420C amp meter

[cflorence]

Hello Everyone, I'm new to the JD crawler world since I just brought home a 58 420C 5 roller with a dozer blade it's serial number is in the 127,000 range. My question is, generator indicates 0 when the tractors running and -10 when I turn on the headlights. I read the service manual and it referes to SM-2013 so with what I have I don't know if the output of the generator or regulator can be turned up. Any thoughts?

Also I'd like to add a PTO and three point lift system, would this be cost prohibitive?

Thanks Chuck Florence, Carthage, NY

[shinnery]

I believe a 420C was designed to be a 6V pos. ground electrical system. Is it still? If somebody converted it to 12V and added an alternator, most of them are negative ground and if the wiring to the ammeter was not reversed it will show discharge when it is actually charging. Get a voltmenter and measure the voltage at the battery terminals.
Bryce

[Lavoy]

Either the battery is in backwards, or the generator needs to be polarized.
PTO is not too bad if you can find one, 3PT is quite spendy.
Lavoy

[Stan Disbrow]

Hi,

Well, if the generator has no output, then the ammeter will read 0 with the lights off and around -10 with the lights on. It should read around +10 with the lights off right after starting, and drop off to near 0 as the generator catches things back up. With the lights on, it should read around 0 as well( just a tad to the plus side).

You could have several issues, and will need to troubleshoot the system. here's the basic theory of how it works.

The system is simple enough. There is a regulator behind the dash panel that's the old two-relay style. So, it has a cut-out relay and a voltage-level relay. There is no current-level relay (which is the third relay in a three-relay regulator). This actually makes life easier.

The way this works, is there are a pair of soft iron plates inside the generator, with the field coils around them. The plates are magnetized and so when the armature spins there is a voltage induced in it, which passes thru the brushes and to the output terminal.

Those plates are what might need the polarization that Lavoy mentions.

Provided that the plates are polarized and the brushes and commutator are good this happens next: When that initial voltage out of the generator is high enough it will cause the cut-out relay in the regulator to pull in and make contact. That now places battery voltage onto the field input terminal of the generator. This puts current thru the field coils which greatly boosts the magnetic field inside the generator and so increases the output.

The cut-out relay exists so that the battery voltage does not flow thru the generator when the machine is off. That would kill the battery, so they cut off, or cut-out, the generator when the thing stops spinning.

Now, as the battery voltage rises to around 7 volts, the voltage regulation relay will open in the regulator. That removes the voltage from the field coils in the generator and the output falls back off. As the ignition (and possibly lighting as well) load drops the voltage on the battery, then the regulation relay closes again and the field picks back up and you get more generator output.

In practice, this opening and closing of the regulation relay occurs fairly quickly and if you listen closely (if you could hear it over the engine popping) you'd hear the thing buzzing as it works.

So much for the theory. Now you need to troubleshoot all this.

You can check for bootstrap voltage, as I like to call it, out of the generator by disconnecting the output terminal and metering it with the engine running. You should see at least negative 6 volts out of it like that, proving that the thing is polarized and the armature brushes and commutator are OK.

If not, then you need to figure out what's wrong inside the generator.

At this point, you could have a good generator but have a regulator issue. Checking that is easy, clip cord from the negative (remember these are positive-ground systems) battery terminal (the starter bolt is an easy place to clip to) to the field-in terminal of the generator with the engine running. If you now get charging, you need to figure out why the cut-out relay is not energizing or making contact.

Note that with the clip cord in place, the regulator relay will have no effect on the system. It will charge too much and damage the battery if you leave the cord in place, so this is a test trick only - it's no good for a workaround. You'd still need to find the root cause and correct it.

This can be rather a pain to work on, too as Deere put the regulator behind the dash, and you can't get that out until you pull the temperature gauge bulb and tube from the cylinder head (the gauges on these are mechanical and not electric). You have to be careful with that temp stuff, too, as it's a nitrogen pressurized bulb, tube and gauge and if you crack anything it'll die and you'll need a new one....

Oh, and the coolant runs out of the hole the bulb goes in, so you either need to drain the coolant or have a suitable plug for the hole in the head. You can cheat by leaving the radiator cap on and, working quickly, get the bulb out and the plug in without having to drop the coolant.

Modern systems use alternators, which have no mag field plates, only field coils, and the regulators and cutouts are solid state and part of the alternator assembly. The field coils are the rotating part, and the armature are the outer coils, so there is no commutator and brushes, just two slip rings and contact wiper arms. They're much easier to work on if you choose to do so or you can just replace the whole thing as a field replacement part and then turn in the core and go on your merry way.

With these old systems, *you* get to troubleshoot and repair it piece by piece.

I'd say 'have fun' but I know all too well that you are *not* going to!

later!

Stan

[Stan Disbrow]

Hi,

In my next dissertation on ancient electrical systems, we'll review the 'three brush' generators as used in the MC which don't have a regulator at all!

For now, we'll skip that one as it can only lead to confusion!

later!

Stan

[cflorence]

Hey guys I read all the great advice on trouble shooting the electrical system but I'm still unsure how to polarize the generator. Any help would be appreciated.

Chuck Florence
58 420C Carthage, NY