herbaderb

I have a real intriguing issue with my 1979 450SL. Normally it starts and runs fine. Except when it sits for about 3 or more days without running. Then it will not start. Everything acts normally (all lights and power comes on) when you turn the key on but nothing happens when you turn the key to the start position. The battery is good, I believe the ignition switch is good because it usually works fine and it's not intermittent, the started has been replaced, it doesn't matter if its in Park or neutral, I've replaced the two big resistors and checked all of the wiring. I don't actually know for sure how I get it going except maybe just leave it in the ignition 2 position for a while,that I basically just keep trying different scenarios with the starter switch until it finally engages the starter. Then I will not have the problem again forever until I don't drive it for 3 or 4 days. Anybody have any ideas what I should check.

Max Blast

So you’re talking about a battery drain?

Take the steering wheel off, take the instrument panel out and unplugged the clock and see if the problem repeats itself.

Mine ran down the battery in three days just from a parasitic draw from a clock.

herbaderb

The battery is fully charged as I have even kept in on a trickle charger when not driving for over a couple of days.

ygmn

So, you turn key and starter does not engage?

When you turn the key to try and start what is batter voltage then?

Any aftermarket alarm or what other electronics?

herbaderb

No change in the battery voltage when the key is turned to the start position.

herbaderb

This car is completely stock with only 50000 miles on it.

ygmn

So when it does not start the starter motor is not spinning is issue?

SO you play with key in ignition switch and eventually you get it to work?

Sounds like ignition switch issue or wiring somewhere or you have aftermarket alarm preventing start.

herbaderb

I know it sounds like a bad ignition switch, I would like to bypass the switch to see if it will spin the starter. Does anyone know where I can get to the switch wiring without pulling out the dash? Or have a wiring diagram.

Max Blast

what is the voltage at rest and what does it drop to with the key in crank position?

herbaderb

When the battery is at rest is reads 12.52V and when the key is turned to the ignition position it reads 12.15V.
When the key it turned to the start position and the starter does not turn, it still reads 12.15V

ygmn

Well soemthing is not telling the starter to spin over.
Be it a broken wire, bad starter or solenoid or ign switch or ??

Max Blast

Solenoid, ignition switch or harness.

try junkyard starting it with key in run position and shorting out the starter solenoid to ground. Caution: do not electrocute yourself.

if it spins then the fault is in harness or ignition sw.

herbaderb

Unfortunately, since the car is now running and being used daily, it does not act up. I would need to let it sit at least 3 days or more to have the problem crop up again. So until I can let it sit and have the problem occur again, I think I will put this link on hold until it happens again.Thanks for the advice and I will open it up again when it happens again.