Walk behind unit electrical problem

[Richie F]

The purpose of this message is to (hopefully) provide some clarification with respect to the comments about diodes, rectifiers, AC and DC. You noted correctly before that the voltage coming from the stator of the engine is AC. Some stator assemblies incorporate a single diode in the harness to provide 'half-wave rectified' AC to the rest of the machine. Some older engines incorporate two individually replaceable diodes to do 'full-wave rectification' of the AC to the rest of the machine. Some embed two in a harness. I'm pretty sure that your Scag's Kawasaki engine uses a 3 terminal 'voltage regulator' to convert the AC stator voltage to the DC used by the machine. AC from the stator goes 'into' the regulator on the two terminals labeled AC, and the rectified or DC output comes from the terminal labeled B+, and ground. I'm pretty sure the wire labeled 'rectifier' on the diagram goes to that B+ terminal of the voltage regulator on the engine.

So, what's the purpose of the diode located across the PTO coil? Well it's merely there to provide protection to the regulator located on the engine. It has absolutely nothing to do with converting AC to DC for operating the PTO. It's called a 'flyback' diode. Why is it needed? Whenever a DC current is flowing through a coil, and it is interrupted by the opening of the PTO switch, it creates a huge voltage spike across the coil. I won't bore you with the engineering reason why. That voltage spike can damage the regulator on the engine because it 'sees' or is exposed to the spike. The semiconductor devices embedded in the voltage regulator can be damaged by this kind of high voltage spike. The diode across the PTO coil absorbs the spike by providing an electrical path around the coil, and the regulator on the engine never 'sees' the spike. The fact of the matter is that it is NOT required to operate the PTO. You can disconnect it and operate the PTO just fine. The chance you take is you damage the regulator when you open the PTO. Many machines don't use a flyback protection diode, particularly ones that have batteries. A battery also absorbs that spike. Under normal operating conditions(PTO on), the voltage present on the cathode side of the diode is always positive with respect to the anode and as a result, there is NO current flowing through the diode. That's why it's not necessary for the operation of the PTO. It's for protection only.

I know you said you checked the diode, but understand if this diode were shorted, it could produce the condition you are seeing. I'd still suggest the following quick test. Disconnect one leg of the diode and then start it. Hit the PTO and see if it still kills it. BTW, the chances of this one test causing damage to the engine voltage regulator are very small. That device is robust and it's the repeated exposure to voltage spikes that causes semiconductor breakdown leading to part failure.

FWIW... Let us know how you make out!

OK thanks for some education on this problem.
If you look at either wiring diagram/schematic posted you will only find one diode in the harness to the clutch.
From the parts break down of the Kawasaki FS481V engine there is only a one wire stator with no 3 terminal voltage regulator.
https://www.jackssmallengines.com/j...s22-4-stroke-engine-fs481v/electric-equipment

So the diode should be a fly back?
This is the way I tested it:
Engine not running.
Disconnected the deck harness plug to the handle switches.
Unplug the connector at the clutch, using my VOM set on the diode symbol, tested the two wires on that plug. Went from A to B one way and it was OL. Changed leads on A and B and got zero volts, showing a voltage path.
I would think if that diode was shorted I would see zero volts in both directions.
Correct me if I'm wrong.
This machine is a pull start and does not have a battery or starter motor.

I'm curious how a single wire wound stator can produce a DC voltage to operate the electric clutch with no regulator ?

I want to make a break out T and see what the output will be. AC or DC.

I do have a sister machine with no problems. I can test it's single diode or install that deck harness on to the problem unit.

I'll leave you with this information.

 

[Telesis]

Yes, I see, this engine does not use a regulator(being that it's a recoil start model). Here is a link to the engine service manual:

http://kawasaki.com/kengine/engines...094-01_fs600v_fs541v_fs481v_english-ebook.pdf

It's interesting that the service manual does not spec an AC output, rather just a stator coil resistance of 4.4 to 15 ohms for the recoil start model (see p 9-6). I do not believe a diode is embedded in the single wire output of the stator coil(if it was they would have a different procedure at the bottom of page 9-17 on testing the coil resistance) So, one side of the stator coil is grounded and the other is brought out via the single lead. That means there's another diode or rectifier somewhere. What's not clear to me based on the wiring diagram from Berts is where the yellow wire labeled 'rectifier' plugs into. Also, it's possible the rectifier diode is what is being depicted at the end of that yellow wire and that plugs into the stator output, point A in the bottom picture on page 9-17. Perfect place for it.

Normally(if there is such a thing), when you test a diode using a VOM on the 'diode' setting, a good diode will read the voltage drop when the diode is forward biased (red on anode, black on cathode) and the value is typically 0.2 - 0.7 VDC. In the opposite direction, the meter will read as an open circuit (typically a '1' or 'OL' , same reading with leads disconnected from diode). A shorted diode will read 0 VDC in both directions. You should double check you don't have 0 VDC in both directions, which would indicate a shorted diode and cause the problem you are having. In addition to testing it on the 'diode' function of the meter, please test it using the Ohms or resistance function. Measure the resistance in both directions. Let us know what you find.

Let us know!

 

[Richie F]

Yes, I see, this engine does not use a regulator(being that it's a recoil start model). Here is a link to the engine service manual:

http://kawasaki.com/kengine/engines...094-01_fs600v_fs541v_fs481v_english-ebook.pdf

It's interesting that the service manual does not spec an AC output, rather just a stator coil resistance of 4.4 to 15 ohms for the recoil start model (see p 9-6). I do not believe a diode is embedded in the single wire output of the stator coil(if it was they would have a different procedure at the bottom of page 9-17 on testing the coil resistance) So, one side of the stator coil is grounded and the other is brought out via the single lead. That means there's another diode or rectifier somewhere. What's not clear to me based on the wiring diagram from Berts is where the yellow wire labeled 'rectifier' plugs into. Also, it's possible the rectifier diode is what is being depicted at the end of that yellow wire and that plugs into the stator output, point A in the bottom picture on page 9-17. Perfect place for it.

Normally(if there is such a thing), when you test a diode using a VOM on the 'diode' setting, a good diode will read the voltage drop when the diode is forward biased (red on anode, black on cathode) and the value is typically 0.2 - 0.7 VDC. In the opposite direction, the meter will read as an open circuit (typically a '1' or 'OL' , same reading with leads disconnected from diode). A shorted diode will read 0 VDC in both directions. You should double check you don't have 0 VDC in both directions, which would indicate a shorted diode and cause the problem you are having. In addition to testing it on the 'diode' function of the meter, please test it using the Ohms or resistance function. Measure the resistance in both directions. Let us know what you find.

Let us know!

Thanks for the engine manual. I didn't have that one.
As far as the single diode is concerned if you look at Berts schematic, on the bottom by the clutch winding symbol you will see the diode.
I must have tested this diode incorrectly.
Using the VOM on it, one way was OL and the other 0 volts.

As far as the rectifier wire goes, look at the other diagram posted by me from Parts Tree.
That yellow wire goes from rectifier to PTO switch. From switch to goes directly to the positive side of the clutch coil.

 

[Richie F]

The more I look at the wiring I'm prone to think that one diode is the problem.
Current comes from rectifier thru PTO to the positive side of the clutch coil.
The diode prevents current to go directly to ground.

If that diode is shorted rectifier current goes to ground and shutting off the engine.

The mag wire is on the same terminal as the ground wire at the main harness plug. Which is terminal C
Blue wire of clutch goes to black wire ground.
Black wire ground is connected to white wire mag.
Grounding mag wire.

Engine shuts off when turning on PTO for the blades.

Then again I could be missing the boat on this.

 

[Telesis]

OK, you are going to love me for this one. I should have done this sooner, and that is read all previous posts several times and print out all the stuff. I mistakenly said earlier that a shorted flyback diode(the one by the clutch) could cause your problem and I was wrong. After printing out the pretty colored schematic on p17 of the SCAG IPL for this unit, the two circuits(1-stator, rectifier, clutch and 2-key sw,operator sw, PTO sw, neutral interlock sw, magneto) are totally independent. If the flyback diode was shorted, that isn't going to kill the spark. My bad. My apologies to the forum.

In one of your previous posts, you identified the symptom when you measured the resistance of the magneto wire(at the engine) to ground and turned on the PTO switch. You found that there was continuity! I'm assuming you had the operator present switch(es) engaged, tranny in neutral, the key switch was ON, and you turned(or pulled) the PTO switch ON. If you have continuity, that's not OK and the magneto is grounded. You should not have continuity with the PTO in either position(when all is OK)!

This is what the "PTO Switch Legend" tells you. When the PTO switch is OFF, H is connected to F which is a 'don't care' because terminal F is unused. C is connected to D which is also a 'don't care' because neither is used. When the PTO is ON, B is connected to C which is another 'don't care' because neither is used. G connects to H which is part of the magneto circuit, and E connects to A which is part of the clutch circuit. Think of the PTO switch(in this instance) as a double-pole single-throw switch.

Berts statements are spot on:

In the off position the magneto is grounded by the key switch.
In the on position the magneto is grounded by the operator switch
In the on position the magneto is grounded by the operator switch

I'd add that in the off position the magneto is grounded by the operator switch if the tranny is NOT in neutral

Sorry for taking you(et.al.) down the flyback diode road. The purpose of that diode and how to test it are still accurate. My recommendation is to go "sniffing" around the operator switch(s) and make sure they are OK(opening up properly). Trace it out as Berts said. There is a very manageable number of wires here. Could be much worse!

 

[bertsmobile1]

OK, you are going to love me for this one. I should have done this sooner, and that is read all previous posts several times and print out all the stuff. I mistakenly said earlier that a shorted flyback diode(the one by the clutch) could cause your problem and I was wrong. After printing out the pretty colored schematic on p17 of the SCAG IPL for this unit, the two circuits(1-stator, rectifier, clutch and 2-key sw,operator sw, PTO sw, neutral interlock sw, magneto) are totally independent. If the flyback diode was shorted, that isn't going to kill the spark. My bad. My apologies to the forum.

In one of your previous posts, you identified the symptom when you measured the resistance of the magneto wire(at the engine) to ground and turned on the PTO switch. You found that there was continuity! I'm assuming you had the operator present switch(es) engaged, tranny in neutral, the key switch was ON, and you turned(or pulled) the PTO switch ON. If you have continuity, that's not OK and the magneto is grounded. You should not have continuity with the PTO in either position(when all is OK)!

This is what the "PTO Switch Legend" tells you. When the PTO switch is OFF, H is connected to F which is a 'don't care' because terminal F is unused. C is connected to D which is also a 'don't care' because neither is used. When the PTO is ON, B is connected to C which is another 'don't care' because neither is used. G connects to H which is part of the magneto circuit, and E connects to A which is part of the clutch circuit. Think of the PTO switch(in this instance) as a double-pole single-throw switch.

Berts statements are spot on:

In the off position the magneto is grounded by the key switch.
In the on position the magneto is grounded by the operator switch
In the on position the magneto is grounded by the operator switch

I'd add that in the off position the magneto is grounded by the operator switch if the tranny is NOT in neutral

Sorry for taking you(et.al.) down the flyback diode road. The purpose of that diode and how to test it are still accurate. My recommendation is to go "sniffing" around the operator switch(s) and make sure they are OK(opening up properly). Trace it out as Berts said. There is a very manageable number of wires here. Could be much worse!

If you can i would be of great benefit if you could go back to your previous post and edit out the incorrect information.
There are a lot of people who lurk and never post when they have a problem.
This post is getting very confusing when as you know the circuitry is high school science simple .
It is a sad & sorry state that people look at a wiiring diagram and their brains go to mush when they are so simple it is not funny.

 

[mwilliams199]

THIS POST IS REALLY OLD BUT MAY KNOW THE ANSWER TO MY QUESTION, DOES THE PTO SWITCH HAVE TO BE GROUNDED TO THE BLOCK WITH NO OTHER SWITCHES IN PLAY?