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1986 420 SEL, gold
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3 Posts
Discussion Starter #1
I have a 1986 420 SEL and am experiencing a new issue. My son drives the car to school and indicates that all of a sudden, the lights dim, the turn signal fades and the radio stops. He can limp the car to the parking lot, and the engine cuts out.

At the end of school, the car did start up and he was able to drive home, however, he just made it to the driveway when the same experience happened, ending with the engine just shutting down.

I'm thinking perhaps this may be an alternator issue? However, I could use any suggestions as to what I should look for and how to test if this really is an alternator issue.

Thanks!
 

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99 E430, 01 E430 Sport, 00 SL500
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3,266 Posts
Handy tool to test battery, alternator.
Peak 12-VOLT BATTERY TESTER | Product Details | Pep Boys
On latter cars, it's usually a bad crank shaft position sensor causing the engine to shut down abruptly when hot. When engine cools down, it starts, runs fine.
Over here,
http://www.autohausaz.com/search/product.aspx?sid=nosiqi45253jhz45y0y5nim2&[email protected]&[email protected]&ye
ar=1986&[email protected]&[email protected]%20Position%20Sensor
it shows two CPS's for your car with huge price difference. I would not mind buying the cheaper one if the CPS is the problem. Is it the problem? I don't know because your son was able to limp the car to the parking. Bad CPS -> engine shuts off completely suddenly at any moment when hot, no limp. Did the car shut off completely? Could it start rigft after the shut off or it had to wait to cool to start again?
 

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1986 420 SEL, gold
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3 Posts
Discussion Starter #4
Further information:

Both times the car shut off, it shut off close to his final destination so he just coasted to the parking stall. In both cases, the engine shut off prior to being placed in park, but after the radio went off, etc.

Because he was at his final destination, he didn't try to restart, so I don't know if it would have restarted if he had tried. I will try to duplicate the issue myself to see if it would restart after it shuts down.

In both cases, the car had only been running less than 4 minutes, from a cold start, so I don't think it should have overheated in that time?
 

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99 E430, 01 E430 Sport, 00 SL500
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So it was engine shutting down completely.
The CPS generally fails because magnet(s) in it "degrades" over time. Degradation coupled with heat (after engine gets warm/hot), CPS fails to cause shut down. It is not the overheat of the engine that caused it to fail.
 

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1986 420 SEL, gold
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3 Posts
Discussion Starter #6
I went to see if I could duplicate the problem and the engine wouldn't even turn over. I checked the charge on the battery (it's new), 11.74 volts. I charged the battery checked it at 12.6 volts, turned the car on and then tested the battery during operation under load of lights and radio. 12.4 and dropping. Looks like an alternator problem.

I checked for voltage drop, both positive and negative to the alternator. Wiring Looked good, no undue resistance, so I pulled the alternator. The voltage regulator had really worn down brushes. I ordered a replacement regulator. I'm going to try this before I go the more expensive route of a new alternator. Hopefully this solves the problem!

Thanks for your input!
 
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