Replacement tach systems

[Jim Rainer]

Anyone who has replacd the Cessna tach system in a 337G or any model with an STC'd replacement or one done with Form 337 paperwork, please let me know what system you used and how much paperwork was involved.

There are a number of manufaturers out there but none I could find with an STC'd system for the 337. Some are "FAA Approved" but my mechanic is leary of exactly what this means.

I am at the point where I think installing a brand new system is preferrable to chasing the old wiring and re-working the mags and possibly ending up with a new or used Cessna tach doesn't make sense when there are brand new systems out there.

Thanks

[Ernie Martin]

I went back and read your earlier post, chasing wires and trying to solve the problem. I wonder if you are frustated by all this and it's influencing your decision. I think that the original system is a good design that should give many years of troublefree service. And I also think that, in the end, fixing it will be far cheaper than finding a new solution.

Ernie

[Jim Rainer]

Ernie, you are right. The system has worked fine for 27 years but now isn't. My problem is I am not a mechanic and don't have enough time to devote to working on the problem. I have confidence in my mechanic who has worked on two Skymasters for me over 15 years. I think he's afraid he may spend untold hours (that I won't want to pay for) and then still have to buy something new or re-wire the old system. I guess we are both trying to take the easy way out and start with something new.

Thanks,

Jim

[kevin]

Jim,

I understand not wanting to throw a bunch of money down a rat hole. But a new indicator will be a lot of labor as well. Here are some thoughts:

First, Jim Stack replaced all of the his engine instruments (pressure, temperature, but not tach, MP and fuel flow). He may have some thoughts about how much work it is to do what you are taking on, but he also (obviously) made a similiar decision to yours, so he may be a good reference point. Mr. Stack, if you are reading this, can you comment? Also, remember that Stack has a VERY good relationship with his local FAA, and that has to have made his project easier.

If it were me, I would remove the tach, and take it either to a local instrument shop, for testing, or failing that, I would take it to my avionics shop and ask them to use a pulse generator to test the instrument itself. If the instrument checks out, the problem seems clearly to be in ships wiring, since the chance of both mag sensors going bad at the same time are low.

If the problem can be duplicated on the ground, have you considered just making a small cable to run directly from the tach sensor (mag) to the instrument? If that makes it work, then you KNOW it is a wiring problem. You can also have your avionics guy use a scope to look at the signal at various points, that may give him a clue as to what is going on.

Getting an avionics guy that you know and trust involved with electrical problems can be very helpful. Many (but certainly not all) A&P mechanics HATE electrical issues, and often want to solve them by total reinstallation, rather than tracing down the problem. That may well not be the case with yours, no insult intended, but it does happen.

Here is what I am getting to. If you put in a new instrument, you probably are considering new wire too. If your instrument and sensors happen to be good, then you are throwing money away replacing them, because either way you will be paying the same or more labor to fix the wiring problem, by installing all new wire.

Bottom line, you should be able to get the instrument tested, to determine if it is bad, rather than buying a new one in case it is bad.

But I *do* understand the frustration.

And, in answer to your original question, I have never seen an aftermarket tach in a 337. Does not mean it is not out there, but it may be hard to find. I would look at Ernie Martin's site on parts sources, maybe ask Gmas on the Yahoo list, and also contact the CPA for their sources, and in general do whatever I could to find a replacement tach of the OEM type, rather than do an STC or field approval. There are a lot of 337s out there being parted out, and since tachs fail so rarely, you should be able to find another. If you install it and it does the same thing, then you know you have a wiring problem.

I realize that I am rambling on here, but seven years of Skymaster ownership taught me that the wiring used in these birds was never designed to last this long, and on my airplanes, the best best for any electrical type problem was some sort of wiring issue, rather than a bad major component. Not always, but very, very often.

Sorry to go on at such length...

Kevin

[SkyKing]

Jim,

An T337H owner I met a year or so ago changed his entire engine instrumentation over to Electronics International units that are manufactured in Eastern Oregon. The finished product looked pretty slick, with the TACHS and MP gages showing both digitial and color dots. HOWEVER, the original TACHOMETER indication system on the 337 is pretty fool proof, with most of the headaches arising where the pigtails from the extra set of points on the magnetos connect with the ships wiring. BEFORE you go out and spend a small fortune testing the OEM gage or investing in new gages from EI, I think it would be HIGHLY PRUDENT to check the wiring at each end of the run where the pigtails are located... all you may have to do is change out the connectors, which are about .50 cents apiece. There's not much to it... JUST TIME!

SkyKing

[Jim Rainer]

Thanks, Skyking. I'm coming back around to that now. Wish I was more of a mechanic myself.

Jim Rainer

[Jim Rainer]

Thank you all for your replies.

For your information I found a shop that will overhaul the tachometer instrument itself - normall $300 - $400 and will check it out first (see below.) BUT he says 99% of the time it is either the clearance setting on the tach points in the mag or the wiring connectors. We are going back and check this out on my rear mag (easy to get to) first before shipping the instrument off.

The shop is Rudy's Instruments in Rudy, AR (just north of Ft. Smith and not on the airport) 479-474-8759. Very nice person and seems knowledgeable on this instrument. Good information to have.

[SkyKing]

Jerry,

Very seldom... I mean to say V-E-R-Y SELDOM does a problem involve the indicator instrument itself, unless something has shorted out internally, and it just doesn't happen. Given the number of years your tachometer indicator has worked without a problem, the FIRST SUSPECT would point to external wiring as we've already discussed and/or the extra set of mag points themselves. I do hope you have a Cessna Service Manual with the details as to the inspection and adjustment of the points, or you'll be shooting in the dark. My bet is that you've either got misadjusted points... or more likely, intermittant connectors at the pigtails which are located about 5 to 7 inches from where the leads comeout of the magnetos.

Let us know what you and your A&P find.

PS... Exhaust everything with the wiring and points BEFORE doing anything with the instrument itself.

SkyKing

[stackj]

Don't overlook any areas where recent repairs have been made on the tach system.

I had a problem with the gauge cluster that Keven mentioned in an earlier post inthis thread. After several months of troubleshooting this intermittent problem, I found a sharp point on a solder connection I had done. This connection was 'protected' by heat shrink... but not well enough. The point was able to poke through the heat shrink just enough to cause an intermittent ground causing me to loose oil pressure indication on the front engine.

Unless you particularly want to get digital tachs for your bird, I'd avoid trying to get a field approval. You need a cooperative inspector AND IA. The way people file suits, I can't blame them...but inspectors don't like to stick their necks out by approving one time installations.

Those who have installed the gauge cluster that I designed will tell you the most difficult part has been getting inspector approval. My inspector asked that I not share the form 337 when I give out details on the installation and I have honored his request. (I may need him again).

Good luck.

By the way, my manuals only cover up to the 73 skymasters. If you have a schematic of the circuit you can share, I'd be glad to look it over, but from the symptoms you describe, it could be almost anything, including two different failures (the front engine tach was acting up way before the rear one).

It is possible you or your mechanic could have caused the problem for the rear engine while troubleshooting the front engine.