Overvoltage relay. - PeachParts Mercedes-Benz Forum

msar · #1 03-10-2004, 08:35 PM

Could someone please tell me what the overvoltage relay does? I've seen it in the manual but am uncertain if the problem I'm having could be caused by a faulty OVR.
My 560sl's idle occasionaly (1x per week) goes to 1700rpm. I got lucky tonight and experienced the problem while working on the car. I put my volt meter on the pins to the idle valve without actually disconnecting the valve. The idle actually fluctuated between normal (650rpm) and high (1700). Correspondingly, the meter showed 5v @ 650 and 0 @ 1700. Interestingly, when I remove the plug from the valve, everything seems to reset and the idle goes back and remains normal until the next time. Other symptoms: this only occurs when the engine is warmed up, I've tried tapping the valve to see if it's getting stuck - no change, I pulled the control module and saw no burned transistors etc.
It seems that the way the valve works is that it closes when power is applied to it. My meter readings show that the power to the valve is fluctuating. Does this narrow the problem down to the module or OVR? Thanks for advice. Mike

txstinger · #2 03-11-2004, 06:19 AM

MSAR - based on your previous post, I do not feel your problem is the adjuster (sticking). Remember - it fails safe and so far it is performing exactly as it is told. 0 volts = high idle and 5 volts = normal idle.

This leaves the controller, sensors, and the OVP voltage faults.

The bad thing here is your problem is intermittent. This eliminates the OVP fuse – that would cause a constant high idle.

So I suggest the next test, see if you can cause the same fault by pulling and pushing on the wires that supply the controller and thumping on the OVP relay.

My guess is faulty contact inside the OVP relay that feed supply voltage to the controller at pin 2 (12 vdc) or ground connection at pin 4 (this should be ground bundle at far right under passenger dash – all brown wires).

Follow the same procedure as before except this time check the voltages at the controller. When your idle goes high, check for 12 vdc at controller (slightly pulling connecter back – exposing the pins) between pins 2+ and 4-. If you have 12 vdc here and your idle is high, chances are bad controller. If no 12 vdc here and your idle is high, chances are OVP contact or wiring.

txstinger · #3 03-11-2004, 06:43 AM

Your power supply pins on the controller are pin 9 (+) and pin 11 (-).

msar · #4 03-11-2004, 09:46 AM

Really appreciate your help. Can you tell me if the OVR is in the fuse box behind the kick panel on the passenger side? Thanks.

RickM · #5 03-11-2004, 10:19 AM

MSAR....if you have a dig camera can you take pics of components' location as you test? This is an extremely common problem and would benefit many.

Where did you find the Control Module?

ctaylor738 · #6 03-11-2004, 10:26 AM

This is the classic chicken-egg problem. Is it a bad controller or bad valve?

The way I diagnosed this on my 380 was to buy a small model railroad transformer. It cost about $30. They generally supply between 0 and 18V. I disconnected the leads to the idle air valve and connected the transformer and adjusted the voltage to get the idle at the right speed. I then measured the voltage and found that it was taking almost 9V to keep the idle at 650. So I figured the valve was bad and replaced it.

That cured most of the problems but I think that the excessive voltage damaged the controller since I have an occasional surge. I am waiting for a controller to come up on e-Bay.

BTW - the OEM VDO valves do come up on e-Bay. I paid $14.61 for mine.

txstinger · #7 03-11-2004, 04:08 PM

Quote:

BTW - the OEM VDO valves do come up on e-Bay. I paid $14.61 for mine.

Man what a steal!!!

msar · #8 03-12-2004, 06:35 PM

No luck finding the overvoltage relay. Spoke with Miller's East. After checking, they said that the MB microfish only show Calif models having the overvoltage relay. Does this make any sense?
By the way, I don't have a digital camera but I will try to explain as clearly as I can the location if I can ever find it!

msar · #9 03-12-2004, 06:53 PM

BTW My 560 is a 1986

txstinger · #10 03-12-2004, 07:55 PM

The OVP relay should be on passanger side footwell, right side (fuse pannel side) behind speaker grill somewhat.

Will be a little larger than the relays in your fuse pannel.

Should have a fuse on top side.

msar · #11 03-12-2004, 09:15 PM

I feel completely dense. I opened the fuse box door on the footwell. There are only fuses, no relays behind that panel. As I look inside the box and then slightly above the box there are 2-3 relays. Some are marked but none have a fuse on them. I'm baffled. Am I still looking in the wrong spot?
I am further confused by reading the wiring diagram on the CD for 1986 560sl's. It also shows that OVR's are only used on California cars.
If, indeed this is true, could any other sensor be sending a signal to the module that in turn would cause the module to react by sending no power (0 volts) to the valve? Do you think it's a correct assumption that none of the sensors can send a faulty signal that would result in having the module cut its output voltage to the valve to zero? Seems like those sensors look at: throttle valve, cis control, electronic ignition control, ac compressor, vehicle speed, ground (hmmm - maybe a bad ground though why is this problem so infrequent?).
So, for instance if the AC sensor detects that the compressor is on, it increases idle speed by 50rpm or so to compensate for the added load on the engine. If that sensor fails, I presume (perhaps incorrectly) that it would send a signal that either the compressor is on when it's really off or conversely, off when really on. In the case where the compressor is off but the sensor tells the module it is on the result should only be an increase of 50rpm - not 1000rpm.
I guess what I'm trying to ask, rather longwindedly, is how could any sensor send a faulty reading that the module interpret to mean that the engine needed to go to 1700rpm? Therefore, if there is no OVR, can there be any other explination other than a faulty module?

txstinger · #12 03-13-2004, 08:08 AM

Quote:

could any other sensor be sending a signal to the module that in turn would cause the module to react by sending no power (0 volts) to the valve?

NO - the inputs only cause the controller to send less or more voltage out to the adjuster.

Use the CD print to double check where the fault lies. When your idle is high, verify the voltage to adjuster, verify output from the controller (is the controller sending 0 vdc or is it sending 4-5 vdc), last check is supply voltage at the controller at the time of fault 12 vdc?

With this info you will know where to go to troubleshoot high idle.

If 0 vdc at adjuster and controller is sending 4-5 vdc, check wiring between controller and adjuster.

If 0 vdc at adjuster and controller is sending 0 vdc, check supply input to controller (12 vdc).

TRICK - remove the controller from its mounting bracket and cut any wire-ties so the controller will freely hang inside the footwell. Remove the plastic backing on the controller connecter. Place a piece of DUCK TAPE in place of the plastic backing. Mark the DUCK TAPE with the pin positions and poke holes with Volt Meter probes allowing you to test the pins without having to remove the connecter. Now, your ready to test all inputs and outputs from the controller without having to disturb the connecter.

txstinger · #13 03-13-2004, 08:14 AM

Quote:

Originally posted by ctaylor738
The way I diagnosed this on my 380 was to buy a small model railroad transformer. It cost about $30. They generally supply between 0 and 18V. I disconnected the leads to the idle air valve and connected the transformer and adjusted the voltage to get the idle at the right speed. I then measured the voltage and found that it was taking almost 9V to keep the idle at 650. So I figured the valve was bad and replaced it.

Nice thinking... I love to see DIY'er find ways to test without the hightech tools.