Location of Idle Control Unit and Overload Relay...1981 380SL

[jim blanton]

Is the ldle control unit and overload relay located behind and below
the glovebox on the 1981 380SL series....any useful hints on replacing
the two ...thanks jim blanton

[Build]

On a '81, the idle control unit is directly behind the glovebox, and the overload relay is in the passenger kickpanel.

There isn't a replaceable fuse on the '81 overload relay, only a fixed one. Mine was blown which causes high idle speed. You can solder a standard 10 amp fuse across the open fixed fuse if yours is blown, or solder a fuse socket in so you can change out fuses in the future.

[jim blanton]

Right where you said they were...had a new relay and idle control unit...pulled out the relay first and it was bad....the new one dropped
the idle to 650 rpm...down from 1500...hummm...tried the new control
and the rpm went up to 950 rpm....so now using the old control ...
also the numbers on the old control are 412 202/002/001 with 8 pins
and the new control has 412 202 012 001 and has 10 pins....wonder what
MB was doing?

[Build]

Interesting. I don't know. I have a new unit as well but don't know the part number. I idle at about 700 rpm. I was told that they changed the idle control sometime during the model year 81 so maybe that's the difference? I guess you could return the new unit.

[starbucks]

can anyone email me a pic of the location of the overload relay? i have a 1981 380sl as well. i have four above the fuse panel and a couple of relays to the left bottom of glovebox. chandays@gmail.com.. i think its one of the ones to the left bottom of the glovebox

[Benz Dr.]

Originally Posted by Build

Interesting. I don't know. I have a new unit as well but don't know the part number. I idle at about 700 rpm. I was told that they changed the idle control sometime during the model year 81 so maybe that's the difference? I guess you could return the new unit.

Not likely. Most places won't take an electrical part back. Standard policy for just about everyone.

[Benz Dr.]

Originally Posted by starbucks

can anyone email me a pic of the location of the overload relay? i have a 1981 380sl as well. i have four above the fuse panel and a couple of relays to the left bottom of glovebox. chandays@gmail.com.. i think its one of the ones to the left bottom of the glovebox

The ' 81 model year 380SL has a small ice cube sized relay that is anodized red. After you remove this relay you can look though a small hole on the underside to see if the fuse is blown. When this relay isn't working the fequency valve won't buzz and the engine goes into a sort of limp home mode type of running. This relay is found behind the kick panel that surrounds the fuse box. The ' 81 and ' 82 model year OVPR are not interchangable.
On the later ' 82 380SL the relay is much longer but is still anodized red. There's a plasic cap on the top with a red 10 amp fuse. If this fuse is blown the idle speed will often increase so always check this fuse first before moving on to the amplifier or controler on the front of the engine.
Sometimes the slide valve inside of the controler is stuck but it's usually the amplifier that's quit working.
After trying a rebuilt amplier on a 560SL with no results, I changed the OVPR even though the fuse was OK and it fixed it, so they can also cause a high idle problem.

[RadioTek]

There's a reason for the circuit to be there, other than to provide revenue for the dealership.

The OVP isn't so much a relay as it is an electronic curcuit. The "heart" of it is a device called a Zener diode, this device is designed to be non-conductive in one direction until a specific voltage is reached, then it will "avalance" and fully conduct.

The end result is that it acts like a relay that closes at a specific voltage. In this particular application, the circuit is designed so that when a specific voltage is detected across the Zener, it avalanches, and blows the fuse. This fuse is also the power source for the ECU. So if the voltage gets too high, the OVP blows the power fuse to the ECU, preventing electrical damage to it. The system is now in "limp" mode, but the ECU is not a smoking ruin (theoretically).

There is a bit of a 'complication' to this: As electronic devices age and undergo heat cycles, their values can change. Teh Zener may trigger at a lower or higher voltage, a resistor may change values so that the voltage and current to the Zener are different. Soldered joints can have micro-fractures or even crystallize. The physical connections in the 'relay' socket can corrode.

The first thing to do while troubleshooting an OVP is to figure out WHY it went on you. Did you try to jump the car and swap the cables? Is the regualtor on your alternator going on you? Are you using the stock aluminum body fuses? Do you have a circuit that is drawing too much?

The stock aluminum fuses in the brass holders are subject to dis-similar metals corrosion, and should be individually pulled, the ends buffed with a coarse cloth (NOT abrasive), dabbed with a thin film of terminal grease, and replaced.

You can do a basic check of the health of your charging system with a voltmeter:
On the DC volts scale that will measure up to 20 volts, measure the battery voltage with the car off. It should read about 13.8 volts. Start the car, and measure agin, it should read a little lower, and start increasing (you pulled some 'juice' out of it, and the alternator has to replace it). Change ove to the AC Volt scale, see if you get a reading. If you get more than a few tenths of a volt at most, you have a diode in the alternator gone on you.

If you have any problems with the readings above, you can go to almost any auto parts store, they will put a special meter on the system that will be able to tell much more accurately where you may have a problem.

Scott