OVP - basic function and test


Senior Member
Sep 6, 2008
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Bromley, London
Your Mercedes
CLK 320 CAB Elegance C208
Thanks to Ian_C from benzworld for giving me permission to use his content.

Mercedes OVP A Basic Primer and Test Process
I am dealing with my second failed OVP and reviewing the various links prompts me to try to consolidate some of the scattered information into this single format. I hope it is useful. See Figure 1. below for a picture of a typical OVP relay.

Over Voltage Protection [OVP] devices are used to protect sensitive electric circuits from damaging high voltage. Two OVP protected circuits in the 124 series Mercedes are the Electronic Engine Control and the ABS [anti-lock brake] circuits. Hard starting/stalling are often symptoms of a failed EEC circuit, often in conjunction with an illuminated ABS light on the instrument panel. The OVP device supplying power to both these circuits is located under the hood, behind the battery, somewhat behind a removable plastic splash panel on the passenger side of the vehicle (North American configuration). It looks like Figure 1. The first thing to check is that the 10 amp fuses on the top of the device are not blown. Assuming the fuses are OK, replace them, and to further test the device; remove it from the car by wiggling and pulling it free from the plug base. Figure 2. shows what it looks like inside with the metal canister removed; but it is not necessary to dismantle the device to test its basic function. Figure 2. below is mainly intended to identify the circuit pins referred to in the next section.

The basic function of the OVP is this: Battery voltage is always present at pin 30 (nominally 12.5v higher when the alternator is charging); and the device is connected to chassis ground through pin 31. With the ignition off (that is no 12v current delivered to pin 15) there should be battery voltage at pins 30a and 30b, but no voltage at the protected circuits 87E and 87L. If there is voltage at the 87E or 87L pins with the ignition off there is an internal short in the device. When the ignition key is turned on, battery voltage is applied to pin 15 (the skinny one) which pulls in a relay switch and battery voltage is then supplied to both the 87E and 87L protected supply circuits.

To test the basic functioning of the device out of the vehicle you only need a 12v power supply (a battery), a couple of (actually 3) jumper wires with alligator clips, and a DC voltmeter. First connect the 12v positive jumper (preferably through a 10 amp fuse) to pin 30, and the 12v negative jumper to pin 31. Use the voltmeter, with its ground lead attached to the negative terminal of the power supply (battery), to check for positive voltage at the other pins. There should be 12v at pins 30a and 30b, but no voltage at any of the other pins [15, 87E, or 87L]. Without disconnecting the jumpers to pins 30 and 31, now connect a third jumper wire from the positive side of your power supply to pin 15 (this is the same as turning on your ignition switch when the device is in the car). You may hear a slight click as the relay pulls in the internal switch, and now when you test the pins with your voltmeter, 87E and 87L should be reading 12v. If there is no voltage or reduced voltage at the protected circuits 87E and 87L this indicates a faulty OVP.
Note that passing this basic test may not be the whole story, as electric devices can fail intermittently, or partially, when in their real working environment. Further testing after heating (hair blower) or cooling (freezer compartment) may be useful if a temperature sensitive intermittent failure is suspected.



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