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Old 07-10-2008, 13:08   #1
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Question Intermittent Tachometers

I have alternator driven tachometers on Volvo MD2030B engines that work "part-time", but I would prefer "full-time" tachs. The sensor wires (gray/black stripe) mount on the "W" post of the alternators. When I did a search on this forum, I found the following post from "Rick" back in 2006:

"Please note that virtually all new engine installations use Hall effect pickups from the engine. If you convert your alternator to an external 3-step regulator in order to properly recover a deep-discharge battery then an alternator driven tach will not function when transiting from acceptance to float and may be intermittent at float from then on."

Since I did convert both alternators from internal voltage regulators, to external 3 stage regulators, Rick's post sounds like it might be my problem.

Has anyone else solved this problem without replacing tachs and senders? I really don't want to replace both of them.

If I do need to replace tach & senders, what type would you recommend that would be an easy retrofit?

Thanks for any help you can provide.
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Old 07-10-2008, 13:43   #2
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This failure mode is shown by SOME alternator/regulator combinations, but it is certainly not true of all. If this is your problem, you might have an easy fix if your external regulator has a "tachometer" output. Many do. Then you attach your tach to that instead of directly to the alternator.

Most Balmar regulators have this connection available. Check your manual.
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Old 07-10-2008, 14:33   #3
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This failure mode is shown by SOME alternator/regulator combinations, but it is certainly not true of all. If this is your problem, you might have an easy fix if your external regulator has a "tachometer" output. Many do. Then you attach your tach to that instead of directly to the alternator.

Most Balmar regulators have this connection available. Check your manual.
GreatKetch, Your advice probably saved me many hours of frustration and lots of my hard earned dollars. Thanks a bunch!

I have Xantrex digital alternator regulators and sure enough the manual says: "connect one end of the white stator wire to the alternator and the other end to the regulator. A Tach Out terminal is provided for connection to your tachometer."

As much as I hate it, I really need to read directions on stuff.
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Old 09-10-2008, 11:01   #4
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I wired up the alternator to voltage regulator on starboard engine and tach didn't work at all. Not sure whether the tach itself is bad or whether the voltage reg isn't compatible.

Before I try to rewire the port regulator I need some advice on which terminal on the alternator to connect to the voltage regulator (VR). Xantrex manual says to connect stator terminal to VR, but the current wiring to the tach has its own terminal labeled "W". Which terminal do you think I should connect? stator or tach terminal?

More background: On the starboard side, I connected the stator first and it didn't work. Then I tried the tach terminal and it didn't work either. It's possible I screwed up the tach by hooking up to the stator first. I called Xantrex and they thought the stator was correct, but weren't certain. Before I potentially make the same mistake on the port side, I would like some better advice. What do you think?
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Old 09-10-2008, 11:39   #5
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An alternator-type tachometer is hooked up (grey wire) to the Stator ďAC tapĒ on the alternator.
This connection can be marked as: AC, AUX, S, R, W, TACH, or nothing at all.
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Old 09-10-2008, 14:24   #6
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An alternator-type tachometer is hooked up (grey wire) to the Stator ďAC tapĒ on the alternator.
This connection can be marked as: AC, AUX, S, R, W, TACH, or nothing at all.
Gord, Thanks again for your help. Unfortunately, I'm a little slow in sorting out this wiring. I do have an alternator-type tach. I'll try to describe my situation in more detail.

The original wiring has a grey wire with a black stripe going from the alternator terminal "W" to the back of the tach. There is another pure grey wire coming out of the tach, but the wiring diagram doesn't show where it goes which is another mystery. There isn't a pure grey wire connecting to the alternator. The alternator also has a small terminal labeled "D+" and I believe that is the stator terminal (when I had the alternator modified to accept an external regulator, the guy who worked on it identified it that way).

The new wiring plan, according to Xantrex voltage regulator manual, has the wire coming out of the tach going directly to the electronic voltage regulator post labeled "tach out" instead of going to alternator. A new white wire comes out of the voltage regulator from a post labeled "STATOR" and connects to the alternator. That's the connection that has me stumped. Does it connect to the "W" terminal (original tach connection) or to the "D+" terminal which I believe is the stator? If I hook it to the wrong one, will I damage the tach?

Gord, you used the term "Stator AC tap". Is that the same as the stator terminal or is it an additional terminal? Do I use "D+" or "W" ...that is the question?
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Old 10-10-2008, 20:09   #7
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Stumped

I decided to hook up the voltage regulator "stator" wire to the "W" post on the alternator. The tach didn't work at all when I started the engine so I don't think the Xantrex voltage regulator is compatible with the VDO tach. Now I'm back to square one since I tried the other "D+" terminal on the stbd engine and it didn't work either.
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Old 10-10-2008, 21:06   #8
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The VDO and any other tach will read the sine wave generated by the alternator when attatched to one of the stator output poles before it is rectified fom AC to something that resembles a DC output. This connection is labled (as Gord mentioned) W etc. Got Volvo? Look for the "W".
The Tach will only work if the Alternator is engergised which means that the new controlers can switch the alt off. = no signal.

Get out the Voltmeter and set it to AC. Connect to the "W" terminal and the ALT. CASE OR ENGINE (gROUND) You will see a signal off the AC tap ("W" Terminal ) if it is there. It will vary with the RPM
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Old 11-10-2008, 07:33   #9
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The VDO and any other tach will read the sine wave generated by the alternator when attatched to one of the stator output poles before it is rectified fom AC to something that resembles a DC output. This connection is labled (as Gord mentioned) W etc. Got Volvo? Look for the "W".
The Tach will only work if the Alternator is engergised which means that the new controlers can switch the alt off. = no signal.

Get out the Voltmeter and set it to AC. Connect to the "W" terminal and the ALT. CASE OR ENGINE (gROUND) You will see a signal off the AC tap ("W" Terminal ) if it is there. It will vary with the RPM
Will, Thanks for the help. I will check the output of the "W" terminal. I got it now "W" for "Wolvo." You also gave me another idea. When I ran the engine yesterday and the tach didn't move, I shut it down right away. The voltage reg has a built in time delay before it activates the field and then "ramps-up" the alternator output gradually. Perhaps the tach will work if I wait until the alternator is fully energized. That would be too easy.
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Old 11-10-2008, 07:52   #10
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Greg, If you have or will change alternators you will want to check / adjust the Tachos with a hand held photo tach or mechanical as the pully ratio or number of poles may change. When checking a tach always adjust it at or around your running RPM and do not try to get the pointers to match at Idle (They may but the running rpm is more important)

VDO made/makes many different styles. Some have a simple potientiometer on the back while others (with the digital hr meter) are adjusted using a button on the back and the LCD on the dial face.

Good Luck
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Old 11-10-2008, 10:57   #11
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Lightbulb Progress

I got the port tach working through the voltage regulator (connected to "W" terminal) and it stayed on consistently for an hour until I shut it down. That's the good news and I think the tach issue is solved on the port side. Will, thanks again for your help. I checked the AC voltage coming into the back of the tach and it showed 16 volts at 1500 RPM so now I know what to look for on the starboard side.

The bad news: The port alternator is being powered by a circuit wired by Captain Ron when Cat Tales was in charter. The real interesting part of this circuit is when I shut off the ignition switch, SOMETIMES everything on the ignition circuit stays powered. I have to physically disconnect two wires from the engine "idiot light" circuit box to depower the ignition. None of this is on the engine wiring diagram so I have some work to do in tracing wires and correcting the "spaghetti" going to the engine instrument panels

When I ran the starboard engine the tach still isn't working although it is wired exactly the same as port. I checked the AC voltage and it was .6volts. Then I realized the starboard alternator wasn't charging so that may be the culprit. That will be this afternoon's project.

Although I have lots more work to fully cure this problem, I think all the great advice from Will, Gord and Great Ketch have given me the tools to finish the task. Thanks again!
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Old 12-10-2008, 10:06   #12
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More Progress

The starboard alternator is working again, but the tach is not. I checked the output of the "W" terminal for the tach and it read 17 volts AC, so my new wiring through the voltage reg is working fine. To prove that, I removed the port tach and plugged it into the starboard tach wiring and it worked fine. Now I know that starboard tach is totally hosed and I need a replacement.

I have one more question for the very smart people on this forum. My tachs have digital engine hour meters and they just blink on an off. When they are on, half of the digits are missing. What provides the input for the hour meter? I have a grey wire going into tach and can't trace it through the wiring diagram. Could that be the input or does it just start counting when ignition switch turns on?
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Old 12-10-2008, 16:08   #13
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Greg, The VDO tachs with the digital Hr meter will begin to count when the power is supplied to the meter 12v + terminal .

Gray is the standard color code (ABYC and other Mfg's) for the tach input. ("W" Terminal)
Blinking Digits etc? Hummm? Look for a small hole in the back of the tach. About 3/16" (3-4mm) Insert a small pin (I use a bamboo scewer from the galley) and see if there is a "click of a switch". If so you can enter a calibration mode if you energise the tach with the switch depresed. See what happens. It may be set on something different and will not read your output.

Does it say Volvo on the front dial face? (Clue)
What is the cutout size of the mounting hole 80, 85, 0r 100mm (Clue)

Pushing the button after teh power is supplied starts the cal program . Waiting a few seconds changes the input to pulse, Alt etc. Play with it and see what happens. You cannot hurt anything and we may get some more clues.

VDO tachs are very reliable and do not fail easily. Do not throw your "Bad" one away just yet.

Will
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Old 12-10-2008, 16:10   #14
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Correction... Energise the tach while the cal button is depressed. See what happens.

(Digital units only)
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Old 12-10-2008, 18:08   #15
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Greg, The VDO tachs with the digital Hr meter will begin to count when the power is supplied to the meter 12v + terminal .

Gray is the standard color code (ABYC and other Mfg's) for the tach input. ("W" Terminal)
Blinking Digits etc? Hummm? Look for a small hole in the back of the tach. About 3/16" (3-4mm) Insert a small pin (I use a bamboo scewer from the galley) and see if there is a "click of a switch". If so you can enter a calibration mode if you energise the tach with the switch depresed. See what happens. It may be set on something different and will not read your output.

Does it say Volvo on the front dial face? (Clue)
What is the cutout size of the mounting hole 80, 85, 0r 100mm (Clue)

Pushing the button after teh power is supplied starts the cal program . Waiting a few seconds changes the input to pulse, Alt etc. Play with it and see what happens. You cannot hurt anything and we may get some more clues.

VDO tachs are very reliable and do not fail easily. Do not throw your "Bad" one away just yet.

Will
Will,

Do you have a Phd in tachometers or something? I really appreciate all your help.

My tach inputs from the "W" terminal come from a gray wire with black stripe, so the pure grey wire is still a mystery.

Here are some answers to the clues. The front face says "Volvo Penta" and the cutout is 88mm so closest to 85mm. The label on top of the tach says "VDO Technik AG Switzerland" and I found the Volvo part # 873992. It has a range of 0 to 4600 RPM. Tach does have the little hole in ithe back and also a slot underneath the hole about the size of a small screwdriver blade. I won't be able to work on it till tuesday so I'll let you know what happens when I energize the tach with the switch depressed.

I hope the bad one can be salvaged because the best price I could find on the internet was $220. YIKES!
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