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Overload Protection Relay Replacement
 
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Pelican Technical Article:

Overload Protection Relay Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$40 to $70

Talent:

**

Tools:

Philips head screwdriver

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Relay

Hot Tip:

Make sure to disconnect the battery

Performance Gain:

Working vehicle

Complementary Modification:

Detail the interior

Many of the older Mercedes-Benz vehicles came with an Overload Protection Relay. This relay with an attached fuse was used to protect the cars sensitive electronic systems from electronic spikes that could damage or destroy valuable components. The relays main job is to stop overloads from a faulty alternator by dumping the electrical spike to ground. It basically works on the idea that no voltage is better than too much voltage to sensitive electronics. The relays were installed in different locations on different chassis so make sure to check the location on your vehicle. On our 300TD it is located up under the passenger side dash below the A- pillar. You will need to remove the under dash panel on the passenger side to access the motor and wiring.

This is one of those spots where over the years your car may have been serviced by multiple people including previous owners and this can lead to parts replaced with different size fasteners and hardware. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches. If something is different on your vehicle please let us know and share your info to help other users. If you have any questions or comments or even a different procedure you would like to share please leave it below and please leave your vehicle information.

Begin by disconnecting the ground terminal from the battery (red arrow) and placing it somewhere where it cannot come in contact with the post while you are working.
Figure 1

Begin by disconnecting the ground terminal from the battery (red arrow) and placing it somewhere where it cannot come in contact with the post while you are working.

Start by removing the large plastic Philips head screws (red arrows).
Figure 2

Start by removing the large plastic Philips head screws (red arrows).

Next remove the trim pieces if you have them and using a Philips head screwdriver remove the three screws (red arrows) holding the panel to the dash.
Figure 3

Next remove the trim pieces if you have them and using a Philips head screwdriver remove the three screws (red arrows) holding the panel to the dash.

Drop the panel down and make sure it is free and clear of everything and remove it from the vehicle.
Figure 4

Drop the panel down and make sure it is free and clear of everything and remove it from the vehicle.

On our 300TD project car the OVP is located at the top of the door opening under the bottom of the A-pillar (red arrow).
Figure 5

On our 300TD project car the OVP is located at the top of the door opening under the bottom of the A-pillar (red arrow).

The wiring connection for the OVP sits in a metal bracket and can be slipped out by sliding it back, but these get brittle over the years and I find it much easier to just wiggle and pull the relay (red arrow) up from the mount and connection (yellow arrow).
Figure 6

The wiring connection for the OVP sits in a metal bracket and can be slipped out by sliding it back, but these get brittle over the years and I find it much easier to just wiggle and pull the relay (red arrow) up from the mount and connection (yellow arrow).

If the fuse on the top of the relay is bad the relay is shot and needs to be replaced.
Figure 7

If the fuse on the top of the relay is bad the relay is shot and needs to be replaced. Installation is the reverse of removal.

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Comments and Suggestions:
Dudley Comments: I have a 1984 300D turbo. I can't find the OVP relay under the dash on the passenger side. The driver's side analysis pin connector is located under the hood. It has no male amp in the cylinder, just the female pin connector. I have not removed the battery yet. My tach is not working and I can't hear the a/c compressor kick in. The tach and compressor were working prior to a mechanic pulling the dash to replace actuators. Am I blind, crazy or both?
December 29, 2016
  Followup from the Pelican Staff: I don't have info going that far back to confirm the location of the relay for you. I would go to the MB site and check if they have a repair manual for your vehicle. it will have the component locations. - Nick at Pelican Parts  

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Page last updated: Fri 1/20/2017 03:09:12 AM