Starting Problems; Electrical

As a wrap-up to this post, I instrumented the airplane as I described below with four DVM's. See the enclosed simplified diagram. After doing all this work to hook-up, I could not get the problem to re-create in two days of trying! While briefly cranking the engine, I got pretty consistent voltages throughout the system that gave me no indication of the problem. A disadvantage with DVM's is the bouncing around of the numbers while it takes a moment for the A/D converter in the DVM to ramp up to the actual voltage, based on the sample rate. I had to somewhat mentally average the numbers. Anyway, I dismantled all this and returned the airplane to normal and will continue to monitor the situation and discuss it with my A&P at the annual, beginning May.

I looked into the ignition switch refurb kit as mentioned by Steve Ells. The ignition switch would be difficult to work on without disconnecting wires since the wire harness doesn't give much room for moving things around. I decided I didn't want to be working on this switch which controls the mags and mess something up.



Thank all for your help on this.

KENT VANDENBERG wrote: Thank you all for your input and suggestions. I have been away from home base for over a week and have not done anything to work the problem, except devise a plan to use four digital voltmeters to read:

1) master contactor input voltage; will be assumed to be the same as battery voltage;
2) master contactor output voltage (aircraft buss voltage); will be assumed to be the same as starter contactor input voltage; should see less than 0.2 volts drop across this contactor under load;
3) starter contactor coil voltage, switched by the ignition switch; should be the same as master contactor output voltage (aircraft buss voltage);
4) starter contactor output voltage; will be assumed to be the same as the voltage to the starter; should see less than 0.2 volts drop across this contactor under load.

Before starting this hook-up, I will identify one DVM as a reference voltmeter and get a calibration bias on the other three relative to the reference DVM so that I can normalize all the voltmeter readings to the reference voltmeter. I'm looking for small voltage sags. With this in place, I will crank the engine and hope for my failed start scenario to occur, and then read/record all voltages (with a helper) and see what is happening in the entire electrical starting chain under load. I think this should point to the problem.

Steve - Thanks for your very helpful suggestion of cleaning the ignition switch contacts, and that it is likely an AD I'll need to comply with anyway. I never thought about the feasibility of taking the switch apart and cleaning, and especially the kit available from A/C Spruce. You may have saved me around $400 (the price of a new ignition switch?)!

Will report results. I hope to do all this tomorrow.

Kent

Four voltmeters will most certainly give you better insight than a light bulb! Be aware you will see voltage drops when load is applied.

You made an observation about the ALT light flashing during the failure, there is where my curiosity lies.

Thank you all for your input and suggestions. I have been away from home base for over a week and have not done anything to work the problem, except devise a plan to use four digital voltmeters to read:

1) master contactor input voltage; will be assumed to be the same as battery voltage;
2) master contactor output voltage (aircraft buss voltage); will be assumed to be the same as starter contactor input voltage; should see less than 0.2 volts drop across this contactor under load;
3) starter contactor coil voltage, switched by the ignition switch; should be the same as master contactor output voltage (aircraft buss voltage);
4) starter contactor output voltage; will be assumed to be the same as the voltage to the starter; should see less than 0.2 volts drop across this contactor under load.

Before starting this hook-up, I will identify one DVM as a reference voltmeter and get a calibration bias on the other three relative to the reference DVM so that I can normalize all the voltmeter readings to the reference voltmeter. I'm looking for small voltage sags. With this in place, I will crank the engine and hope for my failed start scenario to occur, and then read/record all voltages (with a helper) and see what is happening in the entire electrical starting chain under load. I think this should point to the problem.

Steve - Thanks for your very helpful suggestion of cleaning the ignition switch contacts, and that it is likely an AD I'll need to comply with anyway. I never thought about the feasibility of taking the switch apart and cleaning, and especially the kit available from A/C Spruce. You may have saved me around $400 (the price of a new ignition switch?)!

Will report results. I hope to do all this tomorrow.

Kent

Don't you think a bit of troubleshooting is prudent before throwing parts at a problem?