-26-2016, 10:40 PM   #16
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Post #3 .....OP did use one.
Yup. I read that as he checked engine rpm.
Someone introduced pulley sizes etc. Checking alternator rpm will confirm or rule out that set of variables.
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Old 11-27-2016, 05:42 AM   #17
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Who hear adjusts their belts to keep tachs accurate?

I tighten them when loose, but never did or even heard of doing it for the tachs.

Plus I am pretty sure, that even as the belt wears, Rpm remains the same or the ratio changes so little it really doesn't matter.

I thought even pulley size is to keep the Rpm range from being to low or too high, the adjusting screws should be in range or there might be something else wrong
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Old 11-27-2016, 06:52 AM   #18
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Who hear adjusts their belts to keep tachs accurate?

I tighten them when loose, but never did or even heard of doing it for the tachs.

Plus I am pretty sure, that even as the belt wears, Rpm remains the same or the ratio changes so little it really doesn't matter.

I thought even pulley size is to keep the Rpm range from being to low or too high, the adjusting screws should be in range or there might be something else wrong
Probably a couple. Its just another one of those annoying things that's not ever going to be right. The OP is trying to determine why there is a difference between port and starboard. Something as simple as two different manufacturers belts riding the groove higher/lower could make that small difference since the alts are not gear driven.
For me, replace both belts, then find the tach that's most correct using the photo tach. Use that to set the rpm. Sync by ear.
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Old 11-27-2016, 11:36 AM   #19
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Who hear adjusts their belts to keep tachs accurate?

I tighten them when loose, but never did or even heard of doing it for the tachs.

Plus I am pretty sure, that even as the belt wears, Rpm remains the same or the ratio changes so little it really doesn't matter.

I thought even pulley size is to keep the Rpm range from being to low or too high, the adjusting screws should be in range or there might be something else wrong
I agree it would not be a means of fine adjustment but I was pointing it out as a possibility of why he is outside of the range of adjustment at the gauge. Assuming that it was correct at some point and the other engine is configured the same way there must be a reason it changed. If there is a selector for number of cylinders (for gasoline engines), I would expect it to alter the reading by 50 or 100 percent which would be too drastic if he is only 150 rpm off.
My belt theory would not be my first go to to check but I was offering up a explanation that had not been considered.
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Old 11-28-2016, 10:24 AM   #20
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The belt adjustment is for keeping the proper tension on the belt to provide effective load transfer without putting undue stress on the bearings in the alternator, pump, etc.


It's not something to be done to calibrate the tach.
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