edneff

edneff

Well Known Member
Hello,

Today I taxied out to go for a flight, and during the run up, when I selected the first side mag to check (not sure if it was left or right) the tach went to zero rpm and just stayed there. It sounded like a good mag check, but I taxied back to try to troubleshoot. All the wires are connected on the back of the instrument.

Now, this happened to me once before... it was right after an oil change and I ran it up for a leak check and no rpm. So, the cowling came off, and we looked at everything and found a loose screw on the right mag, tightened it. Played with the wires coming out of the tach sender, and generally went over everything. But never did figure out what caused the dead tach, however, on the next start, it came back.

Also, about a week after the oil change, I was doing a runup and the same thing happened, the tach went dead when first switching to a single mag. I had to get the airplane home however, and so we departed and the tach came back during the flight.

This particular incident today, it so happens that I had checked that same lug on the right magneto because the cowlling was already off. I noticed the lug was not attached well, so we crimped a new fitting on it and re-tightened it. Then the tach dies the next time I start the engine. Coincidence?

Is there some relation between the magneto wires and the tach sensor? I know very little about this kind of stuff. If you have any ideas, please speak up. The wire that was loose on the mag was the one with the phenolic under it, not sure what you call it. But in both instances I had also tightened the screw lablel "ground". Also, the large hex bolt that the sensor runs through to get the the crankcase rotates slightly, is that normal?

Thanks
 
"Ed I have a 2-1/4 new tach for sale if you need one. $5.00 off of the new price."

I'm not sure if I need one, or maybe I do. Is it a Van's? If this unit is defective I may want to consider another type.

Also, I started the engine up again this evening, and the tach worked... So I thought let me check the mags, since that's when it failed last time... Sure enough, no sooner had a run it up and just as I touched the ignition/mag switch and began to turn it, Tach went to Zero again! I futzed around with the sender, the wires, etc. Started it two more times and could not get the tach to indicate.

I wish I knew where to start with figuring this out.
 
You sound lost so let me GUESS

Weird with the tach and mag switch???

I'm no Van's tach expert, but I can help with some basics, which may help you start troubleshooting.

First) It could be a common problem, which Van might know off hand, or it could be simple lose wiring, since it seems intermittent.

To answer you question, the mags and tach should be separate if I understand how Van's tach works, it has its own sender, right, mechanically driven off the tach drive, right? The only thing needed is the tach sender is turning (mechanically by engine RPM), sending a signal to the tach and there's power to the sender and gauge.

Check the wring, making sure its getting power to both gauge and sender. The mag thing CAN affect some tachs, when they're used for a TACH signal as many do. However from what you tell me you have a mechanical tach sender, independent of the Mags P-lead.

It could be run-ups on one mags causes a vibration that causes an intermittent connection to OPEN. (I AM WILD A** GUESSING HERE)

I'm not 100% on the working of Van's tach (I don't have one), but I understand it works like most electric tachs....The sender typically receives voltage and as it turns sends a pulse to the tach. If the sender has three wires, power, ground and signal its probably a hall affect sender. (Guess but trying to help here)

Second) You can check the sender by removing it from the engine or cable, than turn it manually (with electric power applied to sender) and check the output signal wire. Connect a voltmeter to the output signal wire and ground and see if it gives you a voltage pulse as you turn the sender. Again not 100% sure Vans gauge works this way. Mine works this way, but it's a different brand. If the sender does give a pulse, its either the wiring or gauge.

Vans' gauges look good and are very reasonably priced. However reliability is not their strong point, from what I read and see. If there's a failure of the sender or gauge a call to Van's will probably solve it. They likely know exactly what's wrong and if there is a common issue, sender or gauge, if they admit it.
 
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Thanks gmc,

that's pretty much what I did today. removed the sender and spun it to see if their was any response on the tach. nada.
replaced my tach with another van's tack from an RV8 builder. Started engine. No rpm.
Called Van's, ordered new transducer. Since the warranty on those is only a year, and the thing was ordered about 18 months ago, paying for a new one.

I'll install it in the next day or two and let you know how it works.

the thing with the mags-- weird. My wife came up with that vibration theory too, maybe there's something to it.
 
Transducer

edneff said:
Thanks gmc,

that's pretty much what I did today. removed the sender and spun it to see if their was any response on the tach. nada.
replaced my tach with another van's tack from an RV8 builder. Started engine. No rpm.
Called Van's, ordered new transducer. Since the warranty on those is only a year, and the thing was ordered about 18 months ago, paying for a new one.

I'll install it in the next day or two and let you know how it works.

the thing with the mags-- weird. My wife came up with that vibration theory too, maybe there's something to it.
Click to expand...

I have a friend on his third Van's tach transducer. Along with changing out the trasducer he ran a blast tube to keep the third transducer cooler and so far (100 hrs later) has had no more problems.
 
tach drive key

Vans gauge has a key at the engine connection that sometimes fails for various reasons. My first one broke. I just ordered a new drive key about $8, replaced it and the gauge worked fine after that for about 250 hours now. However, that would not explain your problem with the change in rpm with mag check.?? My guage is the one for engines without the vacuum pump.

take care and good luck.
 
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That's the same one I have, no vacuum pump. Well, the only thing I can figure regarding the mag check is maybe the vibration with a mag off was the right frequency to bollox up the transducer...
 
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