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Old 10-24-2003, 11:24 PM
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380SL Overload fuse blown

I have a 1982 380 SL with 89,000 miles, its been stored for 2 years and started every few weeks. I noticed that the overload relay fuse was blown and upon installing anouther it blew again immediatly. If you could please tell me what the overload really does and what should I check for . Any help would be greatly appreciated. Thanks

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Old 10-25-2003, 12:52 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
The OVP relay provides power to the lambda control unit and the idle speed control unit. The input to the OVP is alway hot and not fused, so I think the fuse on the relay protects the relay itself.

If it is blowing immediately, I would suspect that the relay has an internal short to its ground. Other possibilities are that either the lambda control or the idle control have either an internal short or one of the feed wires is bad.

On the OVP, the feed wires come off pin 2 and are red/blue, The ground comes off pin 31.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 10-25-2003, 04:50 PM
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Thanks Chuck your respond gave me a good starting point.
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Old 10-25-2003, 05:34 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The real name of the relay is the overVOLTAGE protection relay.

Internal is a zener diode that conducts straight to ground (through the fuse) everything over some voltage like say 16v (may be higher). It neatly clips all spikes with no problem but it won't handle a regularly overcharging alternator or as usually happens an improper jump start. Battery chargers can blow the fuse if the key is turned on while running.

The fuse blows because more than 10amps (I am guessing the fuse current rating) flows at above the zener cascade voltage. (a zener looks like an open circuit to normal voltage but becomes conductive above a voltage.)

Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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