Tachometer adjustment

When I set up my alternator with a double pulley and twin belts last spring, I think it put my alternator off by about 150 rpm: I seem to get the same cruising speed of 7.5 kts at 1650 rpm that I achieved at 1800 the previous summer. I presume I can check with a photo tach, but I'm having trouble finding one I can borrow. Is there an adjustment screw on the gauge in the Pilothouse?

I noted someone in an earlier thread had installed a Tinytach. Is this another option for checking engine rpm?

http://www.tinytach.com/pdf/diesel_instructions.pdf

Jim
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Jim, when I changed my alternator, my tach reading changed 200 RPM even though I used the same pulley. It must have been due to the stator count on the alternator. The stator setting on the back of the tach was correct. I found an inexpensive photo tach for under $30 like this.

Buy Digital Laser Tachometer with Pouch (2.5~99999.9RPM Range / 0.1RPM Resolution)

I used it to verify the tach's accuracy and found it to be within 40 RPM at cruise power of 1800. So it turns out my old alternator was giving me an inaccurate reading of 2000 RPM at cruise and now it's accurate. Knowing this made it worth the $30.

I'd send you mine to borrow but it'll cost more than $30 to send it there and back.

FlyWright said:

Jim, when I changed my alternator, my tach reading changed 200 RPM even though I used the same pulley. It must have been due to the stator count on the alternator. The stator setting on the back of the tach was correct. I found an inexpensive photo tach for under $30 like this.

Buy Digital Laser Tachometer with Pouch (2.5~99999.9RPM Range / 0.1RPM Resolution)

I used it to verify the tach's accuracy and found it to be within 40 RPM at cruise power of 1800. So it turns out my old alternator was giving me an inaccurate reading of 2000 RPM at cruise and now it's accurate. Knowing this made it worth the $30.

I'd send you mine to borrow but it'll cost more than $30 to send it there and back.

Click to expand...

In case it is easier....I bought the same tach on Amazon and i think it was a couple of bucks less.

Our Faria tachs had course and fine adjustment screws... but then it also turned out the tachs were only semi-accurate at the "set" RPMs. We set them at 2000, so we were off at idle and off at WOT.


I've since changed to Aetna digital tachs; the setting is simply the number of teeth on the flywheel. Accurate throughout the full range of RPMs.


-Chris

When you replaced your Faria tachs did you have to run new wires for the aetnas?

woodsea said:

When you replaced your Faria tachs did you have to run new wires for the aetnas?

Click to expand...

No, just used the existing power, ground, and signal wires. Very simple swap, and in fact I worried more about the cut-out sizes than anything else. (Turned out to be a non-issue.)

The Faria gauge also had an extra wire for dash lighting, but the Aetna tach has backlighting built in, so that extra wire became unnecessary.

-Chris

I found my alternator driven electrical tach is 10 percent fast across the whole scale using a $30 photo tach. IE 1800 rpm reading is 1620 actual, which is my normal cruising speed of 6.7 knots. I'm just not ready to fool around trying to adjust a pair of 30 year old tachs.

Alt driven tachs are always off. No way for them not to be.

IF accurate RPM is required ask the engine builder how to hook up a tach sender , and purchase a tach that can read the sender.

After adjustment , this gives most accurate readings .

I check my tachs every season and adjust them if necessary. I had to adjust them both last season when I changed to a Balmar alternator.
It's really not that hard to do. I set the engine to somewhere around 1800 rpm then adjust until I get within about 25-50 rpm. That's close enough.
Then I check them at idle speed and they stay within that 25-50 rpm range.
My experience is that once they are set they hold pretty well.

jleonard said:

I check my tachs every season and adjust them if necessary. I had to adjust them both last season when I changed to a Balmar alternator.
It's really not that hard to do. I set the engine to somewhere around 1800 rpm then adjust until I get within about 25-50 rpm. That's close enough.
Then I check them at idle speed and they stay within that 25-50 rpm range.
My experience is that once they are set they hold pretty well.

Click to expand...

Same here. Our adjustment is on the back. I checked our $11 tach against a $50 tach and it was right on. If the idle speed is off, the tach's mechanical zero or needle rest could be off and that does take some disassembly to set the needle stop. Not hard but just a heads up.

Here's a tutorial that Dave on fryedaze did that shows the disassembly of a VDO tach.

http://www.odometergears.com/documentation/Hour_Meter_How_To_Repair.pdf

Tach/sender for Lehman 120

FF said:

Alt driven tachs are always off. No way for them not to be.

IF accurate RPM is required ask the engine builder how to hook up a tach sender , and purchase a tach that can read the sender.

After adjustment , this gives most accurate readings .

Click to expand...

We have always had issues with the alternator driven tach on our Lehman. Once the batteries are charged the tach pulses as the regulator turns on and off.
Any recommendations for a tach/sender combination for this engine?

Same on my Perkins, Doug. When I increase the electrical load on the batt, the increased alternator output stabilizes the tach. If it's the start bank, I close (switch on) the combiner to share the charges and loads.

I was at Brighouse electric with my winch the other day. They work on starter motors, alternators etc, and they put the double pulley on my alternator last spring. I raised this issue with them and they confirmed what has been mentioned here...that alternator tachs are unreliable and there are too many variables that affect the accuracy. I will probably give them a quick calibration if I can and get a more reliable instrument. That "Tiny Tach shoukd like one option.

Also, it seems to me that engine hours might be affected too. I noticed that sometimes my hour meter indicates more hours at the end of my trip than my recorded hours in my logbook, and the engine isn't sitting around idling more than 15 minutes or so bed or we get underway.


Jim
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I found a digital laser tach on Amazon.ca, for $44 Cdn, including express shipping. More than what it would cost in the US, but I got it nonetheless. I wasn't able to find one at Canadian Tire or Lordco. Princess auto had one advertised for $60, but none in stock.

http://www.amazon.ca/dp/B000I5LDVC/ref=pe_386430_126088100_TE_item


Jim
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Ok. The tachometer is a Motorola model. I dropped a panel down and the back of the tach looks like page 2 of this doc., and I've located the calibration screw, so should be able to figure it out. I'm going to adjust the calibration so it is accurate for 1800 rpm, and then measure rpms at each 200 increment on the dial to WOT.

http://www.westerbeke.com/mdtools/service_bulletins/sb_148.pdf

Jim
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I got one of those Laser Tachometers from Amazon and it works like a hot damn! Here's the calibration curve and it's not linear (no surprise). It's probably some sort of a quadratic relationship. I shall have to see, but I will calibrate the tach at 1800 and resample at different readings.



Jim
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I calculated the % error and they lie fairly close to each other...within about 2.7% margin of error. Here they are:

800 - 12.9%
1000 - 13.1%
1200 - 14.3%
1400 - 16.6%
1500 - 16.1%
1600 - 17.9%
1800 - 18.3%
2000 - 16.5%

Your total average error is 15.7%. Your average error at the 14-1800 RPM cruise range is 17.2%.

Yes, but the % deviation is increasing with RPM. I will calibrate the tach for 0 deviation at 1800 and check the deviations at low idle and at 2400. WOT was 2650 when the tach was accurate before I changed the alternator pulley and threw it off.


Jim
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Are you planning for a new pulley?

No, I already did that last year. I was advised by several people, (a mechanic, a shipwright and the alternator repair outfit I go to) to put a double belt on my alternator and that required installation of a double pulley. That put the calibration of the tachometer "out of wack" and I pretty much guesstimated the cruising RPM's last summer. I figured that that 1600 on the tach was actually 1800, and I was close based on the measurements I took today.

Approximating the engine RPMs from the alternator isn't ideal, I now realize.


Jim
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+1 on the Tiny Tach. I love it.

FF said:

Alt driven tachs are always off. No way for them not to be.

IF accurate RPM is required ask the engine builder how to hook up a tach sender , and purchase a tach that can read the sender.

After adjustment , this gives most accurate readings .

Click to expand...

How does an alternator-driven tach get signal to the display? Doesn't it use a sender of some sort?

-Chris

Tach gets signal from the AC part of the alternator, its really a 3phase alternating current generator, and a connection is given directly to the AC windings. The frequency on this tap is proportional to rpm.

All alt driven tachs should have a fine cal screw to dial in readings.

Ski in NC said:

The frequency on this tap is proportional to rpm.

Click to expand...

Does that mean generally only accurate at a specific RPM level?

Our original analog Faria tachs were adjustable -- sorta -- but the result was that they were accurate at the set point (2000 RPMs was what we tried for), and then not as accurate at the 600 RPM idle and 2600 (actually about 2700) RPM WOT levels. That was using a magnetic sender...

-Chris

A good tach once dialed in will maintain its accuracy within a few percent across the rpm range. All it needs to do is display a reading proportional to the signal frequency, regardless of the source.

A cheap tach often can only be dialed in to be accurate at one rpm. Farias are not known for high quality.

My alternator driven tachs have an adjustment on the back to select the number of stators. I believe mine are set at 8 stators and my RPM at 1800 indicated is actually 1840. I can live with that 2% level of accuracy.

I was able to calibrate it just fine with the laser tachometer and the set screw at the back of the tach. It's close enough to within 2%. With the imprecision of the throttle adjustment, together with the analog tachometer dial, it's close enough, IMO.


Jim
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