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12-02-2007, 12:33 AM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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RV-6 VM1000 RPM Indication Fluctuations
New VAF Member, and first-time poster. I purchased an RV-6 this summer (N600SS, built in '98 by Tom Hallendorf as a "Super Six" with an IO-540). It has a VM1000 that is showing occasional erratic behavior in RPM indication (ex: set at 2100, it goes from 2100 to 1400 to 2100 to 1800, etc,. bouncing around, then stabilizing back at the set RPM...no actual RPM change, just the indication). I have a Lightspeed EI and a Bendix mag, and I've looked for the Tach sensor with no luck. The only lead coming off the mag is the P-lead, and I've traced the EI wires to the EI box under the panel, but all wires seem tightly connected there. Same appears to be the case for all wires going to the brains of the VM1000. If anyone has experienced the same problem and has any recommendations, or can help me narrow my search for a solution, I'd really appreciate it! Thanks, Bob.
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12-02-2007, 10:12 AM
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Join Date: Jan 2005
Location: Wappingers Falls, NY
Posts: 233
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VM-1000 RPM
Bob,
The RPM is is typically taken from a magnetic sensor on a mag. The sensor replaces the cooling plug. There may be other ways as well but I think this is the most common.
As far as the fluctuations go, I'll bet a beer that you have a loose ground. If you have the old screw terminals on the CPU check them often, they do sometimes come loose. I am not familiar with the newer ribbon cable connectors from teh engine to the CPU.
Scott A. Jordan
N733JJ
698 hours & a few loose grounds on the VM-1000 over the years.
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12-02-2007, 01:33 PM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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Scott, Thanks for the info! Looking at the mag, I see two ports...one with the P-lead and the other just has a blank silver cover on it (no wires, etc.). I'll check and tighten the connections on the CPU as you recommended, and see what happens. Will hope for an easy find on a loose ground, and more than happy to oblige on the beer bet!! Thanks again! Bob
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12-02-2007, 02:14 PM
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Join Date: Jan 2005
Posts: 112
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RPM Pick Up Problem
I had a similar problem but with a Slick mag and RMI engine monitor
The RPM signal is derived from the P lead which puts out a large voltage pulse some where in the region of 100 volts.
When the magneto contacts start to wear this pulse tends to have spurious spikes on it which confuse the microprocessor counter in the engine monitor.
This in my case was giving the unsteady RPM readings.
One solution you can try is to have the magneto points inspected and cleaned.
Another solution which may not be possible in your case is to modify the electronic circuit to make it less sensitive to the spurious pulses (spikes). I did this on my RMI Engine monitor and so far with apparent success.
Barry RV6A F-PRVM
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12-02-2007, 06:04 PM
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Join Date: Jan 2005
Location: Palm Beach County, FL
Posts: 304
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VM1000 circa 1998
Have the above on a flying RV6, 180HP carb, CS prop. RPM is fairly stable, but see intermittent drops in fuel pressure to zero which apparently is an erroneous indication (engine kept running), and see very low amp readings soon after start, during flight. Do either of these issues ring a bell with anyone before I start investing a lot of time in troubleshooting? Thanks....
__________________
Link McGarity
Wellington, FL (FD38)
RV6/N42GF bought flying, sold
RV6/N72AT bought flying, sold
B737-800/NxxxAN
Sonex TD w/Aerovee/N732SX bought flying
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12-02-2007, 08:39 PM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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RV-6 VM-1000 RPM Indication Fluctuations
Thanks for the continuing feedback guys!
For Barry and Jeff B (and open to ideas from others) the way I understand it, the RPM signal is either generated by a second pick-up off the mag (not the P-lead), or by a sensor within the EI module. Since I see only the P-lead coming off my mag, I'm assuming (uh-oh) that my VM-1000 is getting the RPM from the EI. Is that a safe assumption? If not, checking the mag contacts sounds like a very good idea.
For wv4i, in addition to my RPM indication fluctuations, I also recently experienced zero Amps on the VM-1000 Amp indicator (and adding load did not raise it off zero), and I also had no EGT at cylinder 3. The Amps problem turned out to be a loose lead at the alternator (now fixed, with good Amps indications), and the EGT turned out to be a bad probe (new one on order now). Neither turned out to be related to the RPM fluctuations, or indicative of a more widespread VM-1000 problem. Just passing along FYI (and not claiming to be a VM1000 guru...but I'm getting smarter!)
Thanks again guys!
Bob
"Rocket" RV Super Six
www.brutalpower.net
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12-03-2007, 12:29 AM
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Join Date: Jan 2005
Posts: 112
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RPM Fluctuations.
Bob,
I am not quite sure I understand what you mean by second pick up off the Mag.
In my case with the Slick magneto the P lead wire goes from the mag to the magneto selector switch. From the selector switch it goes via a small value capacitor (isolation) to the RMI engine monitor to generate the RPM display.
Barry
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12-03-2007, 11:17 AM
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Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
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RV-6 VM-1000 RPM Indication Fluctuations
Jeff and Barry, thanks...I'm probably using incorrect terminology. Looking at the back of the mag, there are two dime-sized silver screw-on caps. One has the P-lead coming from it, and the other is just a plug. I was assuming the second cap was what would be used for the RPM sensor, and since mine is just a blank cover, I thought that meant there was no magneto RPM sensor installed. Just off the phone with Ottis at VM, and he said the magneto RPM sensor would be on the bottom of the mag, not the back, and that is probably the cooling plug that Jeff mentions. So I need to take another look at the mag and see if there is a wire coming from the bottom (I don't think there is). I'll also look to see if I have a similar set up coming off the mag selector switch as Barry describes. If neither of those are the case, then I'm back to tracing the EI wires to the VM-1000 DPU, which all appear tight right now. The Lightspeed Plasma paperwork that I have shows just a little info on the Tach generation wiring, so more homework needed on that. I keep hoping for a loose-wire discovery and a simple fix! All part of getting to know the airplane (and getting smarter on mags and EI!) Thanks again, and I'll let ya know if I find a solution that would help others!
Bob
Last edited by rvmills : 12-03-2007 at 11:21 AM.
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12-03-2007, 05:09 PM
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Join Date: Jan 2005
Location: Wappingers Falls, NY
Posts: 233
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RPM Pickup
Yes, you are right. The dime sized cover can be removed and used with a magnetic pickup for RPM. This is how mine is done.
Scott A. Jordan
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12-03-2007, 05:12 PM
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Join Date: Jan 2005
Location: Wappingers Falls, NY
Posts: 233
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Link,
Check the fuel line for the pressure transducer carefully. I had fluctuating fuel pressure and found a very small speck of dirt in the reducing oriface for the sensor. It doesn't take much.
Scott A. Jordan
N733JJ
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