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  #1  
Old 02-09-2020, 02:17 PM
Jonathan Alvord Jonathan Alvord is offline
 
Join Date: Feb 2018
Location: Prosser, WA
Posts: 123
Default Erratic RPM sensor - Saga continues could it be the Lightspeed

Ok am so perplexed by this. I believe my RPM sensor is built into the Plasma II+ CDI with Direct Crank Sensor. Would that be correct. I haven't not been able to trace a RPM sensor anywhere else. I am running a Slick Mag and the Light speed. for the 4-500 hours my RPM gauge was spot on, now it has been slowly becoming more erratic, now bouncing between 0-3000 (it is def just just sensor/monitor, and not prop). It is wired through my GRT 4000 and and the GRT Horizon WS screens with the first problems showing at the GRT 4000. I have cleaned all the spark plug wires and thought that might have fixed it but to no avail. Now I am thinking it may be the Direct Crank sensor and a bad electrical lead (however the engine is running great and mag checks are normal).

What am I missing? Sometimes I hate electronics.

Jon
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  #2  
Old 02-09-2020, 07:54 PM
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Caveman Caveman is offline
 
Join Date: Nov 2006
Location: Oklahoma
Posts: 685
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Hi Jon,

I have the same set up with a slick mag and Lightspeed Plasma II. On my RV7 the rpm sensor screws in to the side of the magneto. Look for a red hex head with wires coming out of it. If you have the same sensor as me, trace those wires to the firewall and make certain they do not run near any spark plug or high voltage wires. I had an Adel clamp rotate that was clamped on the engine mount and it got too near the ignition wires and produced the same symptoms as you are seeing... with fluctuating rpm.

Joe
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  #3  
Old 02-13-2020, 02:52 PM
Jonathan Alvord Jonathan Alvord is offline
 
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Location: Prosser, WA
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Quote:
Originally Posted by Caveman View Post
Hi Jon,

I have the same set up with a slick mag and Lightspeed Plasma II. On my RV7 the rpm sensor screws in to the side of the magneto. Look for a red hex head with wires coming out of it. If you have the same sensor as me, trace those wires to the firewall and make certain they do not run near any spark plug or high voltage wires. I had an Adel clamp rotate that was clamped on the engine mount and it got too near the ignition wires and produced the same symptoms as you are seeing... with fluctuating rpm.

Joe
BINGO, well almost. I looked at the mag and the sensor wires were laying on the spark plug wires from the mag. Lifted them all off and tied up out of the way. 45min flight and the Tach was rock solid! Thanks for the idea.

Jon
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  #4  
Old 02-13-2020, 07:09 PM
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Caveman Caveman is offline
 
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Glad I could help!!

Joe
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  #5  
Old 02-13-2020, 09:39 PM
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tkirk tkirk is offline
 
Join Date: Jun 2005
Posts: 72
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I also have a lightspeed and a mag set up, and I have had erratic RPM readings a couple of times over the last 12+ years. It sounds like you have your problem solved, but I am curious.....did you happen to turn your mag off when you had the erratic rpm? I have had the erratic rpm issue before (about 2 months ago and about 12 years ago), and it turned out that the lightspeed dsub connectors can get just enough corrosion that they don't make sufficient contact. My rpm alarm went off (it was reading erratic), but the engine appeared to be running fine. I switched off the lightspeed, and the rpm went to a steady/normal reading. I then switched to lightspeed, and turned off the mag and the engine would not hardly run and the rpm was erratic. To fix my problem, I just plugged/unplugged the dsub connectors a few times, and then used dielectric grease on the pins.

Just fyi.....I have a Dynon EMS-D10, so I have rpm indication inputs for both the lightspeed and the mag. The D-10 uses the rpm input of only one source at a time, and it is the one that was selected last. So, when I do a mag check, I always do the mag first, so that the lightspeed will be the rpm source. This way, I know when it may be having this issue.

just another possible solution for others.....
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  #6  
Old 09-03-2021, 08:27 AM
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Dugaru Dugaru is offline
 
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Location: Richmond VA, USA
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Chalk another victory up for the Caveman and Vansairforce: I was having erratic RPM readings, found this thread, and solved my problem. Hugely appreciative.
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  #7  
Old 09-03-2021, 12:12 PM
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AlexPeterson AlexPeterson is offline
 
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It sounds as though the OP's problem was related to a connection on the magneto, but given a few other replies I wanted to post this.

I have dual Lightspeed Plasma III's, direct crank pickup. I installed these in 2007 and they were trouble free for 13 years. A couple years ago, I started seeing rpm indications go to 2x (it appeared), although the readings were very short lived. No pattern I could discern, but it maybe happened for a few seconds on every 4th flight. This summer I was in the pattern doing touch and goes, and on my last downwind, it went to about 2x but stayed for several seconds before returning to normal. I put the plane away, just figuring it was another "glitch".

Upon doing the pre-takeoff ignition check a couple days later, I found one ignition completely dead. I did some double checking to be sure it wasn't anything else in the system, and sent the dead one back. Lightspeed repaired it and I've not seen the faulty rpm readings since (fingers crossed).
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  #8  
Old 09-03-2021, 01:07 PM
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bjdecker bjdecker is offline
 
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Quote:
Originally Posted by AlexPeterson View Post
I did some double checking to be sure it wasn't anything else in the system, and sent the dead one back. Lightspeed repaired it and I've not seen the faulty rpm readings since (fingers crossed).
Alex,

Did Herr Savier provide any root cause information or just swap out boxes?

B
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  #9  
Old 09-03-2021, 02:27 PM
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AlexPeterson AlexPeterson is offline
 
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Quote:
Originally Posted by bjdecker View Post
Alex,

Did Herr Savier provide any root cause information or just swap out boxes?

B
I didn't follow up. My 14 year old system has some obsoleted components, so he "upgrades" the guts, which might be the whole board, dunno. Seems price was reasonable, $275 IIRC. I'd like to have a spare, but they are pricey at something like $1200. Still contemplating that. (Interesting - Klaus said I didn't need a spare, too expensive! I said "yeah, but I'm grounded...")
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  #10  
Old 09-03-2021, 03:04 PM
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bjdecker bjdecker is offline
 
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If you had a spare, the unit would NOT have failed
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