My tach is jacked up

[roostwear]

I have been suspicious of my old VDO tach for some time. It has read a max rpm of 3800. I GPS'd at 65 with an A impeller so, I'm damn sure the tach isn't reading right. Up to 3800 it's fine, it just stops there. In poking around the internet, I found something that said to put a diode inline with the signal lead to a VDO tach. I assume this is for voltage reduction. When I bought the boat, I converted the old points dist to Pertronix and eliminated the ballast resistor, so I don't have any experience running with the points. Has anyone else run into this with a Pertronix igniter? I can't find a direct replacement for the VDOs, So I am hesitant to install a new set just for this.
?????????

[miketsouth]

I have changed out my merc 470 with pertronics, and my stock bubbledeck 454 and it never affected the tach. Bad ground and poor connections have done it to me over and over.

[wsuwrhr]

Are you sure it isn't the engine connected to the tach?
My tach doesn't have the problem you have.
Just my .02 cents.
Brian

[Rexone]

Originally posted by roostwear
put a diode inline with the signal lead to a VDO tach. I assume this is for voltage reduction.
I can't help on your tach problem but a diode won't reduce voltage. A diode is simply a one-way check valve for voltage, in this case probably preventing feedback in that wire.

[roostwear]

Actually, in electrical engineering, a diode has 2 uses. The "check valve" (one direction), and a low amperage voltage reduction in the other. Hence, the need for different values of diodes.
Mike, I didn't see that you carry VDO Vision series gauges, just Autometer, Gaffrig, etc. Can you get VDO?

[King Kuracz]

Rex is right, an ordinary diode is not used for voltage reduction. What you're refering to is a zener diode which is used to limit output voltage to a maximum amount. It does not "reduce" it , it limits it to a stated voltage once the input voltage exceeds that design value. Below that design value it has no effect.
The only "values" associated with a common diode is maximum current amount. Construction and semiconductor material determine it's other charateristics.

[wsuwrhr]

So what happened, did you figure out the problem is WHAT is hooked to the tach?
Brian

[roostwear]

Originally posted by wsuwrhr
So what happened, did you figure out the problem is WHAT is hooked to the tach?
Brian
This would imply your death smoke belching mopar gps's more than me? :D

[Rexone]

We can still order any VDO you need. Just don't stock it any more since they discontinued the chrome and black marine lines.

[wsuwrhr]

Originally posted by roostwear
This would imply your death smoke belching mopar gps's more than me? :D
Yes sir, on the freeway to the river or on the way back.
Brian