MicroMonitor and Van's tach sender

[DLussier]

Hello,
Anybody had any chance using a Van' tach sender part# VTACHGEN 2 with a Rocky Mountain MicroMonitor?

The RPM indication is fine until 2300 rpm then just goes down to about 1350 rpm as engine is increased to 2700 rpm.

My Micromonitor has just been modified to work with a Hall effect sender as per company's instruction and it is the last software version 6.08.

I am in contact with Mr. Mowrer from RMI but we cannot confirm what type of signal is coming out of this sender.

Thank you.

[N402RH]

The Van's sensor is 16 pulses per revolution and I suspect at the higher RPM the pulses are faster than the 555 timer masking circuit can handle. If you shorten the output pulse duty cycle on the 555 IC it should work assuming the software in the unit can be configured for the 16 pulses.


Rob Hickman
N402RH RV-10

[Mike S]

As I recall, the pulse is 8 per prop revolution, due to the cam being driven at half prop speed.

I had to set my tach for 8 pulse/rev to get it correct.

[DLussier]

Following is what you have to know to use the Van's Aircraft tach sender part# VTACHGEN 2 with a Rocky Mountain MicroMonitor according to Mr. Mowrer from RMI:

"Only version 6+ software has the capability of dividing the input pulses by more than 2. So this won't work on earlier software versions (probably the vast majority of unit out there).

You have to modify the MicroMonitor as per the Hall effect Installation Supplement 7/15/93.

FAST RPM ENGINES WITHOUT MAGS:
Remove U24 from circuit whenever pulse train is faster than 7ms (about 3200 RPM with
two pulses per rev). Procedure is, ASSUMING that your tach input signal is from an EI or similar, not a magneto.

Open the top and bottom. Remove the retaining screw. Remove the two mounting screws holding the vertical CPU board, and remove the CPU board. From the CPU board, remove U24 (marked 7555) and discard. Use a SMALL gage bare wire and short pin2 to pin3 on the bottom of the board where U24 was. Use the same type wire to short across C35. This lets the tach signal bypass the U24 circuit.

If you’re interested, a magneto’s firing waveform is a huge, decaying sinusoidal (ringing) pattern. The U24 circuit is a “one-shot” circuit that senses the leading edge of the first voltage excursion, passes that signal along to the CPU, and then shuts itself down for approximately 6 milliseconds to let the ringing die down before looking for the next pulse. At your revs, the one-shot is still shut down when the next pulse comes along, effectively dividing the pulse stream into half."

You need to select 4 pulses per turn in the option page.

I did all the changes to my MicroMonitor and it is now indicating RPM properly.

[almarsh]

Having problem with Cht readings jumping after 6 years of good luck replaced sender no luck that route still reading wrong do I have to put probe in hot water to recalibrate it?

[Glas467]

Check the harness between the sender and the main unit, most likely a connection issue

[Sam Buchanan]

Originally Posted by almarsh

Having problem with Cht readings jumping after 6 years of good luck replaced sender no luck that route still reading wrong do I have to put probe in hot water to recalibrate it?

It is very possible you have a faulty connection at the tray. The gold contacts in the connector can loosen and cause all sorts of weird indications.

Remove the monitor and take a close look at the all the connector pins. Use the manual to determine which pins are for CHT and insure they are making good contact. You might need to gently bend the pins to restore good contact.