Another couple of thoughts!

1. Checking voltage at the fuse terminals will not tell very much as this is on the 'live' side of the relay.

2. To bypass the relay bridge the 30, 87l and both 87e terminals. Beware, the ingition switch may not shut off the engine if you do this. You could use the 15 instead of the 30 but this may overload the ignition switch.

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

I was playing around with this old OVP and noticed that there is no continuity from side to side of the black resistor in the pin 15 ignition circuit. Car wouldn't start when this one was in. Another cause for failure? This one would be immediately noticeable since the car won't start. It's the black one in the middle of the picture.

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I was playing around with this old OVP and noticed that there is no continuity from side to side of the black resistor in the pin 15
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I believe that is a diode, not black resistor...

Switch your meter/leads polarity and re-check for foward bias...

Current will pass through it but it doesn't have continuity either way polarity is checked. Does that mean it's ok?

If it shows no cont. both ways [ polarity wise], it is open.
If so , go to RS and get one [ for about 20 cents] and solder it in there , and go give it a try..

Edit:
Make sure of polarity of the new one . This is what the little stripe on one end of the diode is there for . Put new the same position [polarity] as the one you take out...

Electronics 101 question: No continuity but will pass current? How can that be? I'm lost here unless that means there is a "gate" in there that closes upon current application. By the way, the ID of the whatyoumaycallit in 1N 4003.

Well I did pull the OVP relay fuse on my HFM equipped E220 and guess what? No change to the engine, it started and ran fine, idle speed compensation worked in the normal way, only difference was the ABS light was on.

So I conclude that the HFM engine management computer is not fed from the OVP relay, presumably it has it's own protection from high system voltage.

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

Best discussion on electronics I've come across here in a LONG time. So, allow me to toss in a couple questions for the pros:

1 -- Get out on the Interstate and hit the brakes at speeds in excess of 70MPH in my SL. I get a slight pull to the right front and the ABS light comes on. After that it brakes w/o any pull. Turn the ignition off and the ABS light goes off. Got an ABS computer problem you think?

2 -- Just replaced the Environmental Control Unit (000 822 1203)awhile back ( a $1,500.00 part through Fast Lane and the MB dealership). Managed to find one on the internet for $125.00 so felt much better about doing it. Installed it and it worked fine for about six weeks. Now when I turn the A/C on and pull up to a stop light, the ECU makes a bzzzttt noise, the RPMs surge about 300RPM. You can put your hand on the ECU and feel this is where the relay(?) is sticking. So, the questions are: (1) what's causing me to continue to have this problem with the ECU, and (2) what's the connection between the ECU and the RPM surge? I'd think the latter would be more an idle control valve/unit problem and don't understand what's going on here.

FYI, I replaced the OVP at the same time I replaced the ECU. I've also replaced the idle control valve and idle control unit.

Any assistance here will be GREATLY appreciated.

__________________
Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000

[QUOTE]Originally posted by sublettd
[


2 -- Just replaced the Environmental Control Unit (000 822 1203)]



What the heck is an environmental control unit? I hope I ain't got one!! If so it too will fail or already has!

<
the whozeejigger 4003 is a common generic diode.

Common Diodes allow current to conduct in one direction and restrict flow in the other. That is why the polarity test -- low resistance in one direction, high in the other.
So if one were to put a diode in a dc series circuit, it would conduct in one direction, but if you flipped the leads over, it would not.
Kind of a one way valve in the plumbing world..[ which is a good way to visualize / compare electon flow]

They are commonly used to rectify a/c current into DC current, but have many other applications...

Cap,

The ECU is a "little black box" that mounts on the firewall behind the glove box on my SL. It has two plugs which go into it, one with 12 wires and the other with 4. It controls--so far as I know--the heating and A/C for the vehicle. May control more than that as well (witness my RPM surge when stopped).

My guess is you have one and I hope you never have to replace it. FYI, when I first started looking into the problem the MB dealership here showed a price of about $254.00 for the unit. When I went back to order it, the price had jumped dramatically. That's when I started looking in other places. My guess is their bench stock on the unit(s) is getting seriously low. Same thing happened to the 000 822 0403. It's now over $1,100 at the dealership.

Going to drive a Mercedes you're going to pay the price, or so it seems!

__________________
Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000

It's like a vendor told me once," how can you look someone in the the eye and tell them a distributor cap costs $135?" And you know something else?? In my county a tag and insurance for a fourteen year old Mercedes is as expensive as a three year old anything else!!!

Wait a minute, It seems I'm losing my bias! I'm a BENZ owner!! I should pay more!!

Sublettd, Check your tyres and inflation pressures. Your pulling problem may be related to different rolling radius of the wheels on each side and the ABS is interpreting this as wheels rotating at different speeds. It thinks this is wrong so shuts itself down.

I have heard of this happening before when someone replaced a tyre on one side only and left the part worn one on the other.

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

Cap,

The parts guy told me, "I don't price 'em; I just sell 'em". I'm sure he catches more grief than the average person and is glad to go home at the end of the day. Paid $8k for the car and have put another $5k into it. Just about to get it where it ought to be.

Mick,

Usually keep the tire pressure where it's supposed to be, but will check as you suggest. That's an easy enough fix, but I doubt that's it. Why? Because it doesn't cost enough! I'm starting to get as calloused as Cap. ha!

__________________
Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000

Looking for shorts in the wiring, I just noticed that there is no connector or wire in the OVP plug for terminal 30a. Is that correct?? The terminal connects to nothing??