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P-mag 114 and af-2500. Rpm unsteady

KLax18

Member
P-mag 114 and af-2500. Rpm unsteady- Issue resolved!

Installed two p-mag 114’s on IO- 360 with AF-2500 engine monitor. Put the dioxide between ground and pin 6 per instructions on right mag only. Verified pulses set at 4. Engine starts and runs great. Just rpm on AF -2500 bounces all over the place. Verified rpm with prop counter gizmo. When Prop was at 900, tach read 1300. -1500 but never steady. Constantly bouncing around. Any idea what may be going on?
Huge Thank you to Brad Dement of E-Mag and Rob Hickman of Advanced Flight systems. P-mag generates a 12V rpm pulse from pin-6. The Advanced AF-2500 needs a 5V signal. E-Mag provides a diode if receiving unit needs a lower voltage signal. That arrangement produced an unsteady rpm readout. Rob had and sent an Advanced RPM circuit board part #71,420. Essentially it came with a yellow input wire and then a red, white, black output. The yellow went to pin-6 of P-mag. The red, wht and black went to the AF-2500 analogue board. red went to 5V digital, White to the RPM, Black(which I had to slice with a white extension wire) to the common. (see photos) I then removed the diode from P-mag that was between pin 1 & 6. Solution worked like a charm. I installed dual P-mag 114's. Advanced unit only needs 1 input from 1 P-mag. On 2 flights so far: P-mags are awesome! Same profile where i would see 9.9-10 gph, I was seeing high 8's gph, egt's @1300 or less and cht's high 200's to very low 300's. And it starts like opening one of those trick can of peanuts. Thanks again to Rob & Brad.
 

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The best solution would be to send in the analog module and have it converted to a 12V Tach input. We should have some of our old 12V to 5V interface boards that should work with an AF-2500.


Try the following:

1. Use shielded wire to the EMAG Tach signal, ground the shield at the analog module.

2. Route the Tach wire away from all plug wires.

3. Make sure the 5V diode is correctly installed on the EMAG

4. Try using the other EMAG


Rob Hickman
N402RH RV-10
Advanced Flight Systems
 
To rob

This is before. I took off the old slick 3 wire and just used the single white rpm wire. I tried to mark with yellow in attached picture. You mention shielded and ground at the analogue box. Where do you recommend? What slot.
 

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With the old mags, did you add a resistor on the pLead for input to the EMS for tach signal? If so, did you remove this resistor when you installed the pMags?

Carl
 
Old mags

They were slick lasar mags. Had pick up on right mag plug port. Three wires.
That is all gone.
The only rpm now is one wire to the #6 port of pmag connector. That has the diode installed to the #1 port(ground).

I am running a new shielded dedicated wire from #6 pmag port to the rpm port on af-2500. With the diode like p-mag direction indicate. Took shielding portion and grounded that to mounting screw of analogue module box.
Well, that didn’t work. Still fluctuating between 1200&1500 when idle at @850. Still fluctuates if increase rpm.
 
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Rpm still fluctuating - photo of pmag connection

Diode between pin 1 & 6 for rpm pickup. Still fluctuating.
 

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Test run- pmag left right both results

Got a prop counter to verify actual rpm vs what pmag(s) indicated. In both still had variation. Actual rpm 850. Pmag rpm fluctuated 1000-1400. Went to left rpm actual 819. Left pmag 820. Went to right only: Right actual 808, right pmag read 900-1100 fluctuating. Back to both, actual 960 both pmag rpm fluctuated 1200-1400.
 
What is a prop RPM Counter?

Are you saying you connected another RPM instrument and it did the same thing?

Did you try moving the tach signal wire to the other EMAG?

Rob Hickman
Advanced Flight Systems
 
Rpm

It was just a small box I borrowed from local A&P that I set on the dash to count blade rpm.
Haven’t switched to other mag. Would like to see if I can identify the issue. Just in case others may experience it in future.
 
Another data point that may or may not be useful. My 2500 would jump rpms when connected to the right SDS ignition and leaned. Read ok until I leaned the engine. Switched sensing to a UMA on the left Slick and it read ok.
 
I installed my AFS3400-EE in 2008. I struggled with many occurrences of erratic or non-existent RPM readings in flight for 12 years. I replaced the (magneto-based) RPM sensor twice and sent the display/control unit to AFS once. I'm an electrical engineer who has designed many systems using the kind of TTL level (5 volt) signalling used by the RPM sensor. None-the-less, AFS basically ignored my input and told me there's nothing wrong with the signal levels.

Last year, I had another dropped RPM incident. Finally realizing that the warranty period had long since passed, I decided to open up the controller and see what was up. I found that internal termination and signal conditioning was very wrong. Either the board was improperly manufactured with the wrong resisters or it was a bad design from the start. The board appropriately uses 74HC14 schmitt triggers to condition the digital inputs. However, the existing resister termination didn't allow the inputs to the 74HC14 to reliably reach the levels required in the 74HC14 spec. I corrected the resistor networks on the RPM and Fuel Flow (which had also given me trouble) inputs and haven't had trouble since (knock on wood).

It seems likely the AF2500 uses the same circuit. If so, this might explain your issue.

BTW, I'm not trying to slam AFS here. It's a good product and they've mostly been very helpful and patient when I've brought up technical issues. Their response to this issue is the only complaint I've had.
 
N204RH. Rob. Rpm board

Rob. N402RH. I sent you a private message about instruction below.
 

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P-mag 114 and af-2500. Rpm unsteady- Issue resolved!

Updated original post with solution. Huge :)Thanks to Brad Dement of E-mag and Rob Hickman of Advanced light Systems. Great customer support!
 
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