Tachometer Problem - Most Likely?

[Paul Sharp]

My front engine's tach reading is bouncing all over the place.

1 - What most likely wears out on these? The sending unit or the tachometer itself?

2 - My manuals say that "breaker points" replaces the magnetic pickups in the 1967 models (mine is a T337B), althoiugh the parts manual says "pickup=-tachometer for the part. Are these hard to find?

3 - Does a wiring problem often cause erratic readings, or more likely (as I would suspect) just no reading.

4 - Is there anything adjustable in the readings? Mine are reading off a little when the mechanics compared them (when they're readable) with their own calibrated tach.

Thanks in advance.

[Mark Hislop]

I have had the same problem. In my case, the fix was simple. There are two wires coming out of the mag for the tach. They join the wiring harness through individual spade lug disconnects. In one case, the spade lugs were loose. In another case, one of the wires was almost broken completely through and making contact intermmitantly. Check the wires first before you fool around with the pickup inside the mags.

Mark

[FRED-E]

The PICK-UP screws into the bottom of the mag, but like Mark said it has 2 leads coming out of it with spade lugs on them. The last time I had a problem with mine the wire broke off at the sending unit. I was able to repair it (I won't go into that) and made it work for a while until I could find one.The best thing to do has been suggested, look for breaks in the wires, disconnect the spade lugs and put an ohm meter across them to see if the pick-up is open, while you have the ohm meter on the wires, wiggle them ever-so-slightly to see if you looses continuity, mine was broken inside the insulation. If everything checks out the next step would be to swap the wires on the back of the tach to see if the problem moves from one to the other.
If it turns out to be a bad pick-up and you can't find a replacement right-away I could loan you my old one for a while, it looks like CRAP but it works just fine.

Fred N358 337B 0737

[Bob Cook]

If it is not a broken or loose wire then it is the mag itself.

The tack works on a second set of points and you will need to clean and gap them.

the mags need to be overhauled every 500 hrs.

fyi

bob

[SkyKing]

Chances are you've got a connection with an improper crimp in it. Re-crimp the connections where the short wire leads connect to the aircraft wiring for the longer journey to the tachometer near the mag and you should solve the problem. On the rear of course, you'll have to open up the rear hatch. No big deal, but while you're in there check to make sure your shop put all the screws back where they belong. They are forever famous for leaving things half-assed.

SkyKing

[Paul Sharp]

Ah, lots of good input here. Thanks to all. I'll be checking things out accordingly.

Sure is nice to have a place where we can share this stuff and save each other time and $ trying to troubleshoot.

[Mark Clark]

Guys, Thanks for your assistance. I will begin tracking wires for loose or broken connections for the 2 wires coming out of the mag's. Will report once problem is fixed.

[FRED-E]

What is the Ser # of your aircraft. The maintenance manual said that at ser # 337-0757 that started using the extra breaker points in the mag. Hope this helps.
Fred N358 ser# 337-0737

[Paul Sharp]

Noticed that I didn't ever report back here. The problem was the little wires feeding the tach. Once that was fixed it has worked fine. Thanks to all for the suggestions back then.