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Nov 15, 2017, 01:05 PM
Registered User
I'm trying to set up an MRPM-AC with a Rexcl Ignition on a DLE35RA.
It's wired as per the diagram earlier in this post and set to 2 poles, 1:1 gear ratio.
It works perfectly at idle, however as soon as i increase the throttle it just displays zero. It starts working again perfectly on idle. A couple of times when increasing throttle slowly it seems to work but then displays zero again.
I would appreciate any suggestions anyone may have as i'm out of idea's now.
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Nov 15, 2017, 05:37 PM
Registered User
First thing I would try is to add a pull up resister (3.3kohm) between the Black wire into the MRPM-AC and the +V wire on the Receiver / EXT port side of the MRPM-AC and see if that helps..

Wayne
Nov 17, 2017, 05:13 PM
Registered User

Jeti MRPM-AC with DLE35Ra (Rexcl ignition)


Wayne
Thanks for the advise.
I've been searching all the forums to try to find a solution to this.
Lots of people seem to use this and it just works without adding any resistors so unsure why this isn't working.
I've checked my telemetry logs and it does seem to be picking up some consistent data, however the highest reading it shows is 3050rpm and the lowest is down to 1500rpm. It appears that anything above 2900 ish drops the reading to 0rpm and when the throttle is returned to below 2900ish it kicks back in and reads data correctly.
I'm not 100 sure how this should be set up but the important parts seem to be 2 poles and 1:1 gear ratio. Do you have any idea what the trigger speed refers to? I've tried setting this to below 1500rpm (the speed my engine idles at), default value was 5000rpm i think, and i've tried it at 9000 RPM. Neither value seems to make any difference. Maybe this is for setting an alarm, not quite sure.

Can you offer some explanation to your advise of adding a resistor? It seems you've suggested this before to others but no-one has tried this. I know very little about electronics and resistors so would like to try to get an understanding as to what effect this would have to resolve this.
I see you've offered similar advise to people on the flyinggiant forum, although some years ago. I've seen two posts on there with people who had this working perfectly on DLE engines (no resistors) and switched to DA and had the same problem I'm having. I don't see that anyone resolved it.

Just so we're clear I have a DLE35RA with Rexcl ignition. The MRPM-AC is simply plugged into the EXT on the RX and the other end is connected to the DLE(Rexcl) tacho wire. It only works plugged in one way which i believe is MRPM red - Rexcl Black, MRPM Black to REXCL orange(signal wire). reversing this connection results in it reading '0'RPM
Also if you could be more specific as to where to fit the resistor it would be helpful.

Meanwhile i have an electric model with a Jeti ESC so will try connecting the MPRM-AC in that model to check that it's working correctly for the application that it was actually designed for.

Thanks again for your suggestion and hopefully i can get this resolved so it will help others trying to get this to work.

Micky
Nov 17, 2017, 08:59 PM
Registered User
Quote:
Originally Posted by micky9869
Wayne
Thanks for the advise.
I've been searching all the forums to try to find a solution to this.
Lots of people seem to use this and it just works without adding any resistors so unsure why this isn't working.
I've checked my telemetry logs and it does seem to be picking up some consistent data, however the highest reading it shows is 3050rpm and the lowest is down to 1500rpm. It appears that anything above 2900 ish drops the reading to 0rpm and when the throttle is returned to below 2900ish it kicks back in and reads data correctly.
I'm not 100 sure how this should be set up but the important parts seem to be 2 poles and 1:1 gear ratio. Do you have any idea what the trigger speed refers to? I've tried setting this to below 1500rpm (the speed my engine idles at), default value was 5000rpm i think, and i've tried it at 9000 RPM. Neither value seems to make any difference. Maybe this is for setting an alarm, not quite sure.

Can you offer some explanation to your advise of adding a resistor? It seems you've suggested this before to others but no-one has tried this. I know very little about electronics and resistors so would like to try to get an understanding as to what effect this would have to resolve this.
I see you've offered similar advise to people on the flyinggiant forum, although some years ago. I've seen two posts on there with people who had this working perfectly on DLE engines (no resistors) and switched to DA and had the same problem I'm having. I don't see that anyone resolved it.

Just so we're clear I have a DLE35RA with Rexcl ignition. The MRPM-AC is simply plugged into the EXT on the RX and the other end is connected to the DLE(Rexcl) tacho wire. It only works plugged in one way which i believe is MRPM red - Rexcl Black, MRPM Black to REXCL orange(signal wire). reversing this connection results in it reading '0'RPM
Also if you could be more specific as to where to fit the resistor it would be helpful.

Meanwhile i have an electric model with a Jeti ESC so will try connecting the MPRM-AC in that model to check that it's working correctly for the application that it was actually designed for.

Thanks again for your suggestion and hopefully i can get this resolved so it will help others trying to get this to work
Micky
In the simplest terms the resistor properly terminates the signal so that it transitions properly between a logic 0 and a logic 1. Depending on the device driving the signal wire from tha tachometer interface on the ignition it may or may not be able to transition from 0 to 1 fast enough so beyond a certain RPM the signal looks like noise the mrpm-ac can’t read reliably.

Basically 1 side of the resistor goes from the tachometer output signal wire to a positive voltage source.
My DLE ignition for my 120 and 61 don’t need it but that doesn’t mean others won’t.

Wayne
Nov 23, 2017, 05:05 PM
Registered User
Wayne
Surely if i do this i'll be putting 6 volts to the signal wire from the tacho feed on the DLE ignition and to the black cable from the MRPM-AC (connected to the signal wire of the ignition.
As i said before i don't fully understand electronics, but that doesn't sound like it's going to do much good to the ignition module or the MRPM-AC.
As far as i'm aware the tacho feed from the ignition has earth, positive and signal. If i connect a bog standard digital tacho to this the ignition positive connection supply's the power to the tacho display and electronics.
With the MRPM-AC it gets power from the RX and the black and red leads connect to the earth and signal from the ignition module.
I'm not quite sure that connecting power to the signal wire is correct.

Are you sure about this, as i clearly don't want to damage the ignition module trying to get a tacho to work?
I did recently have the ignition module replaced under warranty and they asked me for the numbers on the module to ensure they sent me the correct one. I understand there are different modules which could explain why some people have it working straight out the box and others can't seem to get it to work.

Having said this i do agree with you that there seems to be a problem with the MRPM-AC reading the signal when revs are increased. It's as if it doesn't know what to read and just reads '0' As soon as it returns near to idle it can read the signal again.

Micky
Nov 23, 2017, 05:25 PM
Registered User
Quote:
Originally Posted by micky9869
Wayne
Surely if i do this i'll be putting 6 volts to the signal wire from the tacho feed on the DLE ignition and to the black cable from the MRPM-AC (connected to the signal wire of the ignition.
As i said before i don't fully understand electronics, but that doesn't sound like it's going to do much good to the ignition module or the MRPM-AC.
As far as i'm aware the tacho feed from the ignition has earth, positive and signal. If i connect a bog standard digital tacho to this the ignition positive connection supply's the power to the tacho display and electronics.
With the MRPM-AC it gets power from the RX and the black and red leads connect to the earth and signal from the ignition module.
I'm not quite sure that connecting power to the signal wire is correct.

Are you sure about this, as i clearly don't want to damage the ignition module trying to get a tacho to work?
I did recently have the ignition module replaced under warranty and they asked me for the numbers on the module to ensure they sent me the correct one. I understand there are different modules which could explain why some people have it working straight out the box and others can't seem to get it to work.

Having said this i do agree with you that there seems to be a problem with the MRPM-AC reading the signal when revs are increased. It's as if it doesn't know what to read and just reads '0' As soon as it returns near to idle it can read the signal again.

Micky
Adding the resistor doens't "connect" the signal to the power line which implies a short circuit. It simply adds a small current source to allow to signal to "pulll up" the signal wire more quickly.

It's up ti you whether you try it or not.
Nov 23, 2017, 05:28 PM
Registered User
Quote:
Originally Posted by micky9869
Wayne
Surely if i do this i'll be putting 6 volts to the signal wire from the tacho feed on the DLE ignition and to the black cable from the MRPM-AC (connected to the signal wire of the ignition.
As i said before i don't fully understand electronics, but that doesn't sound like it's going to do much good to the ignition module or the MRPM-AC.
As far as i'm aware the tacho feed from the ignition has earth, positive and signal. If i connect a bog standard digital tacho to this the ignition positive connection supply's the power to the tacho display and electronics.
With the MRPM-AC it gets power from the RX and the black and red leads connect to the earth and signal from the ignition module.
I'm not quite sure that connecting power to the signal wire is correct.

Are you sure about this, as i clearly don't want to damage the ignition module trying to get a tacho to work?
I did recently have the ignition module replaced under warranty and they asked me for the numbers on the module to ensure they sent me the correct one. I understand there are different modules which could explain why some people have it working straight out the box and others can't seem to get it to work.

Having said this i do agree with you that there seems to be a problem with the MRPM-AC reading the signal when revs are increased. It's as if it doesn't know what to read and just reads '0' As soon as it returns near to idle it can read the signal again.

Micky
Adding the resistor doens't "connect" the signal to the power line which implies a short circuit. It simply adds a small current source to allow to signal to "pulll up" the signal wire more quickly..

It's up ti you whether you try it or not. If done correctly it shouldn't hurt anything, "correctly" being the key word.. Since I don't have your setup in front of me to measure what is actually happening I can't really make any better suggestions.

Wayne
Nov 23, 2017, 06:01 PM
Registered User
Wayne

Thanks, let me check i have this correct.
I solder a small wire to each side of the resistor.
I connect one end to the power from the RX to the MRPM-AC (The red wire)
I connect the other end to the Black wire from the MRPM-AC which goes to the signal connection from the tacho lead.

Micky

I still have the old ignition unit so i think next time i have the cowl off i'll switch them over and try this out on the old ignition. Worst thing that can happen is i blow the old (faulty) ignition or the MRPM-AC.
Nov 23, 2017, 08:57 PM
Registered User
Quote:
Originally Posted by micky9869
Wayne

Thanks, let me check i have this correct.
I solder a small wire to each side of the resistor.
I connect one end to the power from the RX to the MRPM-AC (The red wire)
I connect the other end to the Black wire from the MRPM-AC which goes to the signal connection from the tacho lead.

Micky

I still have the old ignition unit so i think next time i have the cowl off i'll switch them over and try this out on the old ignition. Worst thing that can happen is i blow the old (faulty) ignition or the MRPM-AC.
Correct. Or you could check to see if the ignition outputs power on red wire of the tachometer output with a volt meter. If it does you could just put the resistor across the red and white wires of the tachometer output. As long as one side of the resistor goes to the tachometer output signal and the other goes to power you should be all set. Pull up resistor values tend to vary over quite a range but for this purpose it should be too critical and anything between 3.3kohm and 10k ohm is probably fine depending on what you have available.


Wayne
Aug 17, 2019, 04:43 PM
teamwork, do it my way!
Hey Guys,

Mickey, did you ever find the time to try this solution? I have EXACTLY the same problem on one of my engines, an NGH GF38. I have exactly the same problem and the rpm cut-off is exactly the same..at 3050.
I dont know where I am going to find a resistor where I am but if it worked for you, I will source. one.
Its weird how this sometimes works on other planes and not on others. I really would like to record the rpm telemetry for this engine.
Thanks in advance, and sorry for dredging up an old thread...


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