[Steve0]

Hi guys, I have a Suburban 120 3212H model from (I think) 1991/1992? VERY LOW hours.
20 years ago or so, my father bought this thing off a co-worker who was moving.
Over the last 2 years I have replaced everything that needed replacing, and put back on parts that were missing to make it look nice.
After replacing a 1/2 dozen parts (tires/key ignition/side panels/missing choke lever, I could go on) the last thing I am stuck on is getting the headlights to work (turn on).
We have tested them with 12v manually and they light up with no issue, I have also used a multi-meter on the headlight switch to test continuity and it beeps.
I am attaching some pictures as there is a pair of cables in the wiring that seem to be un-hooked but trying different combinations with them did nothing.
I am definitely more of a mechanic than an electrician so I am wondering if some sort of harness is missing...Any help would be cool. I also reinforced the rear hitch for some serious pulling.

 

[Cnc]

I don’t really know your model, mine is an earlier Bolens. Lacking a wiring diagram, you will need to investigate where each wire goes on the switch, both by colour and continuity, isolate the wires from the switch to the head lights and confirm continuity also confirm the head lights have a proper ground. What’s left at the switch? Should be a wire providing positive power either directly or through a relay. Check for 12 v with engine running ( not sure on your model if lights will normally go one with the switch if it is not running) If not, I would trace the wire by colour and continuity back to it’s source. As for the two un used plugs I would put your meter on 12v dc and check for power, might be for an un used acc. Hr meter, idiot light etc, etc.
Good luck

 

[demonspeed]

An ST-Series! Certainly an overlooked tractor by most Bolens aficionados, but a solid entry-level tractor which could utilize ground-engaging attachments unlike anything in its class today. I had an identical model that my FIL got from a neighbor who was moving decades ago, and like yourself, I put way too much time and money into bringing it back. I digress...

Attached are the only parts to the install "instructions" for the Head Light Kit (30910) that I could find a couple years ago when I was working on mine. I believe mine ended up being a bad ground (and I rewired some of it as well). That said, from the pictures you posted, it doesn't appear as though you have the diode assembly (part number 1744489) which is shown in the bottom-right of the picture I've attached. Let us know if any of this info helps.

 

[Steve0]

Wow thanks for the replies! Yes I know this tractor is very small but for me its what I have to work with at this time, I also have several quads but they are early 90s sport types that weren't designed for pulling stuff and just for going fast. The problem is the headlight switch wiring goes right into the firewall from both sides, just a cluster of wiring tucked under the metal dashboard so to speak.... so figuring out where a ground is and whatnot everything is hidden away. I was hoping it was just an easy fix since this tractor was very popular in different engine sizes for like a decade. Demonspeed, where exactly does the diode go? I wish I had more pictures to show but its at my hunting cabin several hours away and we only go there a few times a month. Also the mower deck sometimes shuts off when cutting the grass for more than a 1/4 hour...might be related to the issue at hand or a bad PTO switch.

 

[29 Chev]

Found a wiring diagram on Jacks Small Engines that may help you
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/troy-bilt/legacy/tractors/3212hr-st-120-s-n-0100101-0199999/wiring-diagram

The second image is modified to reflect the 3212 and 3312 configuration only - appears to be a 7 wire connector somewhere in the harness. Looks like the two diodes are wired so that both halves of the AC wave are used from the alternator - the positive half of the wave charges the battery and the negative half of the wave powers the lights. It appears the lights will only light when the engine is running from the looks of the diagram since the light switch circuit is isolated from the battery since the one diode going to the switch will act as a one way valve - current can flow to the ignition switch from the alternator but no current can flow from the ignition switch to the alternator and the diode going to the light switch.

Hope this helps

 

[demonspeed]

Attached is the best picture I have of any wiring in that area. Anything with blue tape is stuff I repaired. Also attached is a picture showing "before" I messed with the wiring (when the lights did not work) or pretty much did anything to this monster. I had this tractor torn down to just the frame when I redid it all. Fortunately, it's pretty easy to dismantle, but that you've got it at your camp probably makes it a bit more difficult to do. Nonetheless, it looks like that diode will actually plug into the underside that panel where all the other stuff is plugging into under the dash area. Like you, I am more mechanical than I am electrical, so my vocabulary toward explaining this wiring stuff may not be correct.

I also attached a picture of the PTO area that I have -- notice the wire that goes to the PTO... that connector can get easily beat up and full of funk, so I ensured it stayed up in the frame rather than hanging down so that it wouldn't get smacked around by anything. You may want to clean and stuff it up through the hole like I did as that might possibly help with your PTO shutting off. Also check the air gap on the PTO, test the switch, etc... basically all the crummy electrical troubleshooting stuff that sucks, lol

edit: here is a link to my thread from a few years ago when I was trying to figure out my headlights. blackjackjakexxix is an awesome resource for Bolens parts, and he said he may have the diode assembly back then (and I'd be willing to bet he's not had many inquiries on it since then!): https://www.mytractorforum.com/18-b...0-headlights-flickering-how-troubleshoot.html

 

[Steve0]

Found a wiring diagram on Jacks Small Engines that may help you
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/troy-bilt/legacy/tractors/3212hr-st-120-s-n-0100101-0199999/wiring-diagram

The second image is modified to reflect the 3212 and 3312 configuration only - appears to be a 7 wire connector somewhere in the harness. Looks like the two diodes are wired so that both halves of the AC wave are used from the alternator - the positive half of the wave charges the battery and the negative half of the wave powers the lights. It appears the lights will only light when the engine is running from the looks of the diagram since the light switch circuit is isolated from the battery since the one diode going to the switch will act as a one way valve - current can flow to the ignition switch from the alternator but no current can flow from the ignition switch to the alternator and the diode going to the light switch.

Hope this helps

Any idea where the diode would be? Like is at near the engine or up in the firewall or just try to find the alternator and see if a wire is coming out of it and follow it somewhere?

Thanks for your diagram, appreciate the help.

 

[29 Chev]

Any idea where the diode would be? Like is at near the engine or up in the firewall or just try to find the alternator and see if a wire is coming out of it and follow it somewhere?

Thanks for your diagram, appreciate the help.

I don't know where the diode is as I am not familiar with the ST120 model - looking at the schematic I would assume it is between the engine and the connector. It appears that the alternator wire should be orange coming off the engine so I would suggest seeing if you can find that wire and verify you have power from it with the engine running - use a test light or multimeter with one end hooked to ground and the other to the alternator wire. If you have power there (which I assume you do if the battery is staying charged) then see if you have power at the red / white wire at the light switch. If you have power there then the diode is probably ok and if not then the diode could be bad or you have a break / corrosion in the wire somewhere between those two points. I would follow the orange wire to see where it connects to the diodes - thinking from a manufacturing point of view it is probably near or at the connector since it would cost less to run one wire from the alternator to where it connects to the two diodes.

Since you mention that the PTO is having issues as well you may want to first check the ground wire at the engine as both the PTO clutch and lights share this wire for a ground path to complete both circuits. It appears to be a black wire and I would assume connects to one of the engine mounting bolts. An ohmmeter test between the engine and the black wire that connects to the headlights should show 0 ohms resistance. You can also check the wire with a test light or multimeter if you connect one end to the positive post of the battery and the probe to the black wire at the headlights the test light should light or you should get a reading of 12 volts on the meter. Given the issue with the PTO I would suspect corrosion or a bad connection on the ground side may be affecting both the light circuit and the PTO but I could be wrong.

Hope this helps and good luck.



Good luck and hope this helps.

 

[demonspeed]

Since you mention that the PTO is having issues as well you may want to first check the ground wire at the engine as both the PTO clutch and lights share this wire for a ground path to complete both circuits. It appears to be a black wire and I would assume connects to one of the engine mounting bolts.

If the diode is in fact present, then I’m going to guess it is a ground issue. That was my problem, and now that you mention the PTO sharing the same ground, I recall trying to figure out a PTO issue myself.

As mentioned, I shared a picture of what the diode looks like, but I will highlight it below... it will be under the gauge cluster connectors/under the battery tray/behind that “firewall” thing if memory serves me. You will absolutely recognize it when you see it.

 

[Steve0]

Demon yes I get what the part looks like, I was just asking where it's supposed to be located lol. The location is apparently your house with a red circle around it haha. Thanks to both of you for your replies, I will check next Sat if I can get up there. I should have posted this before I took it to camp.

 

[demonspeed]

You’re wrong - not my house.... a random guys’ house that posted that picture on eBay that I swiped several years ago, lol

Actually, I remember sending the guy a message asking if he’d be kind enough to take a picture of the entire instruction sheet so as to help the Bolens community. He never responded...

 

[mowergene]

Here are the important questions: Do you have power at the light switch? If so, is the switch wired directly to the lights? Is the other wire from the lights grounded? If so, your lights will work unless there's a break in a wire. You have no need for a diode in a light circuit. You could wire the switch directly to the battery. This way you could turn on your lights with the machine off.

 

[Steve0]

You’re wrong - not my house.... a random guys’ house that posted that picture on eBay that I swiped several years ago, lol

Actually, I remember sending the guy a message asking if he’d be kind enough to take a picture of the entire instruction sheet so as to help the Bolens community. He never responded...

Was at the cabin this weekend and I cut the grass, the PTO function is all over the place...the mower would work for 20 minutes then turn off for 10 then turn back on for a few minutes then off again.
It was a few acres worth of cutting and when I was down to the last 1/2 acre I had to keep flipping the PTO switch on and off even wiggling the battery thinking maybe it was a cable issue trying to get it to work. Anyway, it did work but it was intermittent to the point it mowed for maybe 30 seconds at a time. Back to the headlights, I found the wire connector you were referring to and its definitely just an empty plug. My question to you is how exactly does the missing part connect on the other side? It looks to me like a drink straw on the opposite end of the plug?

 

[bontai Joe]

I don't know much about Bolens tractors, so I need to ask if your PTO is an electro-magnetic clutch? If yes, it may have an out of spec large gap. The magnetic force required is not strong enough to reliably pull the clutch plates together against the springs that normally keep the clutch plates apart when off. Bouncing the machine while operating can make the clutch engage and disengage. SO can a loose wire connection or a broken wire.

 

[Steve0]

I don't know much about Bolens tractors, so I need to ask if your PTO is an electro-magnetic clutch? If yes, it may have an out of spec large gap. The magnetic force required is not strong enough to reliably pull the clutch plates together against the springs that normally keep the clutch plates apart when off. Bouncing the machine while operating can make the clutch engage and disengage. SO can a loose wire connection or a broken wire.

The terrain is very bouncy on this little mower due to the heavily tilled soil (this is used on an extremely large rye field and a deer food plot that I should be using an actual tractor on, yes I know...but right now its all I have) not mowing a smooth golf course. I know for a fact the battery is just kind of shaking in the compartment. It is a hydro-static st120 3212h from I think 1991-1992. I was going to buy a new PTO switch but a friend told me its probably an electrical issue yesterday over the phone.

 

[Steve0]

Here are the important questions: Do you have power at the light switch? If so, is the switch wired directly to the lights? Is the other wire from the lights grounded? If so, your lights will work unless there's a break in a wire. You have no need for a diode in a light circuit. You could wire the switch directly to the battery. This way you could turn on your lights with the machine off.

I could definitely hard-wire the headlights to work off the existing battery, the battery is brand new and shows 12.3-12.4 volts with a multimeter. I just would like the existing wiring to go back to stock and function like it was intended. This mower was purchased by my father on the cheap because his buddy was moving to another state but it was missing a lot of stuff like it had been worked on by a teenager. Just trying to get it to 100%.

 

[mowergene]

If you hit a bump and the seat switch is momentarily off, that'll shut down the PTO.

 

[Cnc]

12.3-12.4 volts is this static, no load sitting for a few hrs? that is kind of low for a new battery should be 12.7 volts. Maybe your charging system is not putting out enough with the PTO engaged. Check the voltage with the engine running fast rpm no PTO, it should be in the high 13 volts 13.5 maybe even 14volts. Check what the voltage drops to with the PTO engaged also that electro magnetic draws quite a bit, if the voltage drops enough it will disengage after sitting for a bit the battery will recover and it will engage for a short period till it drops again. Since by your description, it seemed to get worse as the day progressed, this may be the cause. This PTO may be the same as used on mine, a 1668, built just a few years before yours, if so there should be a single wire coming down from it and a connector, make sure that connection is clean and tight. On mine there is No air gap to adjust the distance between the electro magnet that gap is taken by the design. The manual does talk about adjusting a gap but that is simply the housing which acts as a brake when the PTO is disengaged.

 

[Steve0]

If you hit a bump and the seat switch is momentarily off, that'll shut down the PTO.

I bypassed the seat kill switch a few years ago, the engine doesn't shut off when I get off to move something out of the way, etc.. Not the safest thing in the world but I prefer it not turning off.

 

[Steve0]

12.3-12.4 volts is this static, no load sitting for a few hrs? that is kind of low for a new battery should be 12.7 volts. Maybe your charging system is not putting out enough with the PTO engaged. Check the voltage with the engine running fast rpm no PTO, it should be in the high 13 volts 13.5 maybe even 14volts. Check what the voltage drops to with the PTO engaged also that electro magnetic draws quite a bit, if the voltage drops enough it will disengage after sitting for a bit the battery will recover and it will engage for a short period till it drops again. Since by your description, it seemed to get worse as the day progressed, this may be the cause. This PTO may be the same as used on mine, a 1668, built just a few years before yours, if so there should be a single wire coming down from it and a connector, make sure that connection is clean and tight. On mine there is No air gap to adjust the distance between the electro magnet that gap is taken by the design. The manual does talk about adjusting a gap but that is simply the housing which acts as a brake when the PTO is disengaged.

Yes 12.3 when sitting for a few hours un-used. I thought that was kind of normal. When I go back up I will charge the battery and see if that temporarily solves the problem maybe?


edit: from searching on another forum it looks like the missing diode demon talked about is definitely why the headlights are not working and may be causing the charging system to not function...This could explain the drop in voltage which is probably why the PTO keeps turning off. If I could only find one! diode p/n 1755616 or 1744489 it appears to be. I emailed Sams bolens because I cant find one anywhere, we shall see.