I have been trying to find information about a pulse-driven tachometer, and came across the old post linked below.
I have the same tach with intact, original wiring. All of the tach sending wires terminate at the (in my case) 1965 McCulloch 75hp... but not at all where you'd expect! The signal is NOT coming from the alternator or rectifier. There is a small plastic switch (transducer) enclosure of some sort that mounts to one of the the starter mount bolts and is fed by a short length of fuel line teed off the fuel pump. It is my guess, that this tach somehow converts fuel pump pulses to RPM. I have been Google'ing the topic of fuel-pulse-driven tachometers, and haven't found anything other than this very old post.
The fule line that drives this " transducer" is teed off of either the #1 or #2 pressure line. I'll update with photo soon.
I took the sender apart - the harness sends the following to the tach: blue = gnd, brown = common, red = normally closed, orange = normally open. the 3 cylinder motor has 2 pressure lines that go to the fuel pump, and the diaaphragm in this sender is attached to one of the fuel pressure lines. Apparently, the pulse that drives the fuel pump makes/breaks the contact for each pulse, and both the make and break are read by the tachometer. Now to find a new diaphragm with an actuator - looks a bit like an accelerator pump on an automotive carburetor.
Here is the sender that converts fuel pulses into tach signals for the Scott tachometer with black, blue, brown, red, orange wires.
blue is ground, brown/red/orange establish the pulses, and other wires into the tach are likely for ignition +12v and backlighting. Diaphragm looks very close to a Holley accelerator pump from a standard auto carb...
First, a caveat. I have never seen a pulse tach system on a Scott motor, but I knew they existed.
You need to check the tubing from the transducer. As I understand the system from the owners manual, it runs off of crankcase pulses, not the fuel pump, at least the liquid gasoline side. The fuel pump also runs on crankcase pulsing so I think the transducer is tapped in on the crankcase side of the fuel pump, not the gasoline side. The fuel pump pulsing would be irregular depending on RPM and load on the engine.
The wiring diagram above of the electrical tach should help you sort out your system a little easier.
Now a little history.
It appears there were three types of tachometers used by Scott outboards.
An all electric tach used from before 1957 to at least 1961. This tach used an extra set of points in the distributor to operate the tach. There is some info available including a wiring diagram I attached that might be useful.
A self contained tach that used rechargeable batteries for power. This was used for small outboards without battery/generating systems. Service Bulletin #338 covers replacing defective batteries.
The pulse tach used in your motor. The only mention I found of it is in the 1965 Owner’s Manual. I have attached that page.
I went through the 1962 to 1965 Parts manuals, the 1962 to 1966 Factory Service , manuals the 1961 through 1966 service bulletins (about 200 pages) with little luck.
I may have missed it, or it may be some document that I don’t have.
My suggestion would be to contact Discount Marine or Laing’s for parts
Discount is the most likely to have the pulse assembly if they can find a parts number for it. I will continue to look for info on that tach.
Sorry about the attachments. Not too clear and inverted. Operator error.
Wow! That is more than I could have imagined! I have parts diagrams, and even the old outboard motor service manual reprint, but I have never seen anything like what you provided. I am more than curious about an owners manual, as I have never seen one reprinted or offered for sale.
I should have clarified that there are 2 pulse lines from the crankcase that power the fuel pump (4 lines in all). The transducer is powered by one of the crankcase lines to the pump. The pressure pulse actuates a diaphragm which looks exactly like an automobile accelerator pump, but with a removable plastic plunger (so as not to beat up the contacts). The plunger extends into a switch enclosure, moving a brass reed off of the normally closed (pink) and onto the normally open (orange) when there is positive pressure. Id does nothing but maintain NC and NO under vaccuum or no pressure. I am a bit baffled why the tachometer would want both the BREAK and the MAKE, since I would think they happen only in pairs.... Almost makes me think the tach considers the amount of time between Break and Make, since it takes little pressure to Break and more to Make.... I was able to find a diaphragm that was close enough to work, since the plunger is a removable, 2-piece part that can be re-used with a new diaphragm.
So that I could corroborate the RPM, I setup a bolt with a wire arm on one of the flywheel puller holes, then mounted an arm with a bicycle bell that would get struck with each pass. I recorded at the bell while cranking the starter for about 5 seconds, and measuring the Hz from the stator with my Fluke. The 12-pole stator read 40Hz, 2400 RPM, and the filtered audio file of the bell allowed me to measure approx 600 RPM, which can be resolved if the stator multiplier is 4.
I'll have to get nearer to water to see what the Tach reads now with the new diaphragm, or build some type of transistor switch to send known freq. pulses to the tach wires.
Thank you for finding this post and sharing the build knowledge that I never knew! May I contact you about some of the documentation materials you have?
Davids in Minneapolis, 1965 longshaft McCulloch 75HP on a 1965 Lonestar Mustang
If you can give me the serial number of your motor I can be more specific as to what manuals you need. I am pretty busy right now with getting the shop etc. ready for winter.
I live just South of Minneapolis in Lakeville MN. We can get together a little later after things settle down for me.
If any moderators see this could you move this thread to the JEGO forum. Thanks.
Would be great to get together! Serial number is 65307541 - 1549 (Electric Start, Long Shaft).
I the meantime, I've noticed: there is a pressure valve (used on pool heaters!) that essentially uses about 2lbs of air pressure to drive a standard honeywell microswitch. Those can be gotten with SPDT contacts, which would mean that the fuel pulse pressure signal would break the NC contact and then make the NO contact... just like the original transducer does. From the limited repair manual I have, the tachometers that were distributor driven, were adjusted (more or less) ONLY so that the distributor lobe ensured the NC contact would break completely, and that the stop would prevent the NO contact from bending/distorting at the highest point of the lobe. Still not 100% clear on whether the TIME between break-make plays any part in determining RPM, or if the tach is just counting the frequency of make/break. The pressure switch assy. is shown in the attachment.