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W124 homemade OVP-relay
The other day my OVP-relay retired resulting in hard to start, very low idle when cold, almost stalling and the ABS light on. When warmed up the idle went to almost 2000 RPMs.

A quick search on the net revealed that the culprit was most likely the OVP-relay. These can be expensive, so I decided to make my own. Afterall the schematics is actually printed on the relay itself.

My 1986 5-speed 300TE has a 9-pin OVP-relay. The earlier 6- and 7-pin versions are very similar in that they have just one internal relay and one 10A fuse. The 9-pin edition has two internal relays, and two 10A fuses.
 
In my adoption of the OVP-relay I decied to opt out on the Zener-diode which is supposed to protect against overvoltage. Since it doesn't kick in until 22 Volts, I figured it is very unlikely that the charging system will reach that high a voltage.
 
What you then need is:
 
  • 1 diode (1N4001 or better). It needs to be able to carry the relay power-on current. Typically 0.16A per relay for a total of 0.32A. The 1N4001 can handle 1A
  • 2 regular relays that can switch at least 10A each. I used a dual 30A relay from Bosch which were integrated into a single unit
  • 2 fuses and fuseholders
  • 2 fuses rated at 10A
 
A bit of wire, cableshoes or even better you should solder the wires. I opted to use cable shoes just to try out my first attempt, but will solder all the wiring later. Soldering prevents bad contact and corrosion which can result in bad connections.
 
The diodes white line marking should face towards the relays 86 connections. The white line is here show on the right side of the diode.
 
1N4001
 
Below are the schematics and a picture of the final result. The car now runs great, and idles perfectly.
 
OVP-original
(original OVP-relay with socket removed from car)
 
OVP-homemade
(final result)
 
OVP schematics
(schematics)
 
Please make a note of where the wires to your original OVP-relay go before you cut the wires at the relay socket. The socket has numbers stamped into it at each connection. Either at the top or side. Note that your wiring might be slightly different.
 
The schematics connections are marked with the pin number, and in parathesis the electrical wiring number, followed by a quick description and then the wire color(s) in the last parathesis.
 
Note that the reference below to relay and fuses are the internal components of the OVP-relay, not the main fuse box. Relay 1 refers to the upper relay, and fuse 1 is the leftmost fuse in the schematics.
 
PinElectrical signal
Source / Destination
Wire color(s)
Comments
 1 30 +B from battery
 Red Unfused, constant battery power
 2 87E Power output from relay 1 and fuse 1  3x Red / Yellow
 Power on when ignition on (fused)
 3 15 Ignition on signal
 Red / Pink
 +B when ignition in 2 or 3 position
 4 30a Constant +B via fuse 1
 Red / Green
 Constant battery power (fused)
 5 31 Ground Brown Grounding
 6 87L Power output from relay 2 and fuse 2
 Red / Blue
 Power on when ignition on (fused)
 7 87E Power output from relay 1 and fuse 1  Red / Yellow
 Power on when ignition on (fused)

 8

 30a Constant +B via fuse 1
 Not connected
 Constant battery power (fused)
 9 87L Power output from relay 2 and fuse 2
 Red / Blue Power on when ignition on (fused)
 
 
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