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.

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.

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

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

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....

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

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

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

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

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

RPM Pickup

Thanks Scott, all makes sense. Buttoned up the plane after installing a new #3 EGT probe, and run-up checks showed no RPM flux. Too gusty to fly at 4SD (Reno-Stead) today, so a longer check for RPM indications will need to wait til the next flight. Meanwhile, still checking for the actual RPM sensor wiring, so if it shows itseld again, I'll be able to hit the source. Thanks again to all for all the great feedback! The VAF forum is a great source for info from pilot bro's!

MP fluctuations

Another fluctuating instrument to watch for is the MP.

In the past I have had the MP fluctuate or be very slow to respond. I was told by VM tech (back in the pre-JPI days) that it was likely moisture in the MP line. You want the transducer as high as possible with respect to the pickup on the engine to prevent this. Though mine is mounted as high on the forward baggage bulkhead as possible, this is still a bit marginal. Removing the transducer and blowing it dry takes only a few minutes and was something I started doing during an oil change while the oil was draining (along with checking the connections on the VM CPU, lubing the tailwheel and a few other routine chores). I started getting lazy and have not done this for the last couple years but have had no further issues with it but can not explain why.

Scott A. Jordan
N733JJ
695 hours

Fluctuations...

Using the VM-1000 on an Aerosport IO-320-D1A, I was encountering fluctuating Oil Temp, Fuel Flow, CHT, & EGT readings after about 50-60 hours. What I discovered is that those crappy little red spade quick disconnects begin to fail in about that time period. (Despite using a GOOD crimping tool) I think one of the root causes is all those corrosive gases running around in the engine compartment eventually attack the attach points and crimps of those fittings enough so that an electrical signal is impeded. The solution? Remove the spade quick disconnects, solder all of the connections, and encase in shrink wrap on ALL FWF engine monitor connections. 430 hrs later, I haven't had one problem since making that change. The only downside is that the connections must be cut to replace a sensor, but for me, that is a small price to pay.

I'm not sure why the engine monitor mfr's still use the spade quick disconnects for that sort of an application. Ease of installation probably... I'm sure there is a more expensive solution that would prevent this sort of thing... YMMV

Fluctuations

Joe and Scott, thanks for the info on the MP and other fluctuations. Haven't seen any yet (knock on wood) but that will sure help if they appear. I'll look for the MP transducer (still doing a lot of learning about where everything is...part of buying, not bulding!)...good info. And I do have the spade connections...the solder/shrink wrap solution does sound time intensive, but sounds worth the effort! At 490 hours TT it is great shape for an almost 10 yr old, but gotta keep the preventitive upkeep going so it stays tip-top! Thanks again for all the great words of wisdom!!

Cheers,
Bob
N600SS
RARA Hangar, 4SD

N733JJ said:

Another fluctuating instrument to watch for is the MP.

In the past I have had the MP fluctuate or be very slow to respond. I was told by VM tech (back in the pre-JPI days) that it was likely moisture in the MP line. You want the transducer as high as possible with respect to the pickup on the engine to prevent this. Though mine is mounted as high on the forward baggage bulkhead as possible, this is still a bit marginal. Removing the transducer and blowing it dry takes only a few minutes and was something I started doing during an oil change while the oil was draining (along with checking the connections on the VM CPU, lubing the tailwheel and a few other routine chores). I started getting lazy and have not done this for the last couple years but have had no further issues with it but can not explain why.

Scott A. Jordan
N733JJ
695 hours

Click to expand...

I've also had quite a few problems with MP. I'm pulling MP off the right rear cylinder, braided flex hose to the firewall and aluminum tubing to the MP sensor. Unfortunately it is a down hill run for a taildragger. I was getting oil fouling of the sintered fitting on the sensor. I guess there is some residual oil that somehow drains thru the intake valve bushing after shutdown. Every 15-20 hours the MP would stop reacting to throttle movement. I would take the fitting out and soak it in Avgas for about 5 minutes.

I finally got tired of this and started looking at changing the routing of the hose to prevent the fouling. I was able to route it up and over the oil cooler instead of around it. Hopefully this will solve the problem. Not enough hours yet to know if it worked.

Karl

Hello, I have had a VM1000 on a Bucker biplane for about 10 years and am installing one in a Harmon Rocket project now. It is one of the first engine monitors but I still like it a lot.

I have had an occasional intermittent read out a couple of times and it has always been the terminals at the "brick" DPU end of the sensors wiring. Those screws somehow loosen over time and need an occasional tightening.

Since your system was working good, it might be as simple as that. Get the manual and figure out which terminals are for the tach sensor and check those carefully, maybe loosen, and tighten those.

As for the tach sensor, it screws into a threaded port on the side of the mag case and is basically a hall effect sensor that notices when a magnet passes by each rotation. No moving parts and only two wires. Hard to imagine it would fail with intermittent readings, more likely a broken wire would fail it completely.

Good luck, don't give up til you find it.

Peter