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Ford 3000 SOS wiring query

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06-09-2018 07:09:16

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Gday everyone,

I have a query for the experts amongst you all!

I have a '65 Ford 3000 Diesel with the SOS tranny. I'm in the process of doing some work on it and I'm replacing the wiring harness with an aftermarket harness.

My query is in relation to the 3 'electrical' wires that are coming out of the transmission cover. I know that 2 of them are for the safety switch and I have that covered, but the 3rd wire has me tricked. It is a smaller gauge wire and it was hooked up to a plain red wire on the original harness. I've looked at all the wiring diagrams in the service manual and there is no entry to what this wire is hooked up to. In the wiring table in the service manual it states that a plain red wire services the "Light switch the side and rear lamps".

Does anyone else have the same setup on their SOS or any ideas as to what it could be?

Thanks in advance


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06-11-2018 04:20:14

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 Re: Ford 3000 SOS wiring query in reply to mickotab, 06-09-2018 07:09:16  
Excellent..... thanks very much Gents.
It all makes sense and it may me pay a bit more attention to the service manual which explains it all in black and white! :oops:
Thanks again and I enjoying reading everyones problems and solutions.



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Sean in PA

06-09-2018 11:02:58

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 Re: Ford 3000 SOS wiring query in reply to mickotab, 06-09-2018 07:09:16  
My '73 4000 S-O-S only has two wires coming out of the top of the trans. I am pretty sure that the 1965+ S-O-S transmissions did not have a low pressure warning light. There is nothing in the schematic in the service manual for one. The only "extra" wiring shown for rthe 1965+ S-O-S tractors vs. the manual tras modles is the illumination bulb for the gear selector so you could see the numbers for which gear it was in at night. A 1965 3000 is a fairly early one so they might have included the pressure warning light back then but I can't find it documented anywhere.

Easiest way to tell for sure is to measure the resistance of that wire to ground when the tractor is not running and again when it is running. f it is a pressure warning light wire then it should read infinite with the tractor not running and close to zero with it running, as the sender is just a switch for a warning light, not a variable sender like for a pressure gauge.

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06-09-2018 14:31:37

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 Re: Ford 3000 SOS wiring query in reply to Sean in PA, 06-09-2018 11:02:58  
third party image

Lube light was discontinued in 1968 with the "new models" - image above is from the repair manual. And yes, the wiring diagram does show the light in a separate box on the same page. If I owned an SOS, I would install one and make sure it works!

Testing of the circuit is backwards from what you describe. The light is designed to turn on with low pressure, therefore resistance should be zero with the engine off, and vice versa. That said, the last SOS I worked on had a faulty switch, which I had to source aftermarket.

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06-09-2018 10:01:32

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 Re: Ford 3000 SOS wiring query in reply to mickotab, 06-09-2018 07:09:16  
Low lube pressure warning light. If it were my machine, I'd have it hooked up and operational. It will warn you of hydraulic issues in your trans long before it becomes an expensive repair.

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06-09-2018 08:46:07

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 Re: Ford 3000 SOS wiring query in reply to mickotab, 06-09-2018 07:09:16  
There is an inset picture of the wiring diagram that shows the SOS part. It is the warning light above the shifter

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Pete black

06-09-2018 07:17:16

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 Re: Ford 3000 SOS wiring query in reply to mickotab, 06-09-2018 07:09:16  
Coming out the transmission it may be for the red warning light on the dash for "low pressure".

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