Tach accuracy problems.

[Richard Carson]

I have a d 180 with a slick mag on the left And a Lightspeed ignition on the right. It appears to be too high on the low end and too low on the high end. The RPMs between 1000 And 2000 appear to be very close. I did put approximately 150 K Ohm resistor in the lead from the Magneto. It could be that I need to drop that back down to 30. Does anyone know if that would affect that type of reading.

 

[jakej]

Any particular reason why you went that high with the resistor value ? Except for the Lyc 540's Ive never seen the need to go above 30 K oHm for Lycomings or their clones.

 

[Richard Carson]

Thanks for your reply Jake. The reason I went above the 30 K resistor was because initially the tach reading would not stabilize at all I believe it turned out to be the pulse rate for the electronic ignition. I eventually et that to 2 And the problem seemed to go away as far as stabilization is concerned. Now I will have to go back and pull everything but the 30k resistor. Thanks for your response because it probably means I'm on the right track!

 

[jakej]

FYI - you don’t need the resistors for LSE or P-Mags, which of those or other type do you have ?

 

[Richard Carson]

I have a slick mag with impulse coupler on the left and I have a light speed on the right I'm aware that I don't need one on the electronic ignition where I've a 10 volt line to the ignition switch. I do believe I need one on the Slick mag however I believe that's what it says in the manual. I will be going to the airport today to pull the additional resistance out of the line. Hopefully that will remedy the situation thank you for your help.

 

[jakej]

As an aside, for others reading this - If you have just 1 x P-Mag then all you need is the1 x tach output wire from it only (regardless of what # 2 ignition system is installed) for rpm display - you don't need 2 x tach outputs in that case & no need for resistors.

 

[Richard Carson]

I was mistaken I had only put a 50k Ohm resistor in the line from the P lead . I had smudged my notes and they said 150. So I went back and put a 30k in. If I were to only use the electronic ignition would I be able to do my normal mag checks?

 

[jakej]

In that case probably you'll only get good tach info from the LSE. How many hours on the slick ?

 

[DBRV10]

A six cylinder with a slick will need 100K plus.

Done a few ;-)

 

[Richard Carson]

I have 63 hours on the Slick Magneto. Hopefully if the weather Isn't too bad today I'll be able to take it up and see if dropping down to 30 K made a different. It appears that the low end is approximately one hundred RPMs too high and the high-end is approximately one hundred RPMs too low. Of course that means when the Dynon is reading 2350 RPMs it's actually 2450 which is not an acceptable situation to be in. I actually gauged this off of the mechanical tach that had been used prior to installing the 180. I have a Lycoming o-235 which has 75% power at 2350 RPM.

 

[Raymo]

Are you using the hall effect module on the LSE or fly-wheel sensor? If the former, the LSE will not report RPMs when doing a mag check. I am not sure if it does when using the fly-wheel sensor. Also, be sure you use the correct inputs for the mag and LSE. There are two sets of tach inputs; you'll need one input from each set.

 

[Richard Carson]

I have the hall effect for the light speed but it has been working reporting RPMs since it was installed with no problem. I will double check the connections to make sure that they're correct. Thanks! I'll let you know what I find out.

 

[Richard Carson]

Flew the machine for an hour today and the tach readings appear to be unaffected by the drop in resistance in the line. I believe the next step is probably to pull the pin on each mag individually and see if that doesn't affect the output. So tomorrow I will pop the pinout for the electronic ignition and see what happens. If anyone has any other suggestions I'm certainly open to them.

 

[jakej]

FWIW- I've never seen any rpm fluctuations with the electronic ignitions eg LSE or P-Mags.
Mechanical tach output (cable driven rpm) can be problematic & are unreliable IMO, I've had many customers say theirs under/over read by ~100 rpm, use a good tach tool to confirm that what the gauge shows is correct or not.
If you still have erratic rpm issues with the LH mag then maybe it's worth having it checked out, I know of another 'poster' here who had similar issues which was found to be the 6 cyl mag, I think by that by changing to another one fixed the issue although the original one met the performance specs.
Personally I've never had to change the recommended 30K ohm value for the 4 cyl Slick mags.
There is/was a device called a Magneto P-Lead Tachometer Signal Stabilizer (P-TACH) available to fix these sorts of problems. message me for more info.