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Over Voltage Relay Replacement
 
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Pelican Technical Article:

Over Voltage Relay Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$60

Talent:

**

Tools:

13mm socket, 13mm, 8mm wrench

Applicable Models:

Mercedes-Benz 260E (1987-89)
Mercedes-Benz 300CE (1988-93)
Mercedes-Benz 300E (1986-93)
Mercedes-Benz 300TE (1988-92)
Mercedes-Benz E320 (1994-95)

Parts Required:

New over voltage relay

Hot Tip:

Check contacts first

Performance Gain:

A proper running engine

Complementary Modification:

Clean your battery posts

The over voltage relay or OVP supplies power to the idle control and cold/warm start circuits. The relay does just what it says and protects these delicate systems from spikes in voltage. Damage can be done to the relay when attempting to jump-start your vehicle with another. If you have jump-started your car and it starts running rough at idle or you are experiencing hard cold or hot starts you may have damaged your OVP.

Like anything on a car they can also just break over time. While most of the damage is done by tiny cracks that develop in the circuit board from heat and vibration, they are also susceptible to corrosion. The relay is located behind the battery under the front windshield and can get fluid in the contacts from the windshield or washer system. It is always a good idea to check the contacts before replacing the relay.

The relay also has a fuse on the top of it. This was superseded by a dual fuse relay later but they are both compatible. Always check the fuse(s) before replacing the relay.

The OVP is located behind the battery which is located under the hood at the rear of the engine compartment on the right side (red arrow).
Figure 1

The OVP is located behind the battery which is located under the hood at the rear of the engine compartment on the right side (red arrow).

Use a 13mm wrench and disconnect the cables from the battery posts.
Figure 2

Use a 13mm wrench and disconnect the cables from the battery posts. Remove the negative or ground cable (red arrow) first and then remove the positive (yellow arrow) cable.

There is a mount at the front of the battery securing it to the tray.
Figure 3

There is a mount at the front of the battery securing it to the tray. Use a 13mm socket and extension and remove the nut (red arrow).

Remove the nut and bracket from the vehicle (red arrow).
Figure 4

Remove the nut and bracket from the vehicle (red arrow).

Use the handles (red arrows) on the battery and pull it straight up and out of the vehicle.
Figure 5

Use the handles (red arrows) on the battery and pull it straight up and out of the vehicle. Batteries are full of acid and can leak and spill. Make sure to use caution and do not spill anything on yourself or the vehicle when removing the battery.

With the battery removed pull the plastic panel (red arrow) out.
Figure 6

With the battery removed pull the plastic panel (red arrow) out. Note: Now is a good time to clean up all the dirt and debris that accumulate in and around the battery tray.

The Over Voltage Relay is located towards the center of the vehicle on the end of the ECU (red arrow).
Figure 7

The Over Voltage Relay is located towards the center of the vehicle on the end of the ECU (red arrow).

The OVR has a fuse on the top of it, which is the first line of defense.
Figure 8

The OVR has a fuse on the top of it, which is the first line of defense. Note: The newer relays have two fuses on the top. If you are replacing the relay both types will work. Always check the fuse(s) before changing the relay. To remove the relay use an 8mm wrench and remove the bolt holding it in place.

Slide the relay out from its mounting bracket and separate the relay from the wiring harness.
Figure 9

Slide the relay out from its mounting bracket and separate the relay from the wiring harness. Always check the connection points on the relay (yellow arrow) and the wiring harness (red arrow) for any signs of corrosion. The area it is mounted in can see a lot of fluid accumulate over the years and this sometimes drips onto the relay and causes corrosion. Installation is the reverse of removal.




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Comments and Suggestions:
Mitch Comments: I have a 1988 W124 and I suspect the voltage relay needs to be replaced after reading others comments. I have got to the stage where I need to remove it but it does not look like the one above, it is not bolted on to other components and has a brass metal strip between relay and frame, unlike one above.Aso I can't unplug from underneath. Help please
January 22, 2017
  Followup from the Pelican Staff: What vehicle are you working on? - Nick at Pelican Parts  
Ernesto Comments: Hello , I have the W124 E220, 1994 model great car and I love it very much. Lately I am experiencing erratic idling at temperature btw 82-85 and in traffic for more than 5-10 mins. When car ready to take off, there is lots of hesitation and sometimes racking. When car temps are at 82 and below, car purrs like a cat. Have replaced fuel pump, fuel filter, spark plug wires, plugs. My mechanic tells me it has to do with fuel mixture formation. Others are suggesting the following-coolant temp sensor, OVP relay, ignition coil and and possibly knock sensor. Can some good mechanic who knows this fault help diagnose this delicate issue before I become bankrupt? Thank you. Ernest
December 11, 2016
  Followup from the Pelican Staff: Is the check engine light on when the problem is present? Sounds like an engine misfire. I would check spark, fuel and compression on all cylinders.
- Nick at Pelican Parts
 
roger Comments: thanks a lot to all member
i found my ke- jetronic control unit has broken
no voltage at pin number 3 on iacv.and the second case is actuator.when i pull out socket actuator the engine can run.so i decide to disconected control unit.has anyone know how to fix control unit.the control unit number is 0 280 800 124 for 4cyl
July 5, 2016
  Followup from the Pelican Staff: I would do a web search for automotive electronic repair services. - Nick at Pelican Parts  
roger Comments: i have trouble with 230e 1989
it was rough idling when starting the car.
every harness i'd checked.i felt confused and i wish all pelican member should help me.thank you
June 26, 2016
  Followup from the Pelican Staff: Sounds like an engine misfire. I would check spark, fuel and compression on all cylinders.
- Nick at Pelican Parts
 
Cas Comments: I am looking for the location of a 1994 S500 MBZ fuel pump relay
May 8, 2016
  Followup from the Pelican Staff: It is in the trunk, on the right side in a relay panel. I don't have a photo to share, use the wiring colors to identify it.

K27 relay - Nick at Pelican Parts
 
Sid Comments: What is the relay that's in front of the green fuel pump relay? Its missing on my 96 W124 E220. Help!!!!
May 8, 2016
  Followup from the Pelican Staff: We don't have that vehicle in the US, so I don't have any info on it. See if you can locate a repair manual. - Nick at Pelican Parts  
earthquakedude Comments: have a 1990 300E and had same problem ABS would come on car would stall would not start again for 30-40 minutes. Was voltage regulator on back of alternator. Replaced it ABS light went off runs great agian.
November 15, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
nizamezre Comments: Hi fren , i got 1 issue about my merc w124 . When normal start , the engine running fine . I means when running engine . . But after I on light and aircond , few minute later ABS light suddenly on and after 10-20 minutes engine stop . After that I try to start again but battery became flat . Can you give me suggestion what I need to do ? Tq
November 9, 2015
  Followup from the Pelican Staff: You either have a charging system issue or something discharging the battery when off. Start by checking charging system voltage and current. - Nick at Pelican Parts  

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Page last updated: Sun 2/19/2017 02:58:35 AM