tachometer display issue

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opie
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Location: Wilmington NC, BF50A Honda, 1999 MacX, "Marin"

tachometer display issue

Post by opie » Tue Mar 09, 2010 6:29 pm

We took the Mac to Lake Wiley, NC/SC the past few days. Never happened before, but the tach would go up to 3200 rpm and then drop down to 1800 and not rise above that even if I went faster. But, the engine sound and speed never changed. The same thing happened when out of gear or in gear. Any ideas? Thanks...

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Buell_S1W
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Sailboat: MacGregor 26M
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Re: tachometer display issue

Post by Buell_S1W » Tue Mar 09, 2010 8:04 pm

Hi there, I presume this is an analogue display gauge? Sounds like a loose or corroded connection rather than a faulty gauge. Check the terminals or plugs @ the back of the gauge & any plugs from the intruments to the engine for corrosion etc. Check any ground wires on the engine for the same. Check the voltage across the positive & neg terminals of the gauge, with the engine running at 1000 rpm expect 14 - 14.5 volts. Let me know how you get on.
Regards, David. :)

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opie
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Location: Wilmington NC, BF50A Honda, 1999 MacX, "Marin"

Re: tachometer display issue

Post by opie » Tue May 04, 2010 4:39 am

I figured out the issue with the RPM suddenly dropping at higher revs.

Short answer: I installed a cheaper aftermarket voltage regulator on my Honda BF50A and the wiring is different for the tach lead.
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Long answer:

My regulator went bad and as best as I can determine, I fried my original voltage regulator by mistakenly turning my 1-2-both-off battery switch to the "off" position while the engine was running. Without regulation, my voltage went up over 16 volts when running. I boiled water out of both my batteries and I had to add water after every trip. I went on the www and found a $50 voltage regulator and installed it. It had 4 wires and the original had 6 wires. The extra two wires were a duplicate ground wire and a separate tach feed. I figured those wires were not necessary. (wrong)

I ran the new reg's two yellow wires to the stator two gray wires. I ran the red wire to 12 v + and the black wire to 12 v -. I connected my old tachometer gray wire to one of the stator gray wires at the yellow/gray new wire nut.

When I installed my cheaper voltage regulator,(link here to installation notes) the tach worked fine. 5500 rpm, no problem. Then I found that the trouble I caused my old batteries did them in and so I bought two new ones, deep cycle, nice. First couple of short trips and no tach problems, Then the funny drop-off of rpms when I got above 3000 rpm, but it seemed that the engine sounded fine, but my mind could be playing tricks and the engine was bad? I kept trying it out and the rpm dropped on the tach but the engine sounded fine.

Thinking a loose wire, I unhooked all zillion wires in the engine, pedestal and boat and cleaned the contacts. I checked for loose ground wires. I used my calibrated shop opto-tach (from my previous working life) and engine rpms were fine. I put on my Fluke 83 meter that can measure rpms and, for sure, the indicated rpms were dropping above 3000. Could not figure it out. I read all over the www on boat forums about rpm's dropping at higher rev's, so was I on to a classic mystery boating electrical story?

I decided it could be a bad tach. I went to Harbor freight and bought one of their $20 tachs called Nitrous. Set it for 6 cylinders and installed it temprorarily in place of my tilt meter on the pedestal. Connected it in parallel with the OEM tach. Same thing. Both tachs tracked perfectly together (surprise to me since the new tach did not have a 3 cylinder selection on the backswitches). And both tachs dropped above 3000 rpm.

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Both tachs tracked nicely together, but the rpm drop was on both of them!

Strange. Then I got to wondering about that missing wire on the new regulator. My guess was that the OEM regulator extra gray wire for the tach is somehow conditioned to block out the functions of the regulator and the effects thereof. By that I mean that the regulator runs up voltage to charge the batteries and depleted batteries take a lot of juice. When a good battery soon gets its fill of voltage and needs no more charge, the regulator somehow shunts or lowers the output voltage or clips it or something so as to not overcharge the battery. It hit me that that is what was happening. The cheap regulator was affecting the waveform that only an oscilloscope could see but was bothering my tach(s) and thereby dropping my rpm.

Ummm. What to do? I decided that the tach was just looking at an up-and-down AC voltage from the stator and a similar up-and-down AC voltage existed on the pulser coil, right next to the stator coil(s) on the flywheel. (Here is where I gambled and a full disclaimer is required. Do not try this at home as it may fry your engine's computer, CDI or ECM. I am not responsible.) I connected the tach wire on the cheap tach to the pulser coil output. So one tach was on the pulser coil and one was on the stator coil.

and here is what I saw, as seen on this video.

Interesting, huh? For those not wanting to look at the video, the tach connected to the pulser coil wires went to 4500 rpm nicely while the tach connected to the stator gray wire dropped to 1800 rpm or so. With no damage done that I was aware of, I rewired my OEM tach to the pulser coil and took out the cheap tach.

And, hooky-malooky, the setup worked. Look ma, over 5000 rpm......
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So, that's my long story and I am sticking to it......

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