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John Deere Tractors Discussion Forum

Positve ground

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jturbo10

01-31-2008 21:07:03

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Starting to baseline the 65 JD 3020 gas I got the other day. The previous owner had moved the battery out of the side saddle and built a box on the left side near the starter. Additionally I assume it came with a generator but it now has an alternator and has aftermarket electronic ignition. It is set up as a positive ground but I don't have any manuals yet to review electrical diagrams. I do have a JD 4020 manual but it has only generator displays. I'm having the starter remanufactured and putting a new 12 volt slim battery in the side pocket and adding new positive and negative cables. Anyone have a 3020 gas that is set up with a positive ground? Any valid reason I should try to change this to a negative ground...would require a new electronic ignition unit for sure. Any suggestions would be appreciated. Thanks.

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johns48jdb

02-02-2008 06:13:21

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Re: Positve ground in reply to jturbo10, 01-31-2008 21:07:03

if it was running the way it was then don't fix it if it ain't broke.

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Texasmark

02-01-2008 06:33:39

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Re: Positve ground in reply to jturbo10, 01-31-2008 21:07:03

Somewhat confused about your alternator and elect ign with pos ground. Is the alternator the automotive AC Delco with internal regulator that are popular? Does your elect ign have a red and black wire to it?

Mark

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jturbo10

02-01-2008 07:29:35

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Re: Positve ground in reply to Texasmark, 02-01-2008 06:33:39

Mark...will be out of town most of today but will get the alternator and electronic ignition numbers/wiring colors when I get back. I'm just as confused as I thought it had to be a negative ground for an alternator setup. The only reason I assumed the ignition was positive ground is they will only work one way and if you hook them up backwards they will fry. If that is not true then I'll have to investigate more. I'm not an electronic genius so I want to proceed slowly before I mess up a good running tractor. Thanks Joe

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Gerald J.

02-01-2008 08:03:00

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Re: Positve ground in reply to jturbo10, 02-01-2008 07:29:35

COMMON alternators are always negative ground. But they can be had postive or no ground. Just not Advance or O'Reilly's autoparts off the shelf. The tractor was originally positive ground.

The electronic ignition box can be had either positive or negative ground and solid state stuff fries faster than you can "oops" with reversed polarity.

The alternator having solid state rectifiers and regulator will also fry instantly with reversed polarity. So if its all working with positive ground leave it or replace both the alternator and the ignition box. Its FAR cheaper to leave it alone.

Gerald J.

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Texasmark

02-03-2008 09:29:33

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Re: Positve ground in reply to Gerald J., 02-01-2008 08:03:00

Agree. Jturbo, you need to walk through this changeover before you hookup or change anything. Also, are you diesel or gas....reason for asking is polarity of ignition coil if you change your ground to neg.

The reason the alternator fries diodes is that it has reverse diodes to ground (and forward diodes to the + terminal but forget that for now). So in the reverse polarity position (like if the + battery terminal in your car becomes -) you have the - voltage applied to the alternator windings which have very little resistance and thru them it is applied to the - terminal of your alternator's rectifier diodes which have the anodes grounded.

That you know is forward bias for them so they allow current to pass. Problem is, with essentially no resistance in the circuit, the current exceeds their capability and they burn up from heat. Like it's 20 or so to 1 over their normal rating.

Mark

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