Intermittant No start issue

[TJT]

I am having a no start issue that occurs only at times. Yesterday my Dad flew the 170 and put it away. He started it twice w/o any problems. I went out to the field an hour later and got nothing. I thought maybe the battery was dead. Usually there is a "thunk" from the firewall when you hit the Master switch but, yesterday nothing. No radios, no starter.

This morning I went out figuring I would have to pull the battery, charge it and have it tested. I tried the Master first and everything worked. I had my "thunk" and avionics. Any ideas? Is the "thunk" I hear the Voltage Regulator? Is my regulator sticking? I am still running a Generator.

Any ideas would be much appreciated.

Regards,
TJT
1951 170A

[GAHorn]

The "Thunk" you hear is the battery master solenoid mounted on the battery box. The big cable comes from the positive battery post to the solenoid....(which is normally OPEN).... and a big cable leaves the other side of the solenoid and goes to your starter-post, and on to your aircraft main electrical buss underneath your inst. panel.

The SMALL screw-terminal on the battery solenoid goes to your cockpit MASTER switch (which in the OFF position is OPEN). When you turn ON your cockpit MASTER switch, it connects that small wire from the battery solenoid to a GROUND (the aircraft metal frame.) That completes the connection for the battery solenoid's electromagnet (it gets its POSITIVE power from that big battery cable attached to itself). The electromagnet SLAMS the internal contacts CLOSED (the "thunk") and completes the connection between the two large cables...allowing battery power to gain access to your starter (so you can activate it with a pull-cable or key), and allowing battery power onto the rest of your aircraft's electrical system.
Here's what the old, original style battery solenoids looked like ( I doubt you have this one anymore):

Here's what the "modern" battery solenoids look like:

Don't be surprised if it has a short diode also attached to it...don't let that distract you...ignore it:

WARNING: WARNING: STAY COMPLETELY CLEAR OF THE PROPELLER WHILE MAKING THE FOLLOWING TESTS! (iF YOUR STARTER HAPPENS TO BE IN THE "START" MODE... AS SOON AS YOU OPERATE THE FOLLOWING TESTS YOUR PROPELLER MIGHT ROTATE! MAKE SURE YOUR FUEL IS OFF AND YOUR MAGS ARE OFF.


FIRST: Check the BATTERY TERMINAL connections are tight and clean. Also check the connections at the battery solenoid, both large ones and the small one. (Brighty-Tighty)
Second: Use a short jumper wire to momentarily connect your battery solenoid SMALL terminal directly to GROUND (the firewall or engine would probably work.) If you now hear the "thunk"... then you've found your problem. If the cockpit MASTER switch still does not work reliably... you have dirty/bad connections in that small-wire grounding circuit thru your cockpit MASTER switch...or you have a bad switch.

HOWEVER; If your small jumper wire does NOT give you the "thunk".... then you have a failing battery solenoid.

I will post pics shortly.

[cessna170bdriver]

TJT,

If you didn't charge the battery between when it didn't work and when it did work, it sounds like a problem with the wiring in the master switch circuit or the master switch itself.

[rupertjl]

I had a similar problem a few years back where I would turn on the master and touch the throttle to advance and the master light would go out. It turned out to be a ground on the back of the panel for the master.

I hate electricity and it hates me back but I've learned that 90% of these types of problems end up being the ground. Might want to check there as well.

v/r,
Jud