The Fordson Tractor Pages Forum

Hi everyone,
I can't find a topic on this exact issue, but basicly, I am nearing completion on my dexta project which is a full re-build (been going on for a while).

It has, a brand new dynamo, wiring loom and lucas voltage regulator, but is not charging the battery.
I have checked the dynamo, and it is working. The wiring loom is new and has no breaks.

I took the top cover off the regulator, because I think it might be faulty. The battery was still connected and I touched one of the contacts very lightly by accident, the contacts joined and then it instantly blew a 15 amp fuse. Its worth saying that I have fused the different parts of the loom incase of shorts etc.

Does this mean I have a wiring connection wrong, or should both contacts in the regulator open and close together and that is why it blew the fuse?

Thanks everyone

Did you polarize the dynamo?
You also need some revs before it will load.
All connections are good?

Yep dynamo is polarised and generating voltage correctly.
I have revved it up and it makes no difference, connections are all tight and all the other wiring, like lights etc work well.

It is just the whole blown fuse thing, it was a dead short because it 'popped' and flashed, but I'm not sure whether it would do this if only one contact is made at the regulator and not both together?

Ive not really done much with dynamo and regulators before

Are you sure the connections are right? D to dynamo out (large connector), E to earth, A to battery negative and F to dynamo exciter (small connector).
Otherwise inspect the regulator and all the fuses.
Sandy

You may have put 12v onto the field windings which will give a very high voltage out on the D line. That's what may have blown your fuse.
Have you ensured that there is a good earth for the battery? Under a light load you may have a connection, can you crank over the engine? Measure the battery voltage using a bare metal part of the chassis rather than the battery post. IMHO
Jerry

Try a quick test bypassing the fuses, the original electrics don't have any fuse. You might find it works fine like that. If so measure the current so you use a big enough fuse.
Sandy

I will check all connections tomorrow but I have already checked them twice!
Its worth saying that it didnt blow the 15 amp fuse until i took the regulator cover off and accidentaly made the contacts touch. As it only charges at 11 amps, surely a 15 amp fuse if sufficient?

Regarding the earth, it has a very good earth, the engine starts very quickly, but I will check the earth that goes to the regulator as it is linked with the horns earth, and now I think of it, i havent actually tried the horn yet

Ok, so I went over all wiring connections today, and they are all good, and in the correct place.
I started the engine and tested voltage on the 'f' and 'd' terminals at the regulator, and there was none when i measured to earth (negative), but when i measured to positive it had voltage, so it was producing voltage as if it should be positive earth. This was strange as I had already polarized the dynamo.
So I polarized it again to operate as negative earth, and without the two cables connected at the dynamo, it was producing voltage correctly.

I re connected the wires, started it up, and it went back to how it was before.

Am I missing something here???

Fordson tractors are positive earth.
Sandy

Yes I know they are from factory, however my major is negative earth. I thought you could kind of convert them to work on negative earth, is this not the case with a dexta?

There must be something wrong with the wiring or regulator as it should be convertible (but not recommended for either Majors or Dextas).
Sandy

Hi,

Imho there is no reason to make a Fordson negative earth. I wouldn't do that personally.

But I get curious about Sandy's statement. Can you explain why it is not recommended? Can't think of a technical reason myself.

rgds

Emiel

The only reason for nor recommending changing the earth is that it involves polarising quite a bit which is a delicate operation. The wiring diagram needs a change to be accurate for negative earth.

Nick, have you tried it with positive earth, as it might work fine.
Sandy

Billy26F5 wrote: ↑

Mon Dec 07, 2020 10:26 am

...... changing the earth ...........The wiring diagram needs a change to be accurate for negative earth.

Not a big problem then. Nor the ammeter working backwards.

When I rewired my Dexta and changed it to -ve earth as I was using an electronic indicator unit and an Alternator. IMHO if you change to -ve earth with a regulator then the only thing you have to do is flash the generator the same way, using a "live" feed from your battery to spark the field winding. This will reverse the residual magnetism polarity so that when the generator is rotated by the engine then there is a small output from the generator. If you have flashed the generator and with both F and D connections removed and you turn the generator then you can measure this small voltage on the D terminal. It is used by the regulator to start the regulation process by feeding this small voltage back into the field windings to increase the magnetism and thus increase the voltage out from the D connection to be higher than the battery voltage which will cause the current to flow into the battery to charge it. As this output voltage rises it causes a relay in the regulator to close it's contact to allow the flow of current into the battery. If you didn't have the relay then the battery voltage will flow back to the generator and cause it to become a motor! The relay has two coils that are energised, one by the battery voltage and one by the generator voltage but in opposition. When the generator voltage exceeds the battery voltage (the condition required for a current to flow into the battery to charge it) then the relay operates and connects the generator to the battery to charge it. IMHO
I converted my Dexta to an Alternator, the wiring diagram I submitted is available at the top of the Dexta page. It is under the WIKI section, second entry.
Cheers
Jerry

I think what I will do, is try it positive earth at the weekend, but I still don't reckon it will work.
Its almost as though when I have polarised it, when I connect the wires back up its being polarised again but not the way I want it. I think if I make it positive earth, it will do the same, which leads me to think the regulator may be faulty?
As I said previously, everything is brand new. I do have the original lucas regulator aswell, so I might try that

It is really frustrating, because if it works, I can put it all together and the re-build will be finished!!

Definitely try the Lucas regulator as it will be better quality.
Sandy

Did you ever manage to sort your charging issue? I’m in the exact same position, new wiring loom, new dynamo, new regulator but not charging despite checking wiring over and over, polarising the dynamo several times, testing the dynamo shows it putting out 14.2v at half revs (only 1.5 - 2 volts at idle, is this normal?).

Im also thinking of trying putting the original Lucas regulator back on to see if this gets it charging.

Do try the original regulator, the new one might well not work knowing how aftermarket suppliers get things wrong.
Sandy

Quick update - installed the original Lucas regulator, everything now working as it should. Just goes to show how poor some of these repro units are.