Aftermarket Rev Counters

[Madmaninshed]

Hi,  My 1900 DG engined water cooled 1986 van has a 2” aftermarket analogue rev counter, fitted by the last owner.  It jumps about, and gives different readings depending on whether accelerating or decelerating (very annoying). There was a similar post on here by someone else who had the same problem a long time ago.  I bought an inexpensive digital replacement, and used the same connections.
The new one is nice and stable, if a little slow to respond. It looks great, but reads almost double (1600RPM at tickover).  The cylinder switch 4,6 or 8 changes the reading on the old analogue one, but does nothing on the new digital one.  Has anyone recently had a similar problem please, and perhaps found a way to get a true steady reading?  The supplier has been very helpful, but has not come up with a solution so far.  Can I check the signal with a multimeter please, and if so, should I set it to volts, amps or ohms please? (Excuse ignorance)
 

[davidoft1]

The switch on the new one is most likely faulty

[The Hairy Camper]

Does the new tachometer come with a trimmer potentiometer for calibration? Looks like a plastic philips head on the back of the tachometer.

If not, you may be better off with a different rev counter. Digital one's probably aren't best suited for our older vans.

[TwinTurbo]

you would need an oscilloscope not a basic multimeter. But the likly hood of the signal being wrong is low as that woul show poor running.

[Madmaninshed]

Thanks guys.  I am hoping it may be a dodgy switch, because the little tachometer will be great if it works.  The SHC rev counter looks brilliant, but I don’t want to lose the analogue clock you helped me to fix.

[TwinTurbo]

why do you even need one?
 

[Shinnster]

i put a vdo tacho on mine, but it also read high. I had to put a resistor (can't remember what one) inline to bring the revs down to the correct reading.

[TwinTurbo]

Strange that a resistor helped as they mostly work on pulse counting.  A higher reasding would normaly suggest the unit is from a car with more cylinders and the pulse devider is not right ( ehy you have them with 4-6-8 switches , although if a VDO was from an AUDI it may be a 5cyl application. 

[Shinnster]

it was a standard generic vdo one, set to 4 cylinders. i had to use a diode inline (not resistor) .
apparently mine was caused by noise interference, which makes sense as my stereo whines like a you know what!