HELP? Tach stopped working while at shop...

[dbier99]

Our RV6A is at the shop having a Garmin G5 installed so we will now have an attitude indicator. As part of the install they moved the existing Tach over a spot or two to a more traditional placement. When the shop fired up the plane for final testing the tach was suddenly dead.

The shop says we have an electronic tach which is simply connected to the mag's P-lead (I'm not familiar with that term) and senses the magneto pulses - it is NOT attached to the mechanical drive on our Lycoming 0-360 engine. They have double checked all connections and can't find the issue. They are now waiting for a callback from the tach maker (UMA).

The tach that Vans has for sale connects to the engine's mechanical drive (https://store.vansaircraft.com/uma-3-1-8-electronic-analog-tachometer-ie-uma-19-811-112.html).

Any ideas what could have caused the tach to suddenly go dead? The shop has also pulled the cowl looking for solutons but has drawn a blank.

ADDITIONAL INFO: The plane has a traditional and an electronic mag. During run-up when the traditional mag is turned off the tach would always drop to zero (which the previous owner said was normal).

 

[Tommy123]

Apparently only the magneto is connected to the tach or someone really screwed up the installation.
Just because someone says it is "normal" does not mean it is working properly. I would get the UMA installation instructions and make sure it is installed correctly.
Both ignitions should be driving the tach.
Also people have a bad habit of not providing enough information when asking questions like this. The tach P/N, what electronic ignition, what magneto????

 

[dbier99]

Apparently only the magneto is connected to the tach or someone really screwed up the installation.
Just because someone says it is "normal" does not mean it is working properly. I would get the UMA installation instructions and make sure it is installed correctly.
Both ignitions should be driving the tach.
Also people have a bad habit of not providing enough information when asking questions like this. The tach P/N, what electronic ignition, what magneto????

Wish I had more detail/info for all of you - but I just bought the plane in August and haven't become intimately familiar with the inner workings/models of all the systems/parts - and the shop has our logs right now. The previous owner did provide a packet of materials that I've looked through but there is no info/materials on the mag, electronic ignition or tach.

The shop is trying to get ahold of UMA to troubleshoot the tach - and without the tach model info I can't help with that. It wasn't until I sent them the link for the UMA tach that VANS sells that I learned mine was electronic with no mechanical connection to the mag itself.

If anyone here has this type of electronic tach and has any insights as to the issue - I'd love to hear it and pass it on to the shop.

 

[PaulvS]

If the tach was working before the shop moved it and now it isn't working then that implies a wiring problem.
But as Tommy said, a tach installation should show RPM for both ignition systems, otherwise how can you check a mag drop during runup?
So I would consider replacing the tach with a new one, e.g. like the UMA from Vans, that reads with a sender directly off the engine drive.

 

[dbier99]

If the tach was working before the shop moved it and now it isn't working then that implies a wiring problem.
But as Tommy said, a tach installation should show RPM for both ignition systems, otherwise how can you check a mag drop during runup?
So I would consider replacing the tach with a new one, e.g. like the UMA from Vans, that reads with a sender directly off the engine drive.

Well of course the shop - that primarily works on certified planes - said that the wiring while safe was not as neat as they would do themselves - and implied that it was probably due to a sub-par witing job. But when they checked all the connections they found no issues. Unfortunately, it seems they have little experience with this type of electronic (versus mechanical sending unit) tachs.

Am I correct to assume that a Lycoming 0-360-A1A engine is going to have the connection for a standard tach sending unit to be connected? If so - as you suggest - I will have that version installed if we can't find the issue with the current tach.

One positive note - the plane also has a JPI slimline digital tach (https://www.jpinstruments.com/shop/slim-line-tach/) - which has worked at idle but been completely unreliable once airborne. I asked the shop to troubleshoot it but they indicated it could be a time consuming process that I'd need to come back for. So, I called JPI tech support and they told me that it was VERY LIKELY a connector crimp issue forward of the firewall - that was easly fixed.

Here's the positive - - while troubleshooting the analog tach the shop also traced the wiring for the digital tach (which apparently has it's own sensor) and think they now have the digital tach working (they say it no longer jumps around at higher revs). I told them I was happy to hear that but would not be comfortable relying on the digital tach until I'd flown with it working perfectly for several hours.

 

[Tommy123]

evaluate which would be less expensive, buying another tach or paying for labor to troubleshot the one that's acting up.

 

[blaplante]

Welcome to the world of Experimental Aircraft!
Few are identical.

Your A&P can't just look up in the Cessna manuals the part number. Downside.
And then is limited to the Cessna part for $500 (if lucky).

It takes more research and knowledge. And consulting the maker of the widget. In this case UMA. Now, I suspect that your UMA tachometer could use any UMA tach sender, such as https://www.aircraftspruce.com/catalog/inpages/umamagtachsend.php?clickkey=10133

With that sender, your pulses will come directly off the tach drive, and you'll no longer have the problem of no reading during mag check. And it might fix your problem. Or maybe the tach died.

Now don't go leap and buy this and have the A&P install it. Find the model of your UMA tach. Confirm that this sender is compatible.... Then, perhaps.

But note that parts for these aircraft, intelligently sourced, can be much less expensive.

 

[AN23]

Early UMA electronic tachs required a switch to be able read both mags. Do you have a tiny switch somewhere that you don't know what it does? If it does, try it!
My 8 originally came with this but I dumped it when I put in the new panel.

 

[dbier99]

evaluate which would be less expensive, buying another tach or paying for labor to troubleshot the one that's acting up.

I'll definitely be asking that question Monday - I'm already a bit afraid of what the extra charges will be.

...I suspect that your UMA tachometer could use any UMA tach sender, such as https://www.aircraftspruce.com/catalog/inpages/umamagtachsend.php?clickkey=10133

With that sender, your pulses will come directly off the tach drive, and you'll no longer have the problem of no reading during mag check. And it might fix your problem. Or maybe the tach died.

Thanks for the sender link and suggestion. I will pass it along to the shop. I'm really hoping they get hold of UMA Monday so we can determine the best resolution. I will be asking the shop for the tach model # so I can post it here - and if needed - pursue an answer from UMA more aggressively than perhaps the shop can do (because I'm obviously not their only focus).

 

[dbier99]

Early UMA electronic tachs required a switch to be able read both mags. Do you have a tiny switch somewhere that you don't know what it does? If it does, try it!
My 8 originally came with this but I dumped it when I put in the new panel.

Unfortunately, no mystery switches - but thanks for the suggestion/diagram! The auxiliary digital tach was the only way to determine the rev drop when cycling the traditional mag during the run-up mag check - and for that it was fine as it didn't start fluctuating wildly until high revs (which hopefully is now fixed as a byproduct of troubleshooting the analog tach).

 

[dbier99]

So, am I correct to assume that a Lycoming 0-360-A1A engine / standard mag is going to have the connection for a standard tach sending unit to be connected?

 

[Ironflight]

So, am I correct to assume that a Lycoming 0-360-A1A engine / standard mag is going to have the connection for a standard tach sending unit to be connected?

Yes….but….it is possible to build the engine and leave the tach drive shaft out - for instance, if you don’t plan to use the mechanical Tach drive because you’re using something that takes pulses off the magnetos. Rare option, occasionally done by homebuilders who have their own ideas about how an engine should be built - but it removes an option for someone later down the line, so I’d argue not a wise choice. The outside of the accessory case will look the same, with a place to screw on a tach cable or sensor…but it may not be connected to the drive train.

Very rare, but legal in the experimental world - highlighting again the need to understand that experimental aircraft require a different level of ownership knowledge - and good documentation!

 

[dbier99]

Yes….but….it is possible to build the engine and leave the tach drive shaft out - for instance, if you don’t plan to use the mechanical Tach drive because you’re using something that takes pulses off the magnetos. Rare option, occasionally done by homebuilders who have their own ideas about how an engine should be built - but it removes an option for someone later down the line, so I’d argue not a wise choice. The outside of the accessory case will look the same, with a place to screw on a tach cable or sensor…but it may not be connected to the drive train.

Very rare, but legal in the experimental world ...

Fingers crossed that the builder of our plane didn't eliminate that from our engine build. I suppose if they did - and the current electronic tach is the failure point - then we'd just need to find another electronic tach that operates in the same way. If we get to that point I'm hoping someone still makes that type of tach. Lots of qustions to ask UMA on Monday!

 

[penguin]

Many regular maintenance shops will not touch experimental aircraft for exactly the reason you have found ...

UMA Has a reasonable website https://umainstruments.com/aviation-instruments/non-tso-engine-instruments/electronic-tachometers/ and are quite helpful. These type of tachos are really straight forward (pulse counters) and usually connect directly to the mag switch https://umainstruments.com/wp-content/uploads/2021/04/Slick-PLEAD.pdf

The other alternative is a sensor unit in the vent hole on the magneto that connects into the tacho.

It may be cheaper to ask the shop to completely re-wire the tacho as fault finding on wiring of dubious quality may be time consuming. There is always the possibility that the gauge has failed...
If you have to buy any new parts you will have to contact a dealer, UMA does not sell direct, https://umainstruments.com/where-to-purchase/

 

[dbier99]

For what it's worth - the shop just sent me a pic of the tach's model number (attached). It's a little hard to make out all the digits but they believe it's 19-806-11G4.

 

[rockwoodrv9]

I have a 19-501-11 if you need it.

 

[blaplante]

Looking up that UMA part number it appears to no longer be a current model. So a call to UMA will be needed to figure out which sensors are compatible.

FWIW, I've found the 'direct to p-lead' pickup to not be real reliable. [as in sometimes go jumpy...] I'd rather use the sensor at the engine tach drive, or one that screws into the mag vent hole.

Here's an odd thought... have the shop confirm they don't have a hot mag. If the p-lead got disconnected somehow from the mag (or on the way to the key switch) then the engine runs but the engine is hot and the mag won't work.

 

[dbier99]

I have a 19-501-11 if you need it.

Thanks for the offer - greatly appreciated. Unfortunately, if we do need to replace it - we need one with the tach hour meter built in (pic attached).

 

[Scott Hersha]

The following UMA electronic tach comes with the 3-wire UMA sender that threads into the side of your Slick mag, and will show RPM regardless which mag is selected, because the mag is still spinning even when selected off.

https://www.aircraftspruce.com/catalog/inpages/tach4cyl.php

If you don’t need to replace the tach gage, the UMA sender can be ordered separately. You would need to check with UMA to see if the sender is compatible with your tach gage.

https://www.aircraftspruce.com/catalog/inpages/uma_sendingUnit.php

I’ve used that sender on my last two RV builds to show RPM on my EFIS engine pages. Works well, easy install.

 

[dbier99]

UPDATE - Issue Resolved!

After relaying suggestions from UMA to the shop (as I suspected, they hadn't gotten ahold of UMA) the shop decided to replace the wire from the mag switch (and tach) to the mag - and the tach started working again. They had previously redone the connections in that line - so I can ony guess that there was a break in the wire. Happy it's resolved and appreciate all the input everyone provided - it's all good learning for me! Now I'm just waiting to see how much the shop will charge me for this issue.

 

[dbier99]

It may be cheaper to ask the shop to completely re-wire the tacho as fault finding on wiring of dubious quality may be time consuming.

You nailed it!

 

[Tommy123]

Thanks for the update.