Alternator driven Tachs

[sailingtoo]

Hello all, This is my first post so be gentle with the flames... I have an older boat with a diesel engine. The tach is driven from the Stator post on the alternator. As you know, the tach "reads" the freq of the AC voltage from the alternator that varies with the alternator speed.

My problem is the tach now reads based on the alternator output. I am connected to the STA terminal on the alternator. When the alternator is putting out max output 50 amps or so, the tach reads fast.... maybe 2500RPM rather than the correct 1500 RPM.... Just a guess there.

With the alternator/regulator connected together so the alternator does not put out max, the tach reads high at first cranking of engine when the alternator gives the high output to compensate for starter motor draw. As the alternator drops back to normal charge, the tach starts to read low - 500 rpm where it should be 1200 rpm?

Has anyone ever seen anything like this happen? Any ideas what can cause this to happen?

An alternator is SUSPOSED to have a fixed freq depending on RPM, not amount of charge.

Thanks to all for any and all suggestions.

Ken H>

 

[Stu Jackson]

Welcome. Hope we can help. We eventually do...

Something sounds backwards in your readings. My experience has been that when under initial load, the regulator is telling the alternator to "put out" and sometimes that results in a slipping belt where the tach reads LOWER until the initial surge of amperage gets into the batteries and the regulator tells the alternator to back off.

I would try a couple of things. First, I agree about the stator, but check your wiring. Should go from the stator of the wiring harness from the alternator to the regulator and then to the tach. The wiring diagrams for switching from an old OEM alternator to a new one with an external regulator on a 1986 C34 with an M25 engine are here: http://c34.org/bbs/index.php/topic,4548.0.html

Second, what engine and alternator and regulator do you have? Specificity helps us help you.

"My problem is the tach now reads based on the alternator output....An alternator is SUPPOSED to have a fixed freq depending on RPM, not amount of charge."

That's not what the tach is doing. The tach is essentially "preprogrammed" so that the difference between the larger engine flywheel and the smaller alternator pulleys are taken into account and the tach is reading the engine RPMs even though the tach is working off the faster rotating alternator RPM. The tach is saying nothing about the state of charge. The engine RPMs and the alternator RPMs are proportional and change based on the amount of throttle you provide, which varies from idle to WOT.

 

[chief14326]

I had a new alternator installed yesterday. If anyone followed my post from Tuesday, I just had the new alternator modified to it's original state; internal regulation. The reason for my question is the Tach. I went from a 55 Amp Motorola, to a 70 Amp Leece Neville. I noticed a significant loss of RPM. I knew the additional load would cause a loss. What would be normal? I have not been out and therefore no load has been placed on the engine yet.

 

[Stu Jackson]

I noticed a significant loss of RPM. I knew the additional load would cause a loss. What would be normal? I have not been out and therefore no load has been placed on the engine yet.

The RPM change has nothing to do with the load, unless your belt is slipping horribly. It has to do with the # of poles on the new alternator compared to the old one. You need to recalibrate your tach.

 

[chief14326]

How do I recalibrate the new alternator? The belt is not slipping.

 

[sailingtoo]

Thank you so much for the quick response Stu - you are correct, it sure sounds weird.

First let me add some info here: I installed a Ford charging system well over 25 yrs ago and have been using basicly the same type system since. This is an alternator with 3 terminals, the BAT (battery), the F (Fields), and a STA (stator). Of course there are ground terminal also. This type system uses an external regulator with:

Regulator F terminal connected to Alternator F terminal.
Regulator "A" terminal connected to Alternator BAT terminal (BAT terminal connected to battery)
Regulator "S" terminal connected to switch to provide battery voltage to regulator only when switch is on.
Regulator has an "I" terminal that is not connected as it is used for charge light indication.

The tach sensor wire is connected to the alternator STA terminal to provide freq to tach for RPM sensing. Same place it's been connected for many years (different alternators of course).

A slipping belt will certainly cause a tach to read lower - after all, the tach is really only reading the freq of the AC voltage inside the alternator before the diodes. The freq of the AC voltage depends on number of stators and how fast the stator is turning - correct?

You mention "... your wiring. Should go from the stator of the wiring harness from the alternator to the regulator and then to the tach."

Now just why does the stator wiring need to go to the voltage regulator? What terminal does the STA wire connect to on the regulator?

Now for my problem now: This system had been charging max for the last yr or so, but since boat was not in use for hours long running, I'd not worried about it since I knew the system was charging from volt meter.

This morning I installed a new regulator so the system would charge correctly - now the system does seem to be charging correctly with high charge right after cranking, then tapering off to normal charge after a couple minutes - indicated by voltage reading and mV reading across shunt for ampmeter. Actual amp meter broken, I'll be installing a new 50mV meter later. Remember, an amp meter only reads the voltage drop across the shunt.

What we're looking for here is under what conditions can the stator change freq based on charge rate? There should be no condition that could happen, but that's what seems to be happening. I do not understand it myself.

I had a hard time writing it up so it made sense - I hope this helps understand the problem?

Thank you all for any and all suggestions - this just doesn't make sense to a simple old sailor.

73 de Ken H>

 

[sailingtoo]

How do I recalibrate the new alternator? The belt is not slipping.

Depending on your tach, there is usually an adjustment on the back with a table to show which number the arrow (or some indication) should point to depending on pulley ratio and type of alternator. Then there is usually a pot or something you can do a fine adjustment with. You DO NEED some way to know what the engine RPM actually is to adjust the tach.

Good luck,

Ken

 

[sailingtoo]

What I would love to do is take my O'scope and actually look at the output signal on the STA terminal to see the freq and wave shape.....

Hey, my DVM has a hz setting - says voltage much be between 200mV and 10V - I could use a high resistor and measure thru that to keep the voltage down - maybe that would give me an idea if the freq is doing what it should be doing.... I'll bet the tach is bad.

Thanks to all for allowing me to talk this thru - and I'm still open to any ideas.

73 de Ken H>

 

[Stu Jackson]

1) The freq of the AC voltage depends on number of stators and how fast the stator is turning - correct?

2) You mention "... your wiring. Should go from the stator of the wiring harness from the alternator to the regulator and then to the tach."Now just why does the stator wiring need to go to the voltage regulator? What terminal does the STA wire connect to on the regulator?

3) Actual amp meter broken, I'll be installing a new 50mV meter later. Remember, an amp meter only reads the voltage drop across the shunt.

4) What we're looking for here is under what conditions can the stator change freq based on charge rate? There should be no condition that could happen, but that's what seems to be happening. I do not understand it myself.

Ken

We're gettin' there, I hope.

1) Seems right to me.

2) The reason I gave you the link to my wiring diagrams, which I hope you've read, is that's what the new external regulators show for wiring. Balmar, Xantrex, you name it, they're all pretty much the same. That's why.

3) An ammeter measures the CURRENT across the shunt, NOT voltage. The smaller 0-25 and 0-50 A ammeters have a small enough current to have internal shunts built right into the meter. Anything larger requires external shunts, like the ones you see with battery monitors like Links and Victrons.

4) I agree, because the charge rate has nothing to do with revolutions of the alternator and, thus, the engine.

Check your wiring is the next step from this part of the conversation.

 

[sailingtoo]

Ken

3) An ammeter measures the CURRENT across the shunt, NOT voltage. The smaller 0-25 and 0-50 A ammeters have a small enough current to have internal shunts built right into the meter. Anything larger requires external shunts, like the ones you see with battery monitors like Links and Victrons..

Stu, if you think about it a minute I think you'll agree that an amp meter than uses an external shunt is only measuring the voltage drop across the shunt. Remember some shunts (not all) will be spec'd as a 50mV shunt for full scale on a specific meter.... For instance the high output alternator I ran for a few years came with a wiring harness and amp meter. The #4 cable from alternator to battery was a set length to give a 200mV voltage drop from one end to the other under a 200 amp load. That allowed the included 200mV voltage meter to show 200 amp max charge. At 100 amp charge (or load) there was a 100 mV voltage drop showing 50% on the meter.

I've removed the faceplate from meters and printed off a new scale depending on what the meter was used for. Very handy thing to do sometimes.

Check your wiring is the next step from this part of the conversation.

Yep, that is my next step, recheck some wiring - it will be next week before I can work on this more - I've got family business taking me out of town for the weekend returning Monday afternoon. Maybe Tuesday I'll have more info and post.

Thank you all - and will be checking posts over the weekend for more ideas. As I understand it, a tach is a simple analog freq meter - it's not rocket science!

73 de Ken H>

 

[Stu Jackson]

Oops, my bad, you're right about the shunts, of course, sorry. They're kinda measuring resistance. The Blue Seas catalog has a good description picture attached, probably hard to read. But not an issue here, an ammeter is an ammeter for our measurement purposes regardless of what they're doing or how they do it!

 

[sailingtoo]

Hey Thanks for the link Stu - that is exactly what my amp meter looks like. It would be a plug in fit for my old meter. I was looking thru my junk box for a meter to "make work" with maybe a new scale, but I'll look more to see if they have what I'm looking for.

Heck, I've even taken the movement from one meter and installed in the housing of a different meter to make it work... sometimes because I'm too cheap to buy new, other times because new just was not available.

73 de Ken H>

 

[sailingtoo]

I just had to test a couple of theories about the alternator - was it putting out a freq? Here is what I found.

I checked the alternator output on the STA terminal using the digital voltage meter with the hz position. The freq sure seems to vary nicely from about 500 hz to 2500hz depending on engine RPM. Wasn't real stable, but this meter isn't stable anyway. I may need to take the scope over next week to check more.

I'm just not sure what the answer is. With the engine charging high at first cranking the tach reads pretty good, and continues to read ok until the engine is idled down so the alternator basicly shuts off - then the tach just won't go back up with max rpm being perhaps 1,000rpm, even with the engine turning between 1500 and 2,000 rpm.

With the DVM connected, the hz on the STA terminal consistently varying with engine RPM from low to high, then back to low. I'm just not sure what the deal is.

I took an old tach and drove it with a function (frequency) generator and can make it read from 500 rpm to 3,000 rpm by varying the freq to the tach senser input from 500 hz to around 2,000 hz. That is just as it should be doing. I connected that same tach to the alternator output and it does exactly the same as the tach that's installed - read high on first engine cranking, then settling back to reading very low as alternator settles down to low charge rate.

Maybe the boat just doesn't need a tach???

Oh, I did get the amp meter working - I connected a 0 to 100mA meter and it seems to be reading pretty decent. I think I'd made the shunt to provide a 50mV signal at 100 amps or something like that. Now after the battery is spun down some, the meter will show abougt 60% (60mA) - that's pretty close to this alternator's max output - I think it's a 65 amp alternator. As the engine runs a bit, the meter drops down to 20%, then on down to between 5 to 10% (or amps) which should be about right.... I think anyway.

Later

 

[sailingtoo]

OK, here is a summary of what I've learned and what the "possible" solution might be - use a mechanical regulator, and here's why. The mechanical regulators actually used resistors to drop the voltage for controlling the charge rate, as well as the "OFF 'n ON" of contacts. The signal to the fields was more of a DC voltage. The new electronic regulators tend to use a PWM (Pulse Width Modulation) scheme to control the fields. The PWM gives a 12vdc ON and a zero volts OFF signal with just varying the time ON and time OFF to give the amount of power to fields to control charge rate. This puts some very complex "waves" on the stator signal and as the battery gets charged causes the stator signal to become very unstable and weak causing tach problems. Ample Power has a special regulator to prevent this - I'll check into that more, but I'll bet it's EXPENSIVE!!!

As I think back on the history of the boat, during the '80's I'm sure I was using mechanical regulators and the tach worked just fine. Then around '90 or so I installed a high output alternator (150amp) along with the solid state regulator that came with the alternator - package designed for boats with tachs driven by alternator. That worked for many years. Then a couple or 3 yrs ago the alternator (I'd went thru a couple of alternators by this time and now had a 65 amp Ford car alternator installed) started charging max output all the time. Since this was only 50 to 60 amps into a 400+ amp/hour battery bank it was not big deal as I was only using the engine for an hour or 3 at a time to get out to go sailing most of the time. BUT I was having problems getting the tach to read correctly - it was reading high all the time and could not adjust down as it should be.

This week I installed a new (electronic - all they had) regulator and that's when the tach started reading very low after charge complete. Now I think the reason is the PWM scheme they use in electronic regulators. The idea is to use a capacitor to smooth out the pulsed signal on the field circuit - would 470 (50 wvdc?) uF work? suggestions please?

I just might take the scope over to boat Tuesday to look at the signals, but I do think ya'll have the answer - find an old mechanical regulator somewhere - they do still make those?
Am I understanding the problem now? I can see where those "pulsed" field voltages could certainly cause problems in the stator signal to the tach.

Stu, I'd sure like your comments and suggestions on my line of reasoning here.

Hey, we just might find a solution yet - We're going to find a solution yet!

73 de Ken H> K9FV

 

[sailingtoo]

Just a short update to confirm the last post - I installed a mechanical regulator this morning and it seems to work. Even after the alternator cut back on charging to a very low output, the tach still read nicely - very steady.

Now if I could just get the tach calibrated - the adjustment on the rear is max'd out to lowest reading. The tach doesn't seem to cover the number of poles and pulley ratio on the boat. The MAIN thing is the tach is reading engine RPM at a steady state.

Ken H>

 

[Stu Jackson]

Let's look at the advantages and disadvantages of alternator driven tachs.

Alt. driven: Not always accurate, driven by the # of poles, a pain to adjust the tach to actual RPMs.

Mech.: actually shows engine RPMs.

How simple and easy. Not so fast.

There are advantages to the alternator driven tachs that many may not have realized:

Wonky Tach Performance: It is GOOD because it tells you something about what's happening down below.

You noted in #13: With the engine charging high at first cranking the tach reads pretty good, and continues to read ok until the engine is idled down so the alternator basicly shuts off - then the tach just won't go back up with max rpm being perhaps 1,000rpm, even with the engine turning between 1500 and 2,000 rpm. I think there is a GREAT advantage to this occurrence. If the tach performs as you've noted it means the belt is slipping!!! That's something I WANT TO KNOW! I've recently tightened up my alternator belt [to specs, and properly] but the poor alternator bracket design on our M25 (with the upgraded alternator bracket) results in regularly loosening belts. I have an alternator belt tensioner and it makes it simple to tighten, but it's a pain to do regularly. So I'm working on installing what a number of others of our skippers have done - a support or turnbuckles arrangement to maintain the preset tension and overcome the poor bracket design.

I also installed a toggle switch to deal with SMALL ENGINE MODE. See: http://c34.org/bbs/index.php/topic,4669.0.html It minimizes alternator belt slippage. Read the link in the link, too.

Tach "perfect" calibration is unnecessary: If your engine is 3,000 rpm and all you can get is 2,800 rpm WOT out of gear, it's NOT a big deal - do the proportional math in your head, or build a simple table in a spreadsheet. After a few days, you'll probably end up remembering it - only two #s anyway, WOT and cruising rpm.

So, knowing the precise engine RPM from a mechanical tach may be just fine, but having one off the alternator can tell you a lot of things a mechanical one can't. And provide you with important troubleshooting information, too.

That's my rant for today, no faraday cages or micro-farads involved!

 

[kloudie1]

C'mon, Stu.. There are the other tachs (my 3 GMF Yanmar) that consist of a small magnet in the flywheel whose passing is recognized by a pick-up coil on the aft, port side of the block.. You could talk about Gauss(es) and stuff !! LOL

 

[sailaway78]

Very Confusing

I have been reading these posts with interest, even though I do not have an alternator driven tach. Lots of what is written makes perfect sense, but some if it really doesnt. My understanding is that the alternator driven tachs work off the number of poles in the alternator, I assume counting the individual pulses. So, alternator output it would seem should have no effect on the tach reading. But what I am totally confused about, is the discussion concerning the regulator. Should not the tach be hooked up to the alternator in front of the regulator, direct to the stator. I can certainly understand that a modern electronic regulator could smooth out the stator pulses, there by making the tach reading off.

 

[Stu Jackson]

Stator wiring

Should not the tach be hooked up to the alternator in front of the regulator, direct to the stator. I can certainly understand that a modern electronic regulator could smooth out the stator pulses, there by making the tach reading off.

I provided the referenced link in an earlier reply to the wiring diagrams I drew for my boat (http://c34.org/bbs/index.php/topic,4548.0.html), AND re-read the regulator's installation manual, the tach wiring goes "through" the regulator. That is NOT to say that perhaps it only lands on a plug, and goes right out again simply because the wiring harness comes back from the "Plug & Play" connector (stator, i.e., tach, and field) on the back of the alternator. I'd ask Xantrex or Balmar.

I know, I know, just following orders is NO excuse, but that's what it says, it's what I did, and it works!