780 Polarity - Always Negative Earth?

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stevegraygb
Posts: 13
Joined: Sat May 11, 2013 8:18 pm
Location: Hampshire

780 Polarity - Always Negative Earth?

Post by stevegraygb » Mon May 18, 2020 3:56 pm

Hi all

This has all the hallmarks of a stupid question, so apologies in advance.

When it arrived with us, our (dynamo equipt) 780 Selectamatic had the battery installed as positive earth. It also had a whole host of electrical issues - which possibly isn't surprising as all online resoruces suggest that this should be a negative earth tractor. Problems included (but not limited to) dash lights always on regardless of key position, battery draining if left connected, all gauges non working, and dynamo spinning up as a motor if battery connected, again regardless of key position.

Even connected as positive earth (which is how I previously thought it should have been up to now) it will crank and start though. I've disconnected the dynamo (and battery!!) for the time being. At present, it won't crank on the key, but will if shorted across the starter solenoid. (Yes, I know- butchery!).

Is there any way likely way this tractor could legitiamately have been positive earth and the electrical problems are all coincidental, or is it more likely that it's always been negative earth, the battery was at some point conected the wrong way, and the control box (and maybe some other things) are damaged as a result?

And can I just re-connect as negative earth, and try again? Or will it have 'flashed over'?

Many thanks in advance, SG.

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ollek
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Location: South West Finland

Re: 780 Polarity - Always Negative Earth?

Post by ollek » Mon May 18, 2020 4:10 pm

The 780 is a Selectamatic and all Selectamatic tractors have negative earth.

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rid54
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Joined: Mon Oct 13, 2014 8:20 am
Location: Vallentuna, Sweden

Re: 780 Polarity - Always Negative Earth?

Post by rid54 » Mon May 18, 2020 8:22 pm

I suppose you could check the system by disconnecting the dynamo and the control box/regulator completely and hook up the battery the right way around (neg. earth). If the battery is charged, the indicator lights, instruments and starter should work. I would then suspect that the regulator is damaged from (very probably) being subjected to a battery connected in reverse at some point. If things seem OK, check out the dynamo for any defects and then try to get hold of a new regulator. There are documents that have in-depth descriptions of electrical system and repair work on the components available through a link I found on this site, lost track of it though, unfortunately.

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cobbadog
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Location: beautiful Coopernook NSW. near Taree. NSW. Australia

Re: 780 Polarity - Always Negative Earth?

Post by cobbadog » Tue May 19, 2020 7:19 am

Yep, positive earth would have been the original set up, but we don't know the history of the tractor before you have owned it either. Over time people do some silly things like replace parts or convert things over to being different to what they should be. We have a grey fergy that runs an alternator and I have renewed the voltage regulator on my 30C and had the auto electrician flash it over to cope with positive earth.
I would suggest to use a multimeter, start the tractor and disconnect the battery and then check the output of the generator/dynamo to confirm that it is positive earth, then check the regulator for the same positive earth. If you find an issue there you can flash it over to being a positive earth and if still no joy then it is a fair guess that it is finished. You are going to have to trace every wire to confirm that it is wired the correct way and you certainly will need to test your ignition switch and any other switches to ensure they too are correctly wired. It is a big job but rebuilding a tractor that has burnt out is a bigger job.

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db2d
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Location: Monmouthshire

Re: 780 Polarity - Always Negative Earth?

Post by db2d » Tue May 19, 2020 7:50 am

As Ollek stated the 780 has negative earth

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stevegraygb
Posts: 13
Joined: Sat May 11, 2013 8:18 pm
Location: Hampshire

Re: 780 Polarity - Always Negative Earth?

Post by stevegraygb » Tue May 19, 2020 9:11 am

Great, many thanks for all the responses , very useful.

I think then that this weekend's job will be to do some checks with a multimeter and expect to have to replace a control box, potentially. Will update as to how things progress...

Thanks again, SG.

philedge
Posts: 324
Joined: Sun Dec 03, 2006 11:27 am
Location: Chester UK

Re: 780 Polarity - Always Negative Earth?

Post by philedge » Tue May 19, 2020 10:50 am

Theres a 780 manual at the link below that has a wiring diagram showing negative earth. I cant think of any reason why someone would convert from the standard negative earth to positive so I guess the battery has been fitted the wrong way round! Good luck sorting out the charging

sites.google.com/site/deerebrown/david-brown-info-ii
'66 880 Selectamatic rat.

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rid54
Posts: 210
Joined: Mon Oct 13, 2014 8:20 am
Location: Vallentuna, Sweden

Re: 780 Polarity - Always Negative Earth?

Post by rid54 » Tue May 19, 2020 2:49 pm

If the dynamo isn't damaged (burnt winding or so) there is no reason to believe it shouldn't be useful with a good regulator. There is a procedure for pre-polarizing it, so the remanent magnetism is "the right way around" for the correct polarity. The regulator (control box) on the other hand, must have been at least partly damaged, as the post suggests the cut-out doesn't work.

If the wiring hasn't charred or is obviously badly tampered with, it should work irrespective of polarity. The only components that are polarity-sensitive are the instruments.

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cobbadog
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Joined: Tue Oct 11, 2011 6:18 am
Location: beautiful Coopernook NSW. near Taree. NSW. Australia

Re: 780 Polarity - Always Negative Earth?

Post by cobbadog » Wed May 20, 2020 7:16 am

Sorry for misleading you about the polarity. It is neg earth.

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