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.

Steve

Hoping someone knows the answer whether the OVP is a voltage regulator or just voltage spike protection, so I know if I need to replace mine. Thanks.

Steve

The Over Voltage Protection relay is basically just a relay that switches power to the fuel and ABS computers. What makes it special is a zener diode connected between the output and ground. If the supply voltage goes too high, say because of failure of the regulator inside the alternator, then the zener conducts and blows the fuse on top of the relay, protecting the computers.

The common cause of failure of the relay is bad solder joints on the printed circuit board inside. If you can use a soldering iron it's easy to open up the relay can and run over all the joints inside.

The N3 component you refer to sounds like the fuel computer, don't forget that your car's KE injection system is a mechanical system with some added electronic aids, one of which is the idle speed control. Most cars will run with the electronics out altogether, but starting and idling may be poor.

Your symptoms are typical of OVP failure, the other give away is illumination of the ABS light.

__________________
Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)

Thanks for the reply, Mick. My ABS light never goes on except perhaps prior to starting. The idle symptoms I have seem different from those others have reported here, so if the zener in the OVP relay is NOT a regulator, I suspect it is ok. As a final test of the OVP, though, I'll check the OVP output voltage when the symptom is occuring - and when not. That should pinpoint any bad contacts in this device.

Opening up the thing is another story! Mine doesn't come apart when I bend the tabs back - the base is loose, but something seems to be holding on higher up. I thought maybe the mounting tab rivets go though an internal heatsink, and might have to be drilled out?

Another thread I found indicated many weird symptoms due to copper oxide deposits intermittantly shorting pins at various connectors. I'm beginning to have suspicions in that vein, since I have already found some past problems where that was a factor. I'll check all connectors that in some way could relate to the idle circuit, I guess.

Steve

You did remove the fuse? That would stop the assembly coming out of the can.

If the bad idling symptoms appear with the ABS light off then it's 99% certain the OVP relay is OK. If you're testing it, check the voltage at the output, it will should be line voltage.

Check the wiring by looking for line voltage at the fuel computer.

I have a diagram showing the same systems for a 300E, email me thro' the forum for details.

__________________
Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)

Mick, I removed the fuse, bent back the tabs, and gently pulled on the base. It seemed free enough away from the mounting tab, but resisted on that side. Since I don't have a spare, I didn't get any rougher than that. I will check the voltage next opportunity to rig something up - I'll set a voltmeter inside to monitor while driving.

Anything you have I'd appreciate. Email is aq433@hotmail.com

Thanks.

Steve

Reporting back on the OVP issue to add a data point to the problem. Now a couple weeks since I replaced the OVP, and the problem disappeared and has not reoccured. Before and after the repair I have replaced ALL the rubber in the idle circuit including the bottom plenum of the mass-flow meter on, with varying effects on the idle. The OVP was replaced separately from any other repair in the sequence, and resolving the issue definitely dates from then.

I assume resistive contacts, from whatever cause, were the issue with the old OVP. I tore it apart with additional resolve, and found little inside to indicate a serious problem. Still, I resoldered the pin contacts, removed and disassembled the relay for cleaning (some contact pitting), and will keep it as a spare.

Steve