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post #1 of 5 Old 04-22-2008 Thread Starter
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Perkins 4-108M Switch details

I am trying to replace my engine switch and Perkins is less than helpful and don't make a replacement. The wires are a mess and I don't know where they terminate. Does anyone have a wiring/terminal diagram for the 4-108m ignition switch.
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post #2 of 5 Old 09-01-2008
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no, I don't know, but I have the same question. My ignition switch on the same motor on my sailboat has quit working and i need to install a new ignition switch, that comes with no schematic or instructions.
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post #3 of 5 Old 09-01-2008
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Ahh! I see I'm not the only one frustrated by Perkins wiring!

I've found that the engine owner's and workshop manuals are not quite exact in this area, since it's obvious that my instrument panel is generally different than those shown in the diagrams.

Which switch do you have? Is it the four position (Off, Run, Preheat, Start) switch, or a two position (Off, Run) with separate starter and kill buttons?
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post #4 of 5 Old 12-13-2008
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switch detail -- Off, Run, Preheat, Start

I got the same problem. I just bought a Perkins Ignition Switch 2846129 and it is a Off - Run - Preheat - Start switch. I think the switch itself has five termanals with one or two of them double. I'm not certain as it's on the boat and I'm at home.
I also cannot find a schematic or anyplace else and would like to know where the many different wires go to this switch. I've got a Perkins 4-108 in a boat. Before the old ignition key froze and I had to buy another, it also had an alarm (maybe a low oil pressure alarm) hooked to it. I put a temporary ig. switch in till I could get the original online. In the meantime, I lost all original info that I had written down before removing all the wires.
any help would be great
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post #5 of 5 Old 04-05-2013
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Re: Perkins 4-108M Switch details

This is five years too late but what the heck...

Diesels do not require any electricity to run per se. On my Valiant (which has separate start and preheat switches) the ignition switch simply applies 12v to the alternator/regulator, to the oil pressure switch, and to the start and preheat switches. The way my boat is wired the instruments are powered by the oil pressure switch--that is, the instruments don't get power until the pressure comes up--no idea why it's wired that way.

If a) you're at all handy, b) not color blind and c) the wires are different colors you could simply check the wire color at the preheater (bolted into the side of the air intake on the manifold). Then look at the small wire on the starter relay. That should give you two of them. The terminal with two wires on it probably goes to the alternator/regulator and down to the oil pressure switch. The power coming into the switch is most likely red, although I would NOT assume it. If you have a multimeter you could switch battery power on at the master battery switch and measure the voltage on each of the wires one at a time. That will allow you to find the 12v that powers everything. The other wires presumably will go to the alternator/regulator and the oil pressure switch if your boats are wired that way and I believe that that's the way Perkins wired the engines.

Then you could use the "OHMS" setting on your meter to figure out how the switch works: In "PREHEAT" position you will get continuity between the switch common terminal and the PREHEAT terminal. In START position you will get continuity between the switch common terminal and the START terminal. In RUN position you will get continuity between the switch common terminal and one of the other terminals. But don't be surprised if the RUN position connects everything together.

Obviously, doing these things will allow you to determine which switch terminal is the common, since each of the positions connects the common terminal to something—preheat, start, etc.

Hope this helps someone sometime.
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