SL/R129: Need advice for MB 500SL
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1990 SL500 R129
Need advice for MB 500SL
Hello all! I have a 1990 500SL R129 with 58,000 original miles. Garaged its entire life. Zero rust anywhere. I bought the coupe 3 years ago. I have the hard top. I started replacing the soft top pistons and have done the 4 largest. The 8 small ones left to go. The biggest issue is this. My car starts up absolutely perfectly if it’s been in the garage for a few hours or more. If I drive it, and it’s a warm sunny day, wherever I park it it may sit for an hour and then when I return it won’t start. Sometimes it doesn’t even turn over. It’s just dead. Then after waiting 10-15 minutes of trying, it’ll start up weakly. Then I’ll start it again and it’ll be roaring like brand new. Mechanic said is either a weak ground between the battery and engine or the mechanical engine itself is responding to overheating. Any advice?
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SL600
In my opinion, that is insufficient information to conduct a targeted diagnostic, as the description points to several completely different potential causes. Or, it’s possible there are simply multiple faults occurring simultaneously.
"Sometimes the starter motor doesn't even turn over" is too vague. If the issue were a weak battery or a poor ground connection, the starter solenoid would still engage, and the lights would dim noticeably. What actually happens?
"The car simply remains 'dead'"—what exactly does that mean? No lights? No horn? No radio?
"After waiting 10 to 15 minutes and making repeated attempts to start it, the engine finally catches"—this could be an indication of a defective fuel pressure accumulator located next to the fuel pump.
"However, initially it runs very weakly"—this could point to either the pressure accumulator or moisture inside the distributor caps. In that case, you would often hear misfires as well; so, what exactly do you mean by "runs very weakly"?
"Sometimes the starter motor doesn't even turn over" is too vague. If the issue were a weak battery or a poor ground connection, the starter solenoid would still engage, and the lights would dim noticeably. What actually happens?
"The car simply remains 'dead'"—what exactly does that mean? No lights? No horn? No radio?
"After waiting 10 to 15 minutes and making repeated attempts to start it, the engine finally catches"—this could be an indication of a defective fuel pressure accumulator located next to the fuel pump.
"However, initially it runs very weakly"—this could point to either the pressure accumulator or moisture inside the distributor caps. In that case, you would often hear misfires as well; so, what exactly do you mean by "runs very weakly"?
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1990 SL500 R129
In my opinion, that is insufficient information to conduct a targeted diagnostic, as the description points to several completely different potential causes. Or, it’s possible there are simply multiple faults occurring simultaneously.
"Sometimes the starter motor doesn't even turn over" is too vague. If the issue were a weak battery or a poor ground connection, the starter solenoid would still engage, and the lights would dim noticeably. What actually happens?
So the lights in the car do come on but they come on weakly. Sometimes they’ll come on and then dim out and the car will stop.
"The car simply remains 'dead'"—what exactly does that mean? No lights? No horn? No radio?
thank you so much for adding the detail. When I turn it over MOST times the electric will come on. Sometimes it’ll be dim. But the electric comes on. There is on occasion where nothing comes on at all. Then I repeat process.
"After waiting 10 to 15 minutes and making repeated attempts to start it, the engine finally catches"—this could be an indication of a defective fuel pressure accumulator located next to the fuel pump.
"However, initially it runs very weakly"—this could point to either the pressure accumulator or moisture inside the distributor caps. In that case, you would often hear misfires as well; so, what exactly do you mean by "runs very weakly"?
"Sometimes the starter motor doesn't even turn over" is too vague. If the issue were a weak battery or a poor ground connection, the starter solenoid would still engage, and the lights would dim noticeably. What actually happens?
So the lights in the car do come on but they come on weakly. Sometimes they’ll come on and then dim out and the car will stop.
"The car simply remains 'dead'"—what exactly does that mean? No lights? No horn? No radio?
thank you so much for adding the detail. When I turn it over MOST times the electric will come on. Sometimes it’ll be dim. But the electric comes on. There is on occasion where nothing comes on at all. Then I repeat process.
"After waiting 10 to 15 minutes and making repeated attempts to start it, the engine finally catches"—this could be an indication of a defective fuel pressure accumulator located next to the fuel pump.
"However, initially it runs very weakly"—this could point to either the pressure accumulator or moisture inside the distributor caps. In that case, you would often hear misfires as well; so, what exactly do you mean by "runs very weakly"?
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1990 SL500 R129
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From: Colorado
G550
If it's convenient, swing by any auto store and ask them to do a load test on your battery (usually no charge for this). They can tell you if your battery is healthy or low on charge.
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SL600
Without electric power no fun. Even with a dead batterie, the alternator should deliver enough power to keep the engine runing. Can you measure the voltage when the engine is runing to check the generator is working? Should be around 14 volt.
Ignition switch? But before you start firing the parts cannon, I'd first check whether the alternator is charging.
After checking the generator check the overvoltage protection relais. Solder joints are often broken; resoldering the circuit board helps.
MOST times the electric will come on. Sometimes it’ll be dim. on occasion where nothing comes on at all.
It doesn’t misfire it just rpms very low
