Tach connection

[TomM]

All:

My rear engine tach stopped working. The way it stopped made me think that maybe the wire came off the connection on the engine. Problem is that I cant get the plane into the mechanic for almost 6 weeks because they are behind.

I have read the maintenance manual and it says that the right mag has a second set of breaker points for the tach. However, that is for serial number 526 and up. Mine is below that and there is no discussion of the tach connection for the early serial numbers.

If a wire has simply come off, I will reattach it. So if you have any information on where to look (pictures would be amazing!) I would much appreciate it. I really hate waiting 6 weeks if it is a simple wire.

Thanks much guys!!!

[DrDave]

Hello: My wiring diagram covers 0001-0759 for the tachometer. The right rear mag has two wires for the tach K-EG5 is a ground wire and K-EG4 goes to the rear pickup terminal on the transmitter. This is mounted mid-ship to the pilot control column. The tach generates an ac signal you can read with your meter. The easiest thing to do is switch the front and rear pickup leads (K-EG4 and K-EG2) and see if the problem follows the wiring. Color-coding the wires at the transmitter and tachometer will help keep it all sorted out.

Dave

[TomM]

Thank you very much! looks to be the pickup after running the trouble shooting.

[JAG]

Tom - also check to make sure the ground wire at the pick-up hasn't broken. Vibration at the engine could cause that wire to fatigue at the terminal end. It usually connects to a screw on the back of the mag (look for black wire.

Jeff

[TomM]

Thanks Jeff. I had to grab a bore scope so I can look at that area better.....I cant even see the pickup on the mag because there is so much stuff in the way and there is no room. I will run the scope through the area, find the pickup and trace the wires to make sure that ground is connected.

Thanks for the additional advise!

[JAG]

Tom,
Maybe this helps (I just changed my rear pick-up this weekend). The wires should come off the mag on starboard side of aircraft - it is the vent hole on the half of the mag closest to the engine, inboard (between the two mags). The ground wire (yellow in the picture) likely goes to a screw in the cover of the same mag, on a small terminal end. The signal wire should come down over the alternator and into the bundle on the firewall (blue in my picture). Sorry about this picture - it is not from this weekend, but rather a very old picture from when I started taking this aircraft apart to rehab it. There is likely one of the Cessna quick disconnects on the signal wire into the bundle.

My tach initially was working, then one day it swung wildly, then a few day later it just gave up. I trouble shot all the easy things first, swapped leads at gauge to see if it was gauge, changed out the box...finally, checked the wiring from the sensor/pick-up. Only thing left was the pick-up. Yes, it is tough to get your hands in there. I went through the left side cowl, and I had the top cowl removed as well as I was doing other work (i advise to do the same. I removed the starter - and disconnected the wires, and fed them up into the area between the mags. I used a stubby 3/4 box end wrench (fed the wires through the box end) and you have just a enough room with the wrench facing fwd, to slide it in and your hand, and was able to get on the sensor and break it free. Do the opposite if you have to reinstall new one. I tried a bunch of other methods, but that seemed the easiest. Could probably remove the vacuum pump to make it easier...but I was lazy at this point. You know what the real pisser is? First flight after the annual yesterday, and my rear vacuum pump failed. So now I have to go and remove that bugger tomorrow anyway. So, my final advice for you is - if you are going to have replace that pick-up for the tach, honestly, buy a new vacuum pump and change it at the same time!!! I am totally serious.

Jeff

[TomM]

As they say, a picture is worth a thousand words! Thanks a ton, that really clears up the access to the area. I am going to get in there and check the wires over the weekend.

I will report back on my findings.

I see you have the same Wolf remote oil filter I do, way better than the F & M ones and way less mess to change it out.

Thanks again, I really appreciate it!!!

[mshac]

Originally Posted by TomM

that really clears up the access to the area. I am going to get in there and check the wires over the weekend.

Tom, if you have the removable engine access hatch on the rear cabin wall, you'd be crazy not to use it. I had to replace my alternator (twice), and I can't imagine doing the job without the access hatch.

I've heard stories of A&P mechanics struggling and cursing to reach something on the rear engine accessory case, then billing (in at least one case) 14 hours labor to replace a mag! All because THEY DIDN'T KNOW THE ACCESS HATCH EXISTED!!!

If you have one, USE IT! All later 337's had it, and there is an STC to add it to early airframes that didn't have it from the factory.

http://www.337skymaster.com/messages...2&d=1597183855

[TomM]

Thanks to all the input I was able to solve the problem. It was indeed a broken ground wire - see attached picture. Put new terminal on it and reconnected to the mag and the tach works again.

My plane is too old to have the rear access panel, but it is on my list of things to add. I can get through one more annual before the mags need to be removed for the 500 hour AD and I already talked to my mechanic at the last annual to be prepared to install the rear access. He said "Gladly!"

The value of this form is amazing, thanks again for all the input!!!

[JAG]

That is great Tom - glad you found it. After about 30 years of working on aircraft, my greatest advice to anybody when starting to troubleshoot wiring problems; "always check the ground first". I can't tell you how many times, from Cessna's to Boeing's, I have fixed a complex or baffling problem all because of a damn ground wire connection.

Jeff