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Old 02-01-2003, 01:31 PM
sbourg sbourg is offline
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Join Date: Jul 2001
Location: SoCal
Posts: 1,303
OVP issues and idle problem

I have been trying to determine if I have a problem with the OVP relay in our '91 190E 2.3. I have been studying the limited info I have and the posts here, and it's function has become clearer, but incomplete. Its primary function appears to be as either a voltage regulator or clamp for a device labeled as only 'N3' on my schematic. If it is a regulator - it is defective. If a clamp - perhaps not. The secondary function is as a relay to disconnect power from the regulator and 'N3' when the ignition is turned off.

'N3' is a multi-terminal device conneced to the EHA, coolant temp sensor, air flow meter, fuel pump relay, throttle valve switch, deceleration microswitch, etc. I think it would be safe to say it is the main lambda control unit. Since my schematic is only good for '89 and earlier, there may be other functions 'N3' connects to I am unaware of, such as the rotary idle speed actuator.

I tested the OVP terminals which connect to the 'regulator' inside, using a current-limited voltage source. I could put well over 20V across the terminals with no significant current draw. If the function is a regulator, it is shot. If it is simply designed to clamp voltage spikes from the ignition or alternator, it may be ok. I'd sure like to know, because $80 is no problem if it's bad - but I don't want to shotgun the issue.

The problem I notice I am having is this:

When I start and drive off to work - 6 am, dark, and usually chilly here in SoCal - the idle settles at about 1200 rpm, but may creep up as high as 2000 rpm or more by the time I get to work (16mi, no traffic). If at any time I stop and turn the engine off, immediately restarting, the idle goes to and stays at normal 700 rpm for the rest of the trip. Any other time of day or night, the problem does not occur. If there are any other related problems, I haven't discovered them, unless mediocre mileage (21-23 mpg) is one. I have cleaned and checked the rotary idle valve, replaced much related air plumbing, and normal idle has improved - but this problem remains.

My questions are, is the OVP a regulator or clamp, and is it bad, and are there any other likely causes for the specific idle problem I am noticing? Thanks.

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