Need help! My 71 280se 3.5 coupe has problems starting when hot (above 190 - degrees). When you turn the key in the ignition nothing will happen, it will act as though there is absolutely no power. When the car cools it will start fine. We have replaced the following: the battery, (did load test checked out ok), the voltage regulator, the connector behind the ignition switch, the starter and solenoid, temperature switch on block, as well as check all connections. The car had an electronic ignition installed over two years ago without any problems. Any suggestions anyone might have would be greatly appreciated as I am going nuts trying to figure this out.

No power meaning not even your acessories go on? The fuel gauge still is at "R"? The fuel pump doesn't buzz? Or do you hear the normal signs, go to start, and perhaps hear a "Click" and nothing more? Or no click?

Some more details please

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Tomguy,

There is power to everything except to the starter. When I turn the ignition to the on position to start the car when the car is hot, I usually get nothing. When the car cools the ignition (starter) will engage. Any ideas why this only happens when the cat is hot?

Thanks

Check to see it if start in neutral -- probably the neutral safety switch that prevents starting it in gear is bad when hot.

It's on the side of the tranny.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Could be the NSS as Peter mentioned. It could also be an issue with the wiring to the starter itself. That's why I asked for more info - it could have been many other things

You're sure the starter solenoid is functioning correctly when warm? Jiggle the gear selector in park while holding the key in "Start" - then try Neutral if park doesn't work.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Peter / Tomguy,

I appreciate both of you trying to help on with this. I will try your suggestion and see if it starts in neutral. The starter and solonoid are new, as well as the voltage regulator. We also tested the battery with a load test and it checked out fine. The problem only seems to happen after the car has been running awhile and gets hot. If I try and start it right away it will start, however if I leave it for a couple of minutes after driving it, it will not do anything when I turn the ignition swith to start, no clicking sound whatsoever. When it cools down the starter will the engage and it will then get power.

If you jumper the starter motor under the hood when hot and it turns then probably ignition switch or wiring to it.

Even though the starter is new or rebuilt you might be experiencing heat soak. Take some water with you and throw it on the starter and solinoid the next time when you normally experience this problem. Then try the starter again in about 30 seconds. It should tell you. Also are you perhaps missing a heat shield if these cars have one?

Update,

I took the car to my Mercedes shop yeterday. After looking at the problem, they noticed that I am getting a 5-7 volt voltage drop to the starter when I turn on the ignition. They are now going through the entire car to see if they can figure out where the drop is occuring. Does anyone have any ideas where they should look?

Thanks

If your aux fan is shorting out, and the fuse for it was directly bridged, it could possibly cause a short at temp but not when the system cools.

That's ALL I can think about. Try unplugging the wire for the aux fan from the 212°F switch (the one with a 1 wire plug by the air cleaner horn)

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Tomguy,

I actually set up a bypass for the aux fan so that I could turn it on manually from inside the car at lower temps. By doing this I bypassed the aux fan fuse alltogether (installed a seperate fuse inline to the aux fan). Question, if the system was bypassed, when the temp switch does go on above 212 and then activates, could this still be causing a voltage drop to the starter? What is this temp switch actually engaging? Relays?

Thanks

Check your battery ground. And also the ground straps from the chassis to the block. When you're starting there is a huge amount of current that must make it from wherever the battery is grounded to the starter ground. Try running a 2/0 wire from the battery negative terminal to a direct ground on the starter and I bet you will get rid of that voltage drop, unless there is an issue with the positive wire that feeds the starter.

Tom Drew

__________________
1972 280SE 4.5

If they do not find the voltage drop quickly you can wire in a cheap relay that will engage at say your seven volts and transfer a full 12v to the solinoid. Still see a lot of old relays used this way. Might also save a lot of trouble shooting time and cost. This also means your old starter was probably good. If you can keep it and just get a truly bad dud to hand in you will have a spare starter. I assume they are reading about seven volts on the solinoid trigger wire when the engine is hot. Not enough to kick the solinoid in in most cases.

The Mercedes shop called me today to come and take a look at something they had found. The headlight switch had shorted out and litterly arced with the metal causing some of the wiring to burn and fuse together. They are assuming this is were I was lossing the 5-7 volts when trying to start the car. As they explained to me the iginition wiring starts out in front, then goes to the headlight switch and from there to the ignition switch. Does anyone know if this is correct?

Thanks,

Dave Benson
1971 280SE 3.5 Coupe

I never followed up with all of you on this. It turned out to be the light switch that was causing the problem. It seems that the power from the battery runs through the light switch. The swich had begun to decay and starting to arc between two of the contacts on occasion. So every once in awhile it would arc not allowing the car to start. Took forever to finally figure out this was the problem.

Dave Benson
71 280SE 3.5 Coupe