John Deere 40C starter relay installation

danlevis

New User
I couldn't find any information online on my particular problem, and I hope someone else can benefit from my experience as I stumbled through this. This should work on any tractor with, instead of a starter relay, a manually operated rod that moves a switch on the starter.

I have a John Deere 40C that I converted to 12 volt. It started and ran fine for about a year; then, suddenly, it wouldn't crank. I love the factory starter circuit: there is no relay, no solenoid, just a knob on the dash; you pull it and a long rod (the "starter operating rod", per the manual) moves a switch screwed to the starter itself:

43833.jpg


Inside the switch, a little copper wedge completes a circuit between two copper pads:

43834.jpg
43835.jpg


I found that the switch was bad; the copper pad was worn away. I suppose there must be an arc every time you start it, and eventually, like any switch, the switch will wear out. I bought another at Tractor Supply for about $12.

Maybe 8 months later, the switch wore out again. "Well", I thought, "I guess I'll just buy a new $12 switch every year."

The next time the 40C wouldn't crank, though, the switch was fine: the copper pad on the starter itself was worn down. A friend suggested that my battery "was too big", as in too many amps, but I don't think that makes any sense. I couldn't figure out how to replace the pad, so I bought a used starter, another Delco 1107127, on ebay for about $100.

After another switch replacement, the pad on the second starter wore down too. Now I had two bad starters and a pile of bad switches. I considered buying a crank handle so I could start the 40C manually, but I think it would interfere with the blade, so I had no choice but to replace one of the pads.

I'm sure I have some of the terminology wrong. What I call the "pad" is the rectangular head of a 3/8 copper bolt; the head takes a 5/8 inch wrench, and a slot is cut into the end of the bolt. A copper nut inside the body of the starter secures it, then copper strips from the field coils are soldered into the slot. Where the bolt passes through the body of the starter, it runs through some insulating washers:

43836.jpg
43837.jpg


Plenty of tractor places sell a "starter repair stud", but they only work on starters with external threads. I couldn't find a direct 3/8 replacement for mine anywhere online, but several places sell a 5/16 bolt (as a "starter stud"). I considered using, but I didn't want to improvise a thicker insulator. Instead, I decided to scrap the original switch setup, and use a stud that would allow me to use a relay. I know it's not as elegant as the original setup, but now the only part that will wear out is a $20 relay with a lifetime warranty, and I'll (let us hope) never have to open the starter again. I replaced the original stud with this:

43838.jpg


It cost about $16 on ebay and is sold as a "Delco Starter Stud Kit". Insulators and washers are included. Another forum suggested using a "soldering copper" to solder the copper strips to the stud, since a standard soldering gun won't get hot enough.

I bought a 2 pound soldering copper but I couldn't get it hot enough with a propane torch. It probably didn't help that I was working outside in 30 degree weather. Finally, I got frustrated, and instead of trying MAPP gas or something else, I just heated up the stud with the propane torch. Surprisingly, it seems to have worked, and I didn't hurt myself or break anything:

43839.jpg


The relay fits many Ford trucks. Ask for a starter relay for a 1986 Ford F-150 with a 300. Conveniently, the bolt holes lined up with the original starter rod hole and another hole just above it, so I didn't have to drill.

43840.jpg


I used a switch from my spares box, but almost any momentary switch should work. I knocked out a plug in the dash that is supposed to take an optional cigar lighter, and my switch happened to fit.

43841.jpg
43842.jpg
43843.jpg


The wiring is just like any starter relay:

43844.jpg


Of course, I had to remove the original starter cable that came straight from the battery, and replace it with 2 cables: one from the battery to the relay, and one from the relay to the starter. I used 4 gauge for both.

I previously installed a fusebox, and now it gets power from the large, always-hot terminal on the relay instead of cluttering up the positive battery terminals.

I've started it 6 or 7 times since, with no problems. We shall see how it works long term. I own a couple of Ford trucks with the same relay, and if it can handle two batteries on a Diesel V-8, I expect it to have no problem on the 40C. I still don't know why the original pads wore out; maybe both of my starters just happened to have pads that were nearing the ends of their lives.

Thanks for reading, and I hope this helps someone with the same problem. I welcome any comments on how I could have done it better.
 
NICE job! I have done a number of those, myself.


An alternative is to drill and tap the original copper pad for a 10-32 machine screw and attach the cable from the solenoid like Delco did on their 10MT starters. You might not think that would work, but they did the same on some larger starter series. It takes a caustom end on the calbe from the solenoid to allow it to connect up.

<img src = "http://i175.photobucket.com/albums/w131/ray_mcavoy/Website_Diagrams/Starter_Sol_zpse580f8f4.jpg">

Note the connection at "M".
 
Ha, I'm a little embarrassed I didn't think of that! Thanks, Bob.

Do you have any idea why both of my pads wore out? Is it just a coincidence? I looked around the Internet, and it looks like I'm one of about 3 people who've ever had this problem.

I'm pretty sure the "battery is too big" theory is nonsense, but I can't really prove it.
 
Not coincidence at all. More like planned obsolescence in the replacement switches. They make them with the shorting bar dimensionally challenged on purpose since it's made of expensive copper. All they really care about is if it works long enough you can't get your money back on it. Were it 1/16 inch longer on both ends it might work for 20 years instead of just 8 months. And they used to do exactly that back in the good old days. Battery too big is some made up stuff to be sure.

I've had the problem, but I didn't post it. I slotted the screw holes so it would mount closer and used it as is. Shimming the switch copper bolt closer is another 'thing to do' to these. Nice write up, excellent pictures, I'm sure it's going to help several too even if you maybe feel a little embarrassed about other suggestions that might have been easier. Removing the 'can't buy a good one anymore' manual/foot switch entirely is a valid solution just the same. IF they just sold a decent switch in the first place, we wouldn't have to do these kinds of things.

I wouldn't have thought of it either, Bob, nice tip there.
 
I was always able to find the original style posts at local rebuild shops.

Your solenoid will likely hold up better ? and now you have a key you can remove to keep kids from starting it accidentally.
 
You know, Mike, I didn't even think of looking anywhere except the Internet. Maybe the phone book would have been a better place to start!

Lee, the switches did seem pretty cheap. I puzzled over them for a while trying to think of a way to "rebuild" them, maybe a longer stud and washers to push the contacts farther towards the starter, like you mentioned.

With the original 6 volt system, the factory switch must have handled twice the amps, and I'm sure you're right, they didn't fail every 6 months. That's how the world works now, I guess...
 
So I was pretty proud of myself for coming up with this starter fix. Then, after about 3 months, the relay stuck closed while cranking: I had to frantically unhook the negative battery cable before the starter destroyed itself. "Well", I thought, "I just got a bad relay." I went to the auto parts store and got a free replacement.

Then, after about 1 month, it happened again. This time it was raining, so prying off the battery cable was even less fun. The guy at the auto parts store said that "limited lifetime replacement" means you only get one replacement for the rest of your life, which was a surprise and a disappointment, but he gave me a free replacement anyway.

I know that people try to use these starter relays for snowplows, and they burn out because they're not rugged enough for almost constant use. I assumed that, since this relay is used on Diesel V-8s, it could easily handle the 40C. So I guess my question is:

Did I just happen to get two bad relays in the space of 4 months, or is there something wrong with my setup?
 
Instead of throwing good money after bad, lets try to understand whats happening...

Your system is designed for 6 volts. You are now running 12 volts. The resistance of the starter (the "load") hasn't changed. To get an accurate calculation, use your multimeter and measure the resistance from the stud to the case, effectively measuring the winding. For the sake of this calculation, we will just assume 0.2 ohms winding resistance for now.

So, we can do a simple calculation using a "voltage triangle". (Google it. It's not in the phone book.)

To solve for current (the only unknown), we must calculate:

Voltage/Resistance

Original 6 volt system:
6/0.2 = 30 amps

New 12 volt battery:
12/0.2 = 60 amps

I don't know what your relay is rated for, or even the original switch for that matter, but it's possible you're trying to put too many electrons through it all at once. Knowledge is power, and knowing is half the battle. Do the measurements and calculations, then check to see if your ratings match up.
 
Thanks, Rudy.

I'm familiar with Ohm's Law. The multimeter says 0.3 ohms for the starter winding, so 40 amps are going through the relay when I'm cranking.

I don't know what the original starter-mounted switch was rated for, but as you point out, with the old 6 volt system, the starter was only pulling half of that, so 20 amps.

For the new relay that I used to replace the switch, I bought the same relay used on my early 1990's Ford van with a 7.3 Diesel. I'm not near the van so can't check the resistance of its starter, but guys on Ford forums claim that their starters draw huge amp numbers, like 500 or even 800 amps. I certainly can't imagine that they draw (or are rated for) less than 40 amps.

So the math seems to check out, unless I'm missing something. It seems extremely unlikely that I got two bad relays, and far more likely that I've done something wrong, I just can't figure out what.

The problem is that when I release the key on the cheap generic switch I installed, the starter continues to crank: I suppose that means the switch could be sticking closed, and the relays are just innocent victims. But since this has only happened twice in 4 or 5 months, I can't think of a way to check that. I used that particular switch because it fit nicely.

I think I'll do this: replace the key switch with something more rugged, and if this third relay (my last free replacement!) fails, I'll buy a continuous duty snowplow relay and try it out.

This relay is sort of exposed to weather, maybe that's the problem? Maybe there's a weatherproof relay out there that I could try, or I can bend a little splash shield out of aluminum.

Thanks again, Rudy. I'm one of those guys who knows enough about electricity to get into trouble, but maybe not enough to actually make things work...
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top