Hey all, Before I take I out and bin it I thought I would ask if anyone knows how to clibrate / fix my rpm guage (one on the left) It's only a cheap unit so thought I would give it a go, set to 4 cylinder mode wired as they normally are to the - of the coil, but it always reads far to high. I've added a inline 10k 1/2w resistor to the signal wire. But I've found conflicting info Is the resistor suppose to be inline with the signal or bridged between the power and signal wire. Cheers all.
Lot of waffle in that link, but it implied the resistor should be used as a pull-up, so wired between the tach signal line and ignition-switched +12V
It may need a capacitor to ground as well as the resistor at the coil end in the signal wire. Its the metres of wire connected to the coil that cause problems. I will think on this some more and make some suggestions tomorrow.
Best to have it closer to the gauge. I think the problem is there’s no internal pull up in the gauge, so the input will “float” when the breakers (or transistor in any electronic ignition unit) are open. A pull-up yanks it up to +12 when nothing’s happening.
I had bizarre readings on my VDO tachometer so I fitted it with an inline diode (1N4007) which cured my problem
Diode in mine like @Valveandy The diode is if the gauge is getting a double signal because of the length of cable...don't know if it affects the digital versions...
Does this imply you're seeing reflections on the long length of wire and the diode is preventing the reflected signals? Whatever the reason, the diode will need to be the right way around.
PS I am not sure why you would need a 7 amp diode, I am just guessing that's what was in @Valveandy 's bits box.
The VDOs don’t like negative-going voltage spikes below the 0V rail for some reason. Adding a series diode chops any spikes off, or at least clamps them. A 1N4007 is a 1A diode, just with a higher (400V?) rating.
Sorry, yes, had to look that up, 1000v PRR, 700 VRev (rms), which is presumably where the 7 comes from.