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.
