You Can’t Stop the Signal

The clouds have parted. Today I got my Kubota mower to start and run normally. Sort of.

When I bought the mower, the seller had no trouble starting it and putting it on his trailer. Then it started acting up on me, and then it quit starting altogether.

For reasons I consider stupid, this mower has a very complicated starting system. I can’t imagine how much worse it would be if it ran on gas.

You can’t just say, “It won’t crank, so it’s part A.” A given problem could be caused by any of several bad parts. I decided to start out by looking at fixes that were easy and/or cheap and/or highly likely to work. When that didn’t work out, I got systematic. I studied and worked with AI, and I came up with 5 possible problems. There are others, but these were highest on the Occam’s Razor list.

I wrote out a list of problems and the ways I should approach them. Then things got worse and worse. I would work on one problem and find out it didn’t help. Then I would work on another one and get the same result. All this time, the list was dwindling, I was suffering, and I was getting closer and closer to the most expensive problem on the list: the mowers $330 electronic control module (“ECM” or “controller”).

Everything has to be computerized these days. My toaster is computerized. It’s not clear WHY everything has to be computerized, but it’s definitely true. Don’t question it.

Cars are full of computers now, and the disease has spread to small ag and landscaping machines. As you probably know, these computers cost a lot, losing one is a crippling blow, and manufacturers stop making them after a while, so you could end up with a useless machine simply because the computer is no good.

Somehow tractors and mowers used to run without computers, but trust me; they can’t run without them now. Just believe. Don’t think.

Originally, I thought my controller was important. I thought Kubota would not put it in a machine unless it did important things. It turns out I was totally wrong about that.

I thought the controller managed everything. Engine speed. Gauges. The transmission. Whatever. Because who would have an expensive, proprietary controller made if it didn’t do important things?

Last night, I went deeper into the starting mystery. I got out the schematic for the system. I wrote up my own analysis. You can read my analysis and understand everything about the starting system.

What I learned was annoying. The controller serves only two purposes. It prevents the mower from starting if the lawyer switches aren’t just right, and it also tells the mower to shut down when you turn the key to the off position.

I’m not kidding. It’s totally unnecessary. You can make the switches and the shut-off solenoid do their thing with analog parts of the kind Radio Shack used to sell.

CORRECTION: I forgot to mention the third function of the controller. It runs a horn that goes off when the mower overheats. You can rely on this to protect your investment. Or you could just look at the temperature gauge once in a while.

The key tries to send a signal to a relay. If it succeeds, the relay relays the signal to the starter, and the mower starts. The controller butts in and decides whether the relay is allowed to send the signal. If even one of the 5 safety switches is in the wrong position, your mower goes nowhere.

A relay contains an internal switch. The controller operates the switch, and the switch sends the signal to the starter. If you short the switch terminals, you cut the controller out of the game. It has no say. It’s like an enraged Karen, screeching at you from behind foot-thick glass. The mower starts and runs.

Shorting is exactly what I decided to do. I need to mow my yard regularly without long breaks due to starting problems. The weather is like the weather in an air fryer. I don’t feel like spending another week or two covered in mower grease, removing and installing parts and finding out it doesn’t help.

Because I always have wire and spade connectors around, like a normal person, I made a short wire with a spade on each end, and I shoved it into the receptacle the relay ordinarily goes into. The mower started and ran like a champ.

The safety switches did not work, so I had to be careful not to do obviously-stupid things. I didn’t start the mower with the transmission or blades engaged. I didn’t jump off the mower while it was moving and lie down in front of it. Things like that. It was challenging.

When I was done, I had to get off and push a little lever on the engine to shut it down. Whew. That was hard. The lever is always exposed, and it’s easy to reach.

Will I ever get around to putting the safety switches and key shut-off in order? Maybe. It should be very simple with cheap components.

If the controller is no good, I don’t think I should replace it, because it costs a lot and could go bad again, and I would rather not depend on Kubota to keep making them. Parts prices keep skyrocketing, and that’s another factor. If the mower can be made to work without a controller, I should fix it so it doesn’t need one.

I don’t want the parking brake switch because I can’t understand why it exists. Why do you need the brake to be on when you start the engine? With the brake off, the mower could roll downhill, but then it could do that whether you start it or not, and if it’s going to roll downhill, you should notice this before starting it, because the mower will be…rolling downhill.

The other switches make some sense. You don’t want the mower starting in gear or has the blades engaged, because it’s bad for the mower and makes you feel stupid. I don’t think that little starter will successfully crank the engine when the transmission or PTO is engaged, though. That would be some starter.

Because you sit on the front of the mower, the seat switch is a good thing to have. It is possible to fall off and land in front of the mower, and without the switch, it could keep on going, and then you would be julienned. The seat has a belt, though, so it’s not like the switch is necessary.

My John Deere’s seat switch shuts the motor off when you dismount, no matter what. That’s horrible. The Kubota only shuts the motor down when the blades are engaged. That’s fine. That’s how it should be done. The mower shouldn’t quit every time you have to get off to move a stick. You should be able to stop the PTO and leave the motor running.

I can make a circuit to shut off the fuel with the key. It’s more complicated than jumping the starter relay, but it’s not hard, either. Or I could just install a shopmade cable that allows me to pull the shutdown lever from the seat.

Bypassing the controller will never, ever cost me any useful function, but it will save me the horror of trying to test it, as well as the horror of paying for a new one if the old one is bad.

Why did Kubota make the controller? They must have had some reason for making an expensive, unrepairable part that does things cheap, easily-repaired parts can do.

Figuring out whether the controller is no good is not easy. Wire connections inside the plug and receptacle can fail. The ground can fail. Relay problems and ignition switch problems may look like controller problems. Kubota’s workshop manual doesn’t list any ways to test a controller, so their mechanics don’t know how to do anything with them.

I guess you can turn your ignition key and see if the controller sends a signal to the starter relay, but it could fail to do that because of a well-hidden problem in the wiring harness. That happened to one guy who wrote about it on the web.

In order to be sure the controller is bad, you really have to buy a new controller, plug it in, and see what happens. If the mower runs, the controller was bad. If not, you learn nothing at all, you still have a puzzle to solve, and you have a dirty part the dealer won’t take back.

The controller is dumb. That’s what it boils down to. It protects Kubota’s lawyers and does nothing a simpler set of components can’t do.

One great thing about learning that the controller is idiotic is that it tells me my fuel-delivery problems have nothing to do with the controller. The mower was slowing down and almost stopping, randomly. I looked this up, and a bad source told me it could be caused by a bad controller. This is not true. It might be true in a car or some other machine with a real computer, but it’s not true in the case of Kubota’s rinky-dink lawyer toy. The controller can’t affect the fuel supply unless it cues the shut-off solenoid at the wrong time, and if it did that, the mower would stop, not slow down.

I thought I needed new fuel filters, so I ordered some. On a recommendation, I also put Clear-Diesel in both tanks. Today, the right tank ran like a Tesla on cruise control, but on the left tank, the mower slowed down. That tells me the fuel in the left tank is probably bad. If the filters were bad, the mower would run badly on both tanks.

I don’t know if Clear-Diesel will help the left tank. If not, I can always dump the diesel in the woods. I mean in a proper receptacle.

I ordered new relays because one of mine tested bad. I later learned the reason it tested bad is that the manual is wrong. It told me to expect an impossible resistance across two terminals. I think the Japanese guy who wrote the Japanese manual also wrote the English manual, and I think he was drunk the whole time.

In case anyone else out there has a ZD326 or ZD331 that won’t start, I’ll paste what I wrote about the starting system. You can find the workshop manual online in PDF form. I don’t promise what I wrote is correct, but I promise it’s a lot better than your wild guesses.

KUBOTA ZD326 STARTING CIRCUITRY

Some of this material is repetitious because it makes it easier to understand. Wire color codes follow at the end.

OVERVIEW

The starting system’s main parts of interest are the ignition switch, ECM (controller), starter relay, starter (includes solenoid), two NO motion lever safety switches, one NO seat safety switch, one NO PTO lever safety switch, and one NC brake safety switch.

N=normally. O=open. C=closed. “Normal” means the mower will not start.

PURPOSE OF ECM

The ECM’s only purposes relevant to starting and running are 1) to monitor the safety switches and either agree or refuse to tell the starter relay to start, and 2) to power the relay that shuts down the mower. When the key is turned to the stop position, the ECM closes the shut-off relay for about 10 seconds, cutting off fuel.

The ECM also makes the horn blow when the mower overheats, but there is an independent temperature gauge on the control panel.

The ECM does not do anything essential, like making the engine run correctly. It can be bypassed completely if necessary. The ECM is not likely to fail, but if everything else has been checked, and the symptoms are consistent with ECM failure, it has to be replaced. It can’t be repaired without extraordinary effort.

A failed ECM can cause the mower to refuse to crank or stop. A failed shut-off solenoid can prevent the mower from starting and stopping, but it can’t prevent it from cranking.

A bad ground or other connection can simulate ECM failure.

The ECM has 9 connections.

OR = positive from PTO switch
BR = positive to starter relay coil
GB = positive from left motion control lever switch
WL = power to shut-off solenoid.
B = ground
YR = positive from seat switch
RY = positive from ignition switch terminal AC
WB = positive from brake switch
RW = positive to shut-off relay coil

RELATIONSHIP OF IGNITION SWITCH TO ECM

The ignition switch has a rotating conductor (terminal B) which is turned by the key and always hot. It is always connected to the R positive wire. When turned to any position other than off (glow plug, start), it is in contact with terminal AC, which can be considered the ignition’s ECM terminal.

Terminal AC is connected to the ECM (controller) and all safety switches (motion lever, seat, PTO, brake). The motion lever switches are in series, so 13 (right) is connected to RY and AC, and 14 (left) is connected to the ECM, providing positive input. The path goes AC-RY-left switch-right switch-ECM.

NORMAL START PROCEDURE

To start, close every switch except the brake switch (levers at rest, PTO off, operator in seat), and open the brake switch by applying the brake. Turn the key partially, letting the glow plug heat if needed. Then turn to the start position and hold until the engine starts.

BYPASSING SWITCHES

The brake safety switch is normally closed (NC). All others are NO. Because it’s NC, the brake switch can be bypassed by pulling the connector and covering it. To bypass the other switches, you have to jump them.

HOW IGNITION SWITCH WORKS IN GLOW PLUG POSITION

When the ignition switch is turned partially, it first connects hot terminal B to both terminal AC and terminal 19. Terminal AC feeds the ECM via the RY wire, and terminal 19 runs the glow plug lamp and glow plug.

Terminal 19 is not significant for starting, running, or stopping the mower. Current goes through terminal 19, the lamp and plug, and then to ground. The mower should start and run fine when 19 is out of commission unless the glow plug is needed.

IMPORTANCE OF TERMINAL AC AND RY WIRE TO STARTING

The RY wire that goes from terminal AC to the ECM sends signal through all safety switches along the way. The safety switches all get positive from RY.

IF TERMINAL AC IS BAD OR WIRE RY ISN’T LIVE, THE MOWER WILL NOT START BECAUSE IT WILL ASSUME A MOTION LEVER SWITCH OR THE PTO SWITCH IS IN THE WRONG POSITION.

THE ECM HAS A SINGLE GROUND, AND IF IT FAILS, THE ECM WILL NOT WORK, CAUSING STARTING AND STOPPING PROBLEMS.

HOW IGNITION SWITCH WORKS IN START POSITION

When the ignition switch is turned past the glow plug position, it disconnects 19 and connects hot B to AC and terminal 50 (BY). Terminal 50 feeds voltage through the BY wire to the starter relay. If the relay is closed, the voltage goes out through the BW wire to the starter solenoid. This is the voltage that tells the starter solenoid to turn.

The starter relay is closed by the ECM (not the ignition switch) through the BR wire. The current exits through the relay coil to the B wire (ground). Don’t confuse the B (black) wire with the ignition switch B terminal.

To start the mower, the ECM needs to see safety switch positive at YR (seat), OR (PTO), and GB (left motion lever in series with right). It needs to see RY positive next to the brake switch WB input. If it sees positive at the input from the brake switch (WB), the mower will not start. WB should be neither positive nor negative when the mower starts. No input.

STARTER RELAY

The starter relay is #6 in the manual’s diagram. It connects to BR (ECM), BY (ignition positive, terminal 50), BW (positive signal to starter), and B (ground) wires. It has 5 connections, one of which is not used. It switches between two poles; off (87A, dead) and on (87, BW). In the rest state, the switch connects the dead pole. Terminal 87 is the BW wire which goes to the starter solenoid.

Terminals 85 (BR) and 86 (B) send ECM juice through the coil. They are connected to the BR (ECM hot output) and B (ground) wires. The B wire is not important for analysis, since it never changes. A positive input has to be connected to BR in order for the relay to close, and the ECM does this. BR exits the ECM next to OR, if the schematic is any indication.

When the relay closes, it connects the relay switch’s BY wire (positive input from terminal 50) to the BW wire that feed the starter solenoid. This starts the mower.

On the schematic, from top to bottom, the starter relay connections are:
1. ECM – BR
2. Ignition switch – BY
3. Starter – BW
4. Ground – B

Jumping BY and BW should bypass the ECM and start the mower without harming anything. UPDATE: I tested this, and it works just fine.

The shut-off solenoid can also be set up this way, but you have to put in a timer or a momentary switch, because if the solenoid is always trying to shut off when the key is in the stop position, the battery will drain.

The manual shut-off lever is easily reached at the right rear of the mower.

TESTING RELAYS

All three relays on the mower are the same. You can test resistances to see if they are working. The manual’s test resistances are wrong.

Resistances: 85-86: ~90 ohms (coil)
87-87A: infinite
87-30: infinite
30-87A: 0 ohms

REMOVING IGNITION SWITCH

Use a pin spanner to turn the outer switch cover to unscrew it. Remove the cover over the motion lever arm switch. Remove all screws from the control panel so it is loose. Push the ignition switch down under the panel, pull it out between the panel and fender, depress the tab on the connector, and remove the switch.

DISASSEMBLING IGNITION SWITCH

There are two tabs on the sides of the switch housing. Pull them outward while pushing up on the key cylinder (keyhole). It helps to put a small object in a vise and rest the key cylinder on it while you pull the tabs and push the switch down.

By comparing the ignition wire colors to the switch’s terminals, you can figure out which terminal does what and how to check resistances to see if the switch is good. Removing oxidation from the copper contacts can reduce excess resistance.

WIRE COLOR CODES

B black
BR black/red
BW black/white
BY black/yellow
G green
GB green/black
L blue
LW blue/white
OR orange
ORL orange/blue
P pink
R red
RB red/black
RG red/green
RL red/blue
RW red/white
RY red/yellow
V violet
W white
WB white/black
WG white/green
WL white/blue
Y yellow
YL yellow/blue
YR yellow/red
YW yellow/white