Need help with tach wiring

I purchased a 10-01076 ISS ELECTRIC TACH GA 3500 from spruce to replace a dodgy one (another make) I had which was picking up its signal from my e-mag tach out put (apparently 2 pulses per revolution)

This new one shows exactly half than actual RPM. So I purchased a sender to bolt on the back of the engine but it doesn't fit as I have a spin on oil filter adaptor. So now I have to order an extension for the sender and it is all going to get messy back there.

So

Can I pick up a signal from the P lead???? Wouldn't that give me the 4 pulses per revolution I am looking for without having to fit the sender to the engine???

thanks in advance

Cgoldy

Colin,
Is there no way to change the pulses per revolution on your tach? This: http://fdatasystems.com/ is the tach which I am using. It is connected directly to two P-mags. It can optionally be connected to the mag switch. The pulses per revolution are programmable. It can be connected to both P-mags or just one, just be sure to properly set the program. Mine is connected to both P-mags. Make sure that you set the pulses to both P-mags, if you use both, or you will get flashing numbers. So far, it has operated flawlessly.

This is the installation manual: http://www.fdatasystems.com/Manuals/T_30_manual.pdf

I have been down this rat hole a couple of times with my Experimental SC clone and a Glassair. Most electronic tachs expect 4 pulses per revolution. If you are using the tach output from an electronic ignition, they typically only provide 2 pulses per revolution, so your tach reads half speed. The first thing I would do is contact ISS. They may be willing to exchange your tach for one set up for pulse count you need. UMA exchanged my tach for one with a modified pulse count at no charge. (didn't even charge me for shipping!)

If that doesn't work, since you have a 2 1/4" tach, I would recommend returning the tach for the UMA 3500 2-1/4 ANALOG TACH unit. But, buy direct from UMA and tell them you need a tach that used 2 pulses per revolution. They will be happy to set it up for you at no extra charge. I found them to be really good to work with. ISS may do the same for you.
http://www.issprogauges.com/Articles.asp?ID=246
http://www.umainstruments.com/
Be aware that the Emag pulses on the tach signal are very short (2 ms), so sometimes are not detected by all electronic tach devices, which may be the source of the dodgy operation of your previous tach. In the GlasAir, this caused an intermittent tach signal on the VM1000 display.

-CubBuilder

That all sounds like good advice. But what would I get off the P lead? I thought I had heard somewhere that you could get a signal from there. Maybe I dreamt it. Maybe it would be 2 pulses as well?

I doubt there is a readable signal on the P-lead of the E-mags, which is why they provide another lead with a tach signal. The P-lead in this case is just for grounding the electronics to shut it down. But if there was a signal, it would also be two pulses per revolution as that is a result of the waste a spark configuration of the E-mags. When using a magnetic pick up in the mag for a tach signal, or a P-lead on a magneto for that matter, you still only get 2 pulses per revolution as that's the number of times the mag pulses to fire per revolution. I don't know why the manufacturers decided to use 4 pulses from a tach drive as their standard, but that's what they seem to have done.

FWIW, I have the electronic UMA tachs in both of my planes, one set up for two pulse per revolution using a magnetic sensor stuck into a magneto and one using a tach drive set up at 4 pulses per revolution. Both tachs are equally accurate and stable.

-CubBuilder

http://www.chiefaircraft.com/iss-dna2501610.html

Sender runs off your tach drive.

Crash

Crash, I haven't seen that transducer before. It apparently goes with the Chief ISS CAT-01 tach. Overall, an economical unit. But does it clear the Casper Oil filter adapter? That's the problem I ran into with other transducers and apparently the same problem cgoldy is seeing.

-CubBuilder

Colin, The following is from page 18 in the P-mag manual. Does this answer your question?

EICAD allows builders to adjust:
Tach Output - Tach output can be set to either one or two pulses per revolution to
match your digital tach instrument. Factory default is two pulses per revolution – 12
volts. Series 114 ignitions change the tach voltage from 12 volts to 5 volts by
installing the diode, provided in your kit, between the tach output (terminal #6) and
ground (terminal #1) on the Control Plug (see wiring diagram). Alternatively, the
diode can be installed on the instrument end of the tach lead.

Peter,

I did ring brad at Pmag and discussed that but 2 pulses is the best we can get. It would be interesting if I had two e mags that I could take pulses from both. 2 + 2 = 4 . Bingo!

Crash

That is the sender I have and it does not clear the filter. Vans will sell me a 12 inch extension which putts it in a very inconvenient place with the problem of finding a suitable mounting that is not just an adel clamp. I could drill another hole in the firewall and mount it behind the panel but all to messy for me. I will ring iss now and find if they will modify the instrument like cub builder recomends. Maybe there is a opening for someone that could make a simple electronic device that would double the pulse. Shouldn't be rocket science!

Colin

I don't know how dedicated you are to making your existing equipment work but for experimental I believe a circuit called a phase lock loop would be the converter you would need to change 2 pulses input to 4 output. Chips to do that are cheap probably under a buck. The feedback part of the loop is usually programmable but you could just use a fixed divider at "n"/2 (I think that's correct. Would actually have to draw it out first). The output of the PLL would need conditioning to deliver pulse width, spacing and amplitude and that would require first measuring what the tach expects in a functioning system with an oscilloscope. The conditioning of the pulse or tracking would be the hard part.

Wow! - Rocket Science