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djohn24

520 tach issues

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djohn24

Looking for help on a 520H tach. I have read over the other post and checked the wiring diagram dealing with the tach. The way mine is wired, doesn't seem to match the wiring diagram. It was working till a few weeks ago the way it is wired right now. I also did the "sniff test" and smell no burned electronics. I just bought a set of gauges off ebay, including a tach. Got that out of the box and the needle was already on 3k rpm. So I pulled the light housing cap off and i could smell burned electronics instantly. So, no luck there!  Adding a picture so you can see how it's wired.

IMG_20150613_102457_527.jpg

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gwest_ca

Do you have the tractor model number and serial number? Will look for some details on this.

Garry

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djohn24

Sorry Garry, I tag has been removed. I believe it is a 93 or 94 based on the onan motor.  I am sure it's not a 88 to 90 model. It has swept front axle, column levers.

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gwest_ca

Now I am confused. The 1991 tachometer circuit shows the grey wire getting the tach signal from the DC output of the regulator. I always thought it got the signal from one tach terminal on the regulator that was tied to one of the AC stator outputs.  Perhaps SOI will see this and chime in. Perhaps the frequency is the same in both the AC and the DC or it is just a plain mistake.

Perhaps one of these clips will help

A second thing I noticed is the regulator test has a chart showing the primary and secondary resistance specs which sound to me something that applies to the ignition coil which has nothing to do with the tach circuit on the Onan.

Garry

Onan 20amp regulator testing SS.jpg

Onan 1991-97 tachometer circuit SS.jpg

Onan tachometer testing SS.jpg

Onan 1990 tachometer circuit SS.jpg

Onan 20amp stator testing SS.jpg

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djohn24

Exactly....I am confused too. But, thank you for trying to help Garry!

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gwest_ca

I would clean up all the connections and verify the gauge is grounded and getting battery voltage. Then let us know where the grey wire is connected on the voltage regulator.

Garry

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djohn24

I did clean connections on the tach and down at the regulator. The 3rd post on the bottom of the tach never had a wire to it. The diagram shows 3 wires. Red/pink, grey and black. I know I tried hooking the grey wire down on the bottom post on the tach and it started to spark. What process should I do to verify the tach is grounded? I will go over it first thing in the morning. Thanks!

 

Dennis

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gwest_ca

The grounds to all the gauges are black. If the post at the bottom of the gauge looks like it is part of the case add a temporary ground to that post and see if that works. The gauge may be already grounded through the mounting but perhaps it is not sufficient because you did get sparking.

Does the gauge not identify the S (Signal or sender) and I (eye for ignition) or + for 12 volt supply terminal locations?

Make sure there is 12 volts DC in the pink wire at the gauge with the ignition key ON. The engine should not need to be running.

Remove the grey wire from the gauge and see what voltage is in the wire with the engine running. Compare it to the battery voltage with the engine running. The clip above states it should be 12 volts DC but I always thought it would be a higher AC voltage from the stator.

Garry

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WNYPCRepair

Not that I claim to know anything about the tach circuit, but if the screw is grounded by the mount, it could be corroded, and I have seen some weird issues from a corroded ground. That was on aircraft many years ago, but the theory is still the same.  :)

 

I would also try a ground wire to the bottom post

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Save Old Iron

The tach wiring off the regulator unit

669463a0.gif

 

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Save Old Iron

The signal to the TACH is off the AC terminal NOT CONNECTED INTERNALLY TO THE B+ TERMINAL.  The orientation of the wiring is correct as seen in the image above.

The TACH signal does not ride on the B+ lead as the battery acts as a "super capacitor" and will filter out any ripple (signal) on the B+ lead.
The TACH signal from the stator is an AC signal which is referenced to ground within the gauge, therefore the gauge must be grounded.

In the OFF state, the indicator pointer on the gauge could end up at any RPM and will not return to zero all by itself. This condition does not warrant the gauge being called defective. The TACH gauge is an "air core" gauge which is not driven by magnets or returned to 0 RPM by any spring mechanism. Air core gauges are used in aircraft applications where a large swing of the indicator is needed and indeed this style of gauge can swing around an entire 360 degree sweep.

Edited by Save Old Iron
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gwest_ca

Thank you SOI.

Garry

Edited by gwest_ca

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djohn24

Ok, everything is wired correctly as SOI  diagram shows. The pink wire is showing 12.27 volts with key on. The grey wire, with engine running shows, 14.47 volts. Like Garry had mentioned. Thinking the voltage would be higher. I did try a ground wire on the bottom post and ran it to a rusty bolt and made sure it was tight. The bracket on the gauge is touching the metal plate on the dash. I made sure that was good and tight. Still no movement in the tach. The odd thing to me is, if this circut is to have a black ground wire as the wiring diagram shows. This does not have one and i see no free black wire hanging anywhere. Like I mentioned before the tach was working till a few weeks ago.

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djohn24

I was wrong. There is a black wire on that bottom post. i cant believe i didn't see it. I will remove that and clean it good. What is the process with a multimeter to check for ground at that post? Forgive me, not very knowledgeable when it comes to this stuff. 

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oldredrider

Set your multimeter to ohms. For a point of reference,  put one lead on ground post of the battery and the other lead at grounding point of negative battery wire. That # is what you are aiming for with a GOOD ground. Should be very low meaning low resistance therefore, a good connection.

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gwest_ca

I was wrong. There is a black wire on that bottom post. i cant believe i didn't see it. I will remove that and clean it good. What is the process with a multimeter to check for ground at that post? Forgive me, not very knowledgeable when it comes to this stuff. 

​One way is to use a voltmeter. Check voltage across battery posts. Now move the black negative lead to the tachometer ground post and you should have the same reading. If it is a lower reading that is what is called voltage drop and is created by resistance (corrosion and/or loose connections) between that point and the battery negative post.

Garry.

Edited by gwest_ca
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WNYPCRepair

I checked the wiring on mine, and it is connected exactly like yours in the picture.

 

If you have a good ground, I would suspect a bad tach, as you had voltage on the other two wires.

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djohn24

Ok, if those steps turn out ok. Is it possible to check the tach for continuity ? If so, wouldn't that detect whether the circut board in the tach is good or bad? I just tried checking ohms on the tach. Set meter at 200 and checked all 3 post on the tach. I tried every combination. The meter remains at 1. Which tells me the circut is open. Or is the test I just tried not a correct way of testing?

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djohn24

That's where I think this is heading WNYPCREPAIR.  I looked on partstree and toro  still has tachs for these. At a cost of $281.37! I bought a set of gauges off ebay and the tach is no good. here is a picture of it.

IMG_20150614_114721_712.jpg

IMG_20150614_114855_873.jpg

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gwest_ca

If there was a way to test the tach it would be in the small clip up above from the Demystification Guide. If all the leads connected to the tach have outputs and the gauge does not work replace it.

The tach lead depends on the flywheel magnets and stator for output and you have no charging system complaints so have to assume they are OK.

Garry

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WNYPCRepair

Ok, if those steps turn out ok. Is it possible to check the tach for continuity ? If so, wouldn't that detect whether the circut board in the tach is good or bad? I just tried checking ohms on the tach. Set meter at 200 and checked all 3 post on the tach. I tried every combination. The meter remains at 1. Which tells me the circut is open. Or is the test I just tried not a correct way of testing?

​I'm no expert, but I don't think you can test the tach by checking continuity. 

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djohn24

Ok. I tried both ways to check for good ground. By Garry's & oldredrider. I have good ground. The issue is in the tach. I won't be giving toro the 281.37 they are asking for a new tach. I have watched a few video's on youtube of guys installing after market tachs on their riding mowers that are for cars/trucks, 4, 6, and 8 cylinder tachs. And they work! I saw on ebay for a fraction of the cost,  tachs for boats that look all but the same as our toro tachs. They may fit right into the dash on the 520s'. If and when I get around to doing this. I will post back on here letting you know if it works. That way if a member needs their tach replaced...it can save the some money. I would like to thank all of you for your help, Garry, WNYPCRepair, Save Old Iron, Oldredrider. Thank you again men for your help!

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Save Old Iron

 I bought a set of gauges off ebay and the tach is no good. here is a picture of it.

IMG_20150614_114721_712.jpg

 





The tach from my 520 was also defective - with that 1980's thru hole electronics burning smell.



520HTachometerAutopsy003-1.jpg


Certain parts of the circuit board are burned beyond recognition and recreation of the copper traces on the board would not be practical.



I believe the circuit schematic may be similar to the one published by the IC chip manufacturer.

cs189schematicparsed.gif






tachpcbfuncareasupdt.jpg




tachcircuitboardburnedcomponents.jpg

Edited by Save Old Iron

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WNYPCRepair

Man, that is cooked. Capacitors look fine though, usually they are the first to go in my line of work
 

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Save Old Iron

My recollection is the caps tested defective due to high ESR series resistance. Physically, they did not vent, just dried out.

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