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My point in all of this is that if the sensing circuitry is voltage sensitive you need to have some protection on it as it will certainly see more than 50 volts. In fact, what my scope shows is actually less than w/o the scope because the scope's capacitance, however small, will tend to dampen the swings slightly. So, were it me, I'd put a 25 volt zener across the input with a resistor ahead of it so it isn't trying to deal with the coil's wrath.
In other words, there are ways around the problem at hand. So, if you want to run one of the diesel tachs we can design an input circuit that will protect it. In fact, you probably need the same circuit I do for my air/fuel ratio meter - the one we had on your truck last year. That's 'cause the tach input on my meter wants a 5v square wave input, and goes bananas with the signal from the coil.
So, let me think about what/how to do this. I'm not saying it'll be easy, but it can be done. And, we can prove it on the bench before subjecting your tach to it.
I believe I will go with the 4000 RPM tachometer. The 3500 RPM tach would have been cool, but is not necessary. What puzzles me is that it was thought to come from a gas engine truck and does not have the yellow and red zones which are on all diesel tachometers which I have seen?
The 4000 RPM tachometer and the 4500 RPM tachometer both have what seems to be the "gasoline" engine markings on the terminals.
I have an open style 6K tach and a closed style 6K tach somewhere in my storage. When I get a chance I am going to examine both and see how the terminals are marked. My first reaction was hey, the 3,500 tach came from a 370 gas truck so how could it not be for a coil signal... but then I remember from my reading on diesel tachs that a lot of medium duty diesel trucks (not necessarily Ford) took their tach signal from the flywheel teeth. It is a very long shot, but it is at least possible that the 3,500 tach was set up that way, and therefore compatible for both gas and diesel versions of the same truck (?) That seems a bit far fetched but with a part that rare it is good that you are cautious with it. I am curious though, how much difference in voltage there is between gas and diesel signals. There was a thread on the diesel forum from a guy who transplanted a diesel into an 8th generation F150 and claimed the gasser tach actually worked. He said it was reading low but it was working. I don't know if the reverse would end as well...
Okay, as promised here are a couple of photos to compare the terminals on an open style and closed style 6K gas engine tachs. As you can see the closed style has the cast "G" and "B" on the right, and ink stamped "C" and "8" on the left, not S+ and S- like the 3500.
Today the mail man brought me a cool surprise... David sent me a 4500 tach, presumably the non-diesel one with no yellow or red zones on the face. It is cool to see these unusual parts in person. Thank-you David for this museum piece!
By coincidence I visited the junkyard last Friday and I decided to snap a couple shots of the truck that the 4000 tach came from before it gets crushed. Here it is just for visual interest or in case any of the door jam info. is useful to anyone researching medium duty truck tachs:
The windshield trim, on the old truck, looks salvageable, Jonathan.
Do you know of anyone who needs it? I could probably pull over a dozen decent sets at that junkyard. I wish I could save it all but I don't have enough trucks to put it on!
Another strange thing is the "Power Wagon" hood emblem. I thought that was a Dodge thing, unless somebody just bolted it on there...
Do you know of anyone who needs it? I could probably pull over a dozen decent sets at that junkyard. I wish I could save it all but I don't have enough trucks to put it on!
Another strange thing is the "Power Wagon" hood emblem. I thought that was a Dodge thing, unless somebody just bolted it on there...
No, but I hate to see good pieces go to the crusher.
The windshield trim might be really useful. David gave me 4 of the 5 pieces in very good condition, for which I'm eternally grateful. But, since they do have some minor scratches I thought about getting NOS ones - until I discovered that a full set is about $500. So, I ordered the 5th piece to complete the set, and even then it was pushing $100 with the part and shipping.
The windshield trim might be really useful. David gave me 4 of the 5 pieces in very good condition, for which I'm eternally grateful. But, since they do have some minor scratches I thought about getting NOS ones - until I discovered that a full set is about $500. So, I ordered the 5th piece to complete the set, and even then it was pushing $100 with the part and shipping.
Oh wow! That's really expensive. My yard sells trim in two categories: less than 6" and more than 6". Since the windshield trim is one continuous trim line it is considered one piece even through it breaks into segments. I got the whole set for $20. I guess I should probably buy a spare or two or three... sorry for the thread jack David
On a more relevant note David tells me the 4,500 tach came from a Diesel engine medium duty. I don't know for sure where the signal came from but if it had a 6.9/7.3 IDI it might read correctly in my truck. I will try it out when I get my engine running
Thank you so very much, Mr. NumberDummy! Is it possible that a NOS one still exists as obsolete? The best I can tell from looking over the information you provided, the 4000 RPM tachometer like the one I have in my truck is: 80/81 F600/800 [gas] *D8HZ 17360-B. A photo of it is on the 1st page of this thread.
Thank you so very much, Mr. NumberDummy! Is it possible that a NOS one still exists as obsolete?
The best I can tell from looking over the information you provided, the 4000 RPM tachometer like the one I have in my truck is: 80/81 F600/800 [gas] D8HZ-17360-B. A photo of it is on the 1st page of this thread.
Notice in the description (column right) that E1HZ-17360-EreplacedD8HZ-17360-B
No Ford dealer or obsolete parts vendor has either one.
btw: I have my P/M feature turned off, but Mr. Clean aka Mr. Fixit, since he's a mod, can P/M me, but then I have to email him back with my answer.
