# How to wire up carb heater box on MTD 5/24 with Tecumseh HS50-67324K



## unknown1

I recently acquired an old-school snowblower MTD 5/24 2-Stage 
Model: 312-610E000 (1992 I think)
Engine: HS50-67324K

It did not have a carb heater box on it. It also did not have any kill switch terminals on the throttle lever.

I have ordered a used carb heater box.. it's coming in the mail. I've attached a picture of the box. It has a green wire on it. I hope it's the right box.

I also have attached a picture showing a side back view of the (dirty) engine with the carburetor removed. You can see the throttle lever and you will notice there is no kill wiring to shut off the engine when the lever is in the bottom position. I might add one.

I have two questions I'm hoping you can help me with.

1) When I attach the carb heater box, where does the green wire go?
Can you recognize any position on the engine photo which is already pre-wired as the correct place to connect it? Or will I have to find a way to route it up to the ignition coil grounding terminal?

2) If I decide to connect a throttle lever kill terminal. I assume it will act as a second way to shut-off the ignition by grounding the coil using the throttle lever as a path to ground. If that's true, the throttle lever kill terminal will therefore go to the same location as the green wire from the box. Is that correct?

I guess I could (and maybe should) strip the machine a little further until I can see the coil and then trace the wiring (if there is any) from the coil's grounding terminal and down to the carburetor area.... but I was hoping that someone might be familiar with this old machine and immediately know the right answer.

Initially, I guessed I might connect it to join the black wire visible in the center of the picture.. but I just measured the impedance between that wire and the engine body and got zero.. in other words, that black wire is already grounded. If that's true.. it can't be coming down from the coil kill terminal can it otherwise the engine would not run.

By the way, my logic here assumes that the key in the carb box (when I get one) and the throttle lever kill-switch (if I add one) will both operate the same way... by simply grounding the coil's "kill" terminal. Have I got that right?

I have only found one video showing carb box replacement on the web and it ends abruptly with the green wire dangling. It never goes on to show where that gets hooked up... pity. 

Thanks to anyone who can answer these questions for me and confirm whether my logic is correct or not. I'm really not at all familiar with these machines but I have been watching lots of videos to try to get up the leaning curve.


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## Jackmels

Here's what it looks like. Heater Box Kill wire goes to screw on bracket. Bracket with hole is attached with Lower Shroud Bolt. I have that throttle with wiring assembly available. You need to remove the flywheel to change it. PM if interested.


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## unknown1

Thanks for the photo!

It looks pretty much like my initial guess.. except your vertical wire is green and mine is black.
But I don't see how this can work UNLESS that black terminal post holding your green wires is made of plastic or some other non-conducting substance to prevent those green wires from being grounded until EITHER the throttle is pushed all the way down OR the carb-box key is pulled. Am I right? Is it plastic?

On my machine I think someone just put a regular metal bolt (therefore electrically grounded) into the place where those terminals should go... in which case.. if I understand how this stuff works.. they must have disconnected the terminal from the coil up high otherwise the ignition would be permanently grounded and therefore "killed".

I'm going to have to disassemble further to get this clear in my head so I can actually SEE which wires go where in my machine and see for certain which are connected, which are dangling, which are grounded and which are electrically isolated (or SHOULD be). 

I will try to get to that later today.


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## skutflut

stuart80112 said:


> Thanks for the photo!
> 
> It looks pretty much like my initial guess.. except your vertical wire is green and mine is black.
> But I don't see how this can work UNLESS that black terminal post holding your green wires is made of plastic or some other non-conducting substance to prevent those green wires from being grounded until EITHER the throttle is pushed all the way down OR the carb-box key is pulled. Am I right? Is it plastic?
> 
> On my machine I think someone just put a regular metal bolt (therefore electrically grounded) into the place where those terminals should go... in which case.. if I understand how this stuff works.. they must have disconnected the terminal from the coil up high otherwise the ignition would be permanently grounded and therefore "killed".
> 
> I'm going to have to disassemble further to get this clear in my head so I can actually SEE which wires go where in my machine and see for certain which are connected, which are dangling, which are grounded and which are electrically isolated (or SHOULD be).
> 
> I will try to get to that later today.


The key is plastic, When inserted, it opens the contacts to isolate the coil wire from ground. When removed, it connects the two contacts, routing the coil wire to ground through the green wire. You need to find the wire from the coil and bring it to the contact on the key switch opposite the one with the green wire. Green will go a ground point. If your throttle control has a ground point available, you can add another piece of wire from the key switch coil wire, to the throttle switch, so that the coil wire gets grounded when the throttle is closed all the way.


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## unknown1

Figured it all out thanks. Will post a final answer to my own questions in a moment to help the next person who hits this.


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## unknown1

EDIT: By design, the ignition "coil" has an electrical terminal on it that can be used to "kill" or "stop" or "disable" the engine.
When that terminal becomes grounded, (intentionally or accidentally), the engine will stop. See the photo in message #1 of this thread showing that terminal on the "coil".
There are various switches and electrical safety devices used on snowblowers designed to stop or disable the engine.
If these are incorrectly wired, or wires are shorted out, you can end up having a machine that will not start...or.... conversely... will not stop.
Now back to my original posting (before edits).........


Basically my thinking was right. That terminal post holding the green wires *IS* made of plastic (or nylon or whatever). The important thing is that it does not conduct electricity.

I've annotated Jackmels photo and my photo and attached here.
In these pictures, the cable marked with the red arrow goes up to the kill terminal on the ignition "coil".
If that cable ever makes it to ground... (intentionally or accidentally)...the spark will stop and the engine will be killed.

In Jackmel's photo. The cable marked with the green arrow is just one way of killing the engine. In this case, when the throttle lever is pushed fully down, the cable marked by the red arrow gets grounded via the cable marked by the green arrow which is grounded via the metal throttle lever handle.

In both pictures, the terminal marked by the yellow arrow MUST BE ISOLATED FROM GROUND. Otherwise your engine will NEVER run (assuming everything is connected right up at the "coil"). 

That's achieved by having a plastic (or nylon) insulator on the attachment bracket.

I added a couple of photos of that bracket (assembled and disassembled) to show the square plastic terminal mount that keeps those green wires isolated from ground.

By attaching the green carb box wire to this isolated terminal post we will have TWO ways of killing the engine. (assuming I add the connection terminal to my throttle.. Jackmel already has one).

If I really wanted to, I could add any number of kill switches using (for example) a single throw toggle switch, or an instantaneous push switch or any other electrical mechanism that would ground out those (currently isolated) green wires. (Might be handy to have a hidden kill switch if it was a snowmobile or go-kart to stop someone riding off on it).

Hope this helps somebody some day.


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## ELaw

That's a great writeup!

Just FYI some Tec engines use a somewhat different setup. That setup uses a throttle like in Jackmel's image with a stop contact built-in but the stop contact has a little male spade connector on it, and the wire from the key (the one on the carb cover) goes directly to the spade connector. With that setup, the little bracket on the side of the cover isn't present.

The attached image shows that setup. One green wire goes to the magneto under the cover, the other goes to the key. That photo is of an HMSK110.


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## skutflut

stuart80112 said:


> Basically my thinking was right. That terminal post holding the green wires *IS* made of plastic (or nylon or whatever). The important thing is that it does not conduct electricity.
> 
> I've annotated Jackmels photo and my photo and attached here.
> 
> The cable marked with the red arrow goes up to the kill terminal on the ignition "coil".
> If that cable ever makes it to ground... the spark will stop and the engine will be killed.
> 
> In Jackmel's photo. The cable marked with the green arrow is just one way of killing the engine. In this case, when the throttle lever is pushed fully down, the cable marked by the red arrow gets grounded via the cable marked by the green arrow which is grounded via the metal throttle lever handle.
> 
> In both pictures, the terminal marked by the yellow arrow MUST BE ISOLATED FROM GROUND.
> 
> That's achieved by having a plastic (or nylon) insulator on the attachment bracket.
> 
> I added a couple of photos of that bracket (assembled and disassembled) to show the square plastic terminal mount that keeps those green wires isolated from ground.
> 
> By attaching the green carb box wire to this isolated terminal post we will have TWO ways of killing the engine. (assuming I add the connection terminal to my throttle.. Jackmel already has one).
> 
> If I really wanted to, I could add any number of kill switches using (for example) a single throw toggle switch, or an instantaneous push switch or any other electrical mechanism that would ground out those (currently isolated) green wires.
> 
> Hope this helps somebody some day.


Too many kill switches can be a problem.. too many places for a wire to get damaged and go to ground.


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## unknown1

Just to document the final solution.
These images show the wiring of the carb box key kill switch for machines with two different configurations....
a) For a machine that does have a throttle kill switch (giving two ways to kill)
b) For a machine that does not have a throttle kill switch (only one way to kill)


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## unknown1

I do not have a kill terminal on my throttle. This made it difficult to stop the engine under normal usage.
Instead of buying a different throttle, I decided to add a toggle kill switch and mount it up next to the gear shift.
This now lets me shut off the engine without pulling the key or the spark plug boot.

Photos:
https://goo.gl/photos/xMT5vFu5Ltrg1UxL6

Look at the white wires underneath the carburetor box area for appropriate wiring.
One wire goes to the isolated terminal mentioned earlier. This shares the connection with the carb box key and (most importantly) the ignition "coil" kill wire.
The other wire goes to a reliable ground. In this case I used the bracket mounting bolt.

The switch is a sealed waterproof military-spec Honeywell toggle capable of taking a direct hit from a hose pipe. Overkill..but reliable.

Honeywell switch here:
http://www.newark.com/honeywell-s-c/...77v/dp/11J4976

I made a decal by copying the artwork from a Tecumseh carburetor box


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## unknown1

Finally.. for completeness, if (like me) your throttle does not have a kill terminal, you could order Tecumseh part #610973 (about $6 shipped on eBay)

These pictures show how that would look fitted beneath the throttle lever.
https://goo.gl/photos/9az7mF3bq9WtEPF58

As always.. the wire goes to meet the ignition "coil" kill wire (perhaps along with the wire from the carburetor box key) onto the isolated terminal mentioned previously.
Notice that the body of the terminal is plastic or nylon to ensure that the green wire is not grounded UNLESS the metal throttle lever is pushed down against the the metal "springy hook".


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## unknown1

A more complex version of kill circuitry involving auger switch, traction switch and a grounding relay is over on this thread..

http://www.snowblowerforum.com/foru...8569-new-guy-troubleshooting-toro-5-24-a.html


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