Voltage drop in the ignition switch when key is "on"

[ZIMMERMANN JEan-Luc]

Hi

to day, the engine of my 359C refused to start.
Car is cranking, but engine doesn't fire.

i'm suspecting an electrical problem

I have a brand new battery (Optima Red) in the car and could measure a solid 6,35V on it. Also no ground problem.
But at the ignition coil, i measured only 5.6V (coil is also new)

Could this voltage drop be the cause of my problem ?

After investigating a little bit more, it seems that the ignition switch is the cause of this voltage drop.
Is it possible to disassemble it in order to clean the contacts or is it necessary to replace it ?

Thanks in advance for your help and greetings from France

Jean-Luc

[Doug McDonnell]

Lots of old threads about your problem. I like David Jones suggestion of jumping B+ on the voltage regulator to the coil If car then starts problem is your ignition switch. Most common ignition switch failure would be to not turn the starter over though. Larry Markham and others rebuild switches. Since you have a C do your door lock and ignition match? If not you can buy a new switch and locks as a kit with 4 keys. Dick Weiss also repairs them. I would try the jump 1st though. No sense having your car down for weeks repairing the switch to find out the switch is not the problem. Report back with results.

[David Jones]

Jean-Luc, 5.6 volts with ignition on and points closed is normal because of the volts drop across the coil. Rotate the engine with the distributor cap off and watch the points open then measure the voltage. It should now read the same as at B+ on the regulator. Try the jumper method from B+ to the coil but the problem is likely elsewhere. You need to have someone crank the engine over with the key and check if you still have power at the coil when cranking in case there is an open circuit at the switch only while cranking though this is not likely.
While the distributor cap is off you can test for a spark from the coil by rotating the engine until the points are about to open then with the HT wire from the coil removed from the distributor cap, place the HT wire end about 3 to 5mm away from ground and break the points by moving the rotor. You should see a spark jump the gap to ground. This will tell you that your ignition system is working.
Let us know what you find.

[Mervyn Hyde]

Jean Luc,if you are sure it is the switch read through this viewtopic.php?f=4&t=40763.

Then fit a relay on your starter. That can allow even weak old ignition switches to last longer and protects good ones. The relays are cheap and can even be made at home, if you have the circuit diagram.

[ZIMMERMANN JEan-Luc]

Jean-Luc, 5.6 volts with ignition on and points closed is normal because of the volts drop across the coil. Rotate the engine with the distributor cap off and watch the points open then measure the voltage. It should now read the same as at B+ on the regulator. Try the jumper method from B+ to the coil but the problem is likely elsewhere. You need to have someone crank the engine over with the key and check if you still have power at the coil when cranking in case there is an open circuit at the switch only while cranking though this is not likely.
While the distributor cap is off you can test for a spark from the coil by rotating the engine until the points are about to open then with the HT wire from the coil removed from the distributor cap, place the HT wire end about 3 to 5mm away from ground and break the points by moving the rotor. You should see a spark jump the gap to ground. This will tell you that your ignition system is working.
Let us know what you find.

thanks you guys.
the switch is definitively ok, unlike my first guess.
It's like David wrote: same voltage (6.15V) on the coil as at B+ when the points open and about 5.6 when points are closed.
Ignition seems to be OK, but the car still doesn't start
I'm continuing my investigations.
My next try will be to add a relay on the starter solenoid, but without great hope. Problem is likely elsewhere ...

[Doug McDonnell]

There used to be a great starter troubleshooting template at Cyberwerkstatt but I can no longer find it. Maybe someone printed it off while it was on the web? http://www.cyberwerkstatt.com/Members_O ... ooting.htm

[David Jones]

Jean-Luc. Make up a jumper wire with a piggy back connector like in this link.
http://www.mouser.com/ProductDetail/Pan ... MYQAvD_BwE
They should be available at almost any accessory auto store. Connect it and the original wire to the solenoid connector and run an extra cable through to the engine compartment. Do this with battery disconnected. Now reconnect the battery and have someone turn the starter switch. Read the voltage at the end of the cable extension in the engine bay. It should read around the same as you got at the coil previously. If not there is a fault at the ignition switch supplying power to the solenoid. Now take that same wire end and touch it to B+ on the regulator. The solenoid should operate and the starter should turn. If it does not then you have a bad starter or solenoid unless there is a loud click and thud then your starter bushing is bad.
Let us know what you find.

[ZIMMERMANN JEan-Luc]

today, i could manage to restart the car.
It was not an electrical problem as i first guessed.
After having checked more or less everything, i decided to replace the spark plugs ... and what a miracle, after that, the engine started gently )))

Positive thing is that now i know that the electric system is in good shape

thanks to all for your suggestions and tips

[Mervyn Hyde]

Good news Jean-Luc. I would still fit a starter relay on any car I had.

[ZIMMERMANN JEan-Luc]

Relay installed, static ignition timing set to 5°

car is starting perfectly now

Thanks all !!!