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Old 04-24-2016, 07:04 AM   #1
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Lehman 120 Tach Issues - Different Readings

Hi all. I only have about 10 hours on my new to me 78 Marin Trader 34. I am still trying to find the "sweet" spot for cruising RPMs. So any advise for that one.

But main question for this post: I noticed my upper helm gauge is showing 200 Rpm more than lower gauge. They both "stick" a bit upon start up as well. Before I start chasing down wiring etc. I wanted to see if others have had similar experiences.

Not sure if original gauges, but the look like they maybe.

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Old 04-24-2016, 07:12 AM   #2
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My upper is about 4 to 500 more.

Some can be adjusted, but if the internals are sticking....not sure any adjustment will be permanent.

I planned on replacing all upper gauges....even possibly changing to digital tach because the standard Lehman alternator costs as much/more to rebuild than other new ones so the new tach may not have an alternator take off.
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Old 04-24-2016, 07:38 AM   #3
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Greetings,
Mr. rg. I've found that the three Lehmans I've had so far seem to like about 1250 & 1750 RPM. Just about the same spot as the Motorola tach face "rivets". Bob Smith suggests that 1750 RPM is indeed the "sweet spot" as well.

Mr. ps. The hour meter on one of our Motorola tachs is non functional. I contacted Lauderdale speedometer and they said an internal gear is probably dead and they are NOT available. Do you have a source for same?
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Old 04-24-2016, 08:07 AM   #4
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RG: I'd start and see which tach is accurate, then see if you can adjust the one that's not reading correctly.

Digital Photo Laser Tachometer Tester Non Contact RPM Tach Motor Handheld Tool | eBay

As far sticking upon start up, some tachs are powered from the alternator and take a few seconds to come up.
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Old 04-24-2016, 08:14 AM   #5
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Quote:
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...The hour meter on one of our Motorola tachs is non functional. I contacted Lauderdale speedometer and they said an internal gear is probably dead and they are NOT available. Do you have a source for same?
RTF: We bought new gears from these guys and were very easy to repair our VDO tachs, thanks to Trawler Forum member Dave on m/v Fryedaze. They maybe able to help with your Motorola's.

www.OdometerGears.com
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Old 04-24-2016, 09:12 AM   #6
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Greetings,
Mr. LM. I have contacted Odometer Gears by phone and they are unable to supply for Motorola. Thanks.
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Old 04-24-2016, 11:23 AM   #7
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Quote:
Originally Posted by raymond.goff View Post
Hi all. I only have about 10 hours on my new to me 78 Marin Trader 34. I am still trying to find the "sweet" spot for cruising RPMs. So any advise for that one.

But main question for this post: I noticed my upper helm gauge is showing 200 Rpm more than lower gauge. They both "stick" a bit upon start up as well. Before I start chasing down wiring etc. I wanted to see if others have had similar experiences.

Not sure if original gauges, but the look like they maybe.

RG




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Www.libertytrawler.com

RG,
don't know why but it seems to be very common that lower and upper tachs differ.
When purchased our AMS we noticed that the upper reading is in the range of only 75-80% of the reading at the lower helm. At the end I checked both by a stroboscopic measurement of the real engines rpm and found that the lower tach shows 115% and the upper tach 90% of the engine rpm, so both are somehow misleading.
Signals come from the alternators, not from the mechanical tachometer adopter as on our previous boat. Was unsure whether the readings at constant engine rpm will change with the load condition of the batteries but couldn't measure any significant changes during some random checks.
Instead of an calibration of the tachs (don't know if it is even possible) I just prepared two tables "translating" the readings to engines rpm, one for the lower and one for the upper helm. Works fine ...

We prefer to run our twin FL120 in a range of 1300-1750 rpm.

And yes, the above given figures of reading differences are average values of starboard and port engines, there is also a deviation between both sides ...



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Old 04-24-2016, 04:40 PM   #8
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Quote:
Originally Posted by RT Firefly View Post
Greetings,
Mr. rg. I've found that the three Lehmans I've had so far seem to like about 1250 & 1750 RPM. Just about the same spot as the Motorola tach face "rivets". Bob Smith suggests that 1750 RPM is indeed the "sweet spot" as well.

Mr. ps. The hour meter on one of our Motorola tachs is non functional. I contacted Lauderdale speedometer and they said an internal gear is probably dead and they are NOT available. Do you have a source for same?
No source yet...
have to look into it...still undecided whether to try and do a 2 alternator setup or just one and go with something closer to 100 amp rather than 65 or whatever mine currently is....

If I can get away with more inexpensive alternators...say one wire jobs...then I will need to figure out a tach setup...there are so many cheap optical ones now....I might rig something simple and inexpensive. Pretty much the last 10,000 miles is start, set throttle, arrive, slow down, shut off.

I could probable get away with no tach and just draw a line on the throttle quadrant and check every once and awhile with a handheld...
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Old 04-25-2016, 06:52 AM   #9
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"I could probable get away with no tach and just draw a line on the throttle quadrant"

Many throttle handles have a ball and a spring to adhust throttle resistance to movement.

These frequently press against a removable curved bit of metal.

A ball hole for idle and one or two for the various cruise speeds could allow throttle reset by feel.

We use this system for idle as a fully closed throttle will shut down the engine.
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Old 04-25-2016, 07:06 AM   #10
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No source yet...
have to look into it...still undecided whether to try and do a 2 alternator setup or just one and go with something closer to 100 amp rather than 65 or whatever mine currently is....

If I can get away with more inexpensive alternators...say one wire jobs...then I will need to figure out a tach setup...there are so many cheap optical ones now....I might rig something simple and inexpensive.
All you have to do is take off of the R terminal on the new alternator. If there isn't one you can do like I did on my old Dodge/Cummins truck and tap off one of the rectifier leads inside the case.
Some tachs come with a mag pickup you mount near the harmonic balancer.
Lots of options available.
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