520 Tach issues with a Kohler 18 magnum

[smoreau 658]

I have a 1990 520 with a Kohler 18 magnum engine in it and I am trying to get the tach to work. The first problem I have is the labels on the back of the tach are missing, or rubbed off. I have found ground as that is easy. but there is 3 more terminals with no labels on them. So I hooked up power to the longer post on the upper right side of the back of the gauge, and ground to the upper left and the gauge went from around 1500 to 0. Next I hooked up to one side of the stater before it went into the regulator. that gray wire I tried on both bottom terminals with know response from the tach. I checked connections from the tach to the stater connection and they ohm .02 ohms, so they check out fine. With the engine running I have 14 volts dc on the gray signal wire and 30.1 volts ac. That doesn't make sense to me. With the engine off and the key on power is sent to the center wire on the regulator and there is 12 volts dc at the gray signal wire. Know if I take the gray signal wire and scratch it on the threads on the lower right terminal of the tach, the needle will jump up to about 400 rpms for a second and drop back down to 0. So I think the tack works, just getting the wrong type of signal from the stater. Next I thought that it was back feeding from the charge (center) terminal at the regulator, so I unplugged it and still the same results. The signal wire is just spliced in to the stator wires and shrink wrapped to keep moister out. Any ideas? I am stumped as the charging system is putting out 14 volts at full throttle and about 15 amps according to my amp clamp. So I am to the point that I need

[sorekiwi 761]

I see you have already been to Chucks site Scott. Did you see the thread there dealing with the Onan tach?

[smoreau 658]

Yes I did, and it didn't help much. I think that the regulator for the kohler might be the issue. Thinking of trying a onen regulator and seeing if that works.

[Save Old Iron 1,566]

Yes I did, and it didn't help much. I think that the regulator for the kohler might be the issue. Thinking of trying a onen regulator and seeing if that works.

Scott, Thanks for stopping by at SOI U. Sorry the thread didn't give wiring pinouts. The post on SOI University was based soley on thought of an autopsy or "maybe I'll repair this". I have not yet added hookup or theory diagrams to the Tachometer Theory section of the site (but that is easy info to add). Try this link

Having said that, give me some time tonight to post a brief explaination of the hookup for the tach. I'ts basically just a DC powered analog frequency counter. The AC signal comes from on leg of the stator in relation to DC ground. Signal conditioning circuitry inside the gauge "squares up" the AC wave from the stator into DC pulses. The DC pulses drive a classic frequency to voltage circuit. I'll get the wiring setup from the back of my gauge and post in the morning.

[Save Old Iron 1,566]

and P.S.

The tach is an air core type meter movement, the same commonly used in aircraft. The meter pointer is free to move 360 degrees with no mechanical stop. The pointer could end up anywhere on shutoff, and float around while power is turned off. First step I would take is remove the illumination bulb from the back of the meter and give it the ole "sniff test". If you smell burned electronics, the circuit board is fried, like the one over at SOI U.

[smoreau 658]

I did give it the smell test last night, and it has no burnt smell, I think I have the wrong type of signal coming from the stator or something like that. I plan on bringing home my better meter (fluke 87) and going from there. For some reason it is back feeding from the battery on the charge charge wire. I will also try and post the wire diagram that I am using. I know others want to do what I am doing and this in theory should work.

[Save Old Iron 1,566]

"With the engine running I have 14 volts dc on the gray signal wire and 30.1 volts ac. That doesn't make sense to me"

Remember, in a Kohler RR setup, the B+ output from the RR is also electrically common with one of the AC terminals. I was surprised to find this to be true during my RR autopsy (posted a few years back on RS). If you happen to have the signal and ground inputs to the tach connected to the B+ and AC terminals which are electrically the same, you are not applying any signal across the tach inputs.

I wonder,

is the stator in the Onan referenced to ground where the magnum stator is floating? Only grounded thru the regulator?

I wonder,

If you take a standard 24 VAC transformer and connect it to the S and ground while the I terminal is powered at 13 VDC, the 60HZ signal should give you a ???? RPM indication. Guess you would need to calculate how many poles on the 520 stator / how many AC cycles per revolution. Looks like one of us has to hook up the scope to a stator and check the frequency.

[smoreau 658]

Very interesting on the kohler regulator. I didn't know that one side is hooked to positive like you said. I think I will try the other side of the regulator and see what that does, But it makes more sense why I had 12 volts on the signal wire with the key on, engine off. I just got home from work and I will do some tinkering with it and report back.

[smoreau 658]

This is what was wrong with the way I had it wired! I flipped the plug over on the regulator as it was the 3 pin flat style so the one + lead that is connected to the stator as Chuck stated in the above Quote was not on the signal wire and the tach now works!! I am very happy it was something simple and not cost expensive if you know what I mean. As for how accurate the tach is will be another story. I have a Snap-on digital timing light at work and it has a tach signal that is driven off the plug wire, on a car it would times the spark signal by two because it is a 4 stroke engine and only has a spark on every other revolution of the crank shaft. On the Kohler magnum twin sparks every revulsion of the crank being that it has a magneto on the fly wheel it would spark both plugs at the same time, one cylinder on the power stroke and the other cylinder on the exhaust stroke. So I would have to cut the # on my snap-on timing light in 1/2 to get a correct reading, Rite? please correct me if I am wrong as I will bring that home tomorrow and see how close it is.

Thanks Chuck!! You helped me out a lot with all the threads you have on hear and your site. I really appreciate your time and help

Scott

[Save Old Iron 1,566]

I am very happy

So I would have to cut the # on my snap-on timing light in 1/2 to get a correct reading, Rite?

Scott , all that logic would work if the pulses where being taken from ignition related components. The pulses being measured by the tachometer are being produced by the windings in the stator. It is the FREQUENCY of the AC signal off the stator being monitored - not the frequency of the ignition pulses (trigger events or actual spark events). This is why I mentioned the frequency counter in the previous post. The stator does produce a known number of AC cycles per revolution (the flywheel is synchronized with crankshaft movement) BUT, but the number of AC cycles is much greater than 1 or 2 per revolution of the crankshaft. If you look at a stator, there are typically 18ish segments of the stator, and 6 magnets and each magnet with 3 poles per magnet. Someone into wind turbines could probably do the math on this but for me, a frequency counter on the AC leads from the stator is where I would go.

Hopefully you have a 1 to 1 relationship between a Kohler stator and an Onan stator. That would be sweet. Otherwise, several components would need to be changed inside the gauge to accomodate it to its new home.

Glad to have helped with this one. It's why I do those autopsies. Sometimes the knowledge gained is not immediately obvious but can be put to use years later. In this case, it was the fact that the B+ and one of the AC lugs are electrically the same. That knowledge gave my neighbor a no cost repair on a corroded RR unit a year back and now helped today with your project - very nice.

If you would update your post at SOI U, I'll backfill it with some scope patterns off the stator at various engine speeds and we will have a gained some great experience in the tachometer section.

Thanks for getting back to us so quickly. I'll send your SOI U diploma as soon as they're back from the printers !!

[smoreau 658]

Yes I will as soon as I am able to log in again, Keeps cycling back to the log in page. In the above post I want to hook up my timing light and compare the two tachs to see how close thy are. Thats what I was thinking of using it for. I had a old stator out of a KT17 that the magnets come apart on and it destroyed it. I was going to count the coils and compare the # to a onen. Chuck, I am a auto mechanic and I do a wide range of repairs from major engine repairs, to front ends , and finding shorts, opens and poor connections in automotive wiring. This one had me baffled because I didn't know that little info about the Kohler regulators had two terminals connected inside. I was going to use the right side wire in my set up to connect to when I hooked up the signal wire, but the left side had a fray in the wire and my thinking if fixing two things at once should work as both wires from the stator should have a AC signal. That Idea is what got me to here. So I learned a few new things and thats Great! Thanks Chuck

Scott

[smoreau 658]

Well today I got the idea to check ebay for pics of stators for kohlers and onens. Well they look the same! same amount of poles and actually are almost identical. So when I got home tonight I checked the tach on the 520 and compared it to my snap-on timing light wit a digital tach on it. The match perfectly! So the tach in any 520 will work with any kohler engine with a 18 pole stator. just make sure you hook it up to the right side or not combined terminals on the voltage regulator. I hope this tread helps anyone thinking of hooking up a WH tach to there Kohler engine

[Save Old Iron 1,566]

That will save me a trip up to the barn with the o'scope !

Well done sir.

[Road-Track 39]

Thanks for the info . I had wanted to install a 520 tach on my C-145 and had asked if it would work before, but no one was sure. Now I believe it will.

[Save Old Iron 1,566]

The tachometer works on the frequency of the AC signal off the stator. The stator frequency is dependent on the number of segments or poles on the stator and the number of magnets on the flywheel.

So far, only 18 pole stators and 6 magnet flywheels appear to be compatable with the 520 tach gauge. Confirm you have this 18 / 6 pole vs magnet relationship in your engine prior to purchasing the tachometer.

[smoreau 658]

Good point about the magnets Chuck.

[Road-Track 39]

I just looked at the pictures of my 1983 C-145 Kohler K321as 14 HP stator and fly wheel magnets and found 18 coils and 6 magnets.

Looks like a candidate :eusa-think:

[Martin 2,132]

was talking about this with scott at the show this last weekend, and might try to install a tach on my c125 if it has the right stator setup. one question though, is there a replacement aftermarket tach that could be substituted for the 520 tach? what would i need to look for (specs wise) to be sure i had a compatible tach for this application?

[smoreau 658]

I would check flee bay for one, but I don't know of any that use the stater to show rpm. Harbor freight has a small tach, but never tried to hook one up to a single cylinder engine.

[Save Old Iron 1,566]

is there a replacement aftermarket tach that could be substituted for the 520 tach?

Martin, are you looking for a replacement with the same diameter? I think Stewart Warner has a similar size gauge for diesel boating applications. I'm not sure the calibration would be correct for a single cylinder engine. There are digital meters out there but not of the same physical dimensions of the 520 gauges.

[Martin 2,132]

im looking for the same diameter, not interested in a digital one, if i cant come across a good 520 gauge i was wondering if any of the same 2" diameter aftermarket ones out there could be a replacement. i will do some more research on the sw diesel gauge and see what i can find out. being a diesel gauge, do you think it would be worth a try? would the signal from a diesel engine in theory be the same as the kohler/onan method? does the diesel work on an alternator signal? i guess im just not really sure what specs to look for, if i can pick up one cheap, it would be worth a try, though........questions, questions.......

im only interested in doing this if i can find the right size gauge to fit the 2" opening where an hour meter on the c125 dash would go.

[Save Old Iron 1,566]

I did not pay close attention to the details on the diesel gauge but we know its not a mechanical gauge and we know its not getting a signal from the ignition coil !!

I was looking for the original manufacturer of the WH gauge in order to grab a schematic to fix the fried unit I have in my 520. My googling led me to the Stewart Warner site. Several unanswered emails later, I gave up on Stewart Warner customer support.

[JackC 617]

<<I was looking for the original manufacturer of the WH gauge in order to grab a schematic to fix the fried unit I have in my 520.>>

How do you take them apart?

Can the lens be replaced?

A "how to for dummies" on adding a 520 tach to a twin or single cylinder Kohler would be nice if you have nothing better to do.

The 520 tach is the right size and they are available.

Thanks for the great posts.

[Save Old Iron 1,566]

How do you take them apart?

Can the lens be replaced?

Thanks for the great posts.

You are welcome.

Most gauges have metal bezels that are crimped around the back edge of the bezel ring. Using a small screwdriver or pick, the edge of the bezel can be straightened out and the glass and the bezel can be removed. Not difficult, but finesse is everything on a maneuver like this. The glass face, gauge face and pointer can be cleaned and the bezel carefully re crimped back down around the gauge body.

[JackC 617]

Good to know, thanks.