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126 820 20 26 Green Relay driving me insane

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2K views 7 replies 7 participants last post by  sushipaste  
#1 ·
So in my fuse box position B there is a green 5 pin relay, standard looking enough with the usually 85/86 and then30/87/87A layout. Everywhere I look there is a diagram showing 85 and 86 is power and ground and the 30 to 87 circuit is normally closed and when the control circuit 85/86 is energized with 12v it switch's power to 87/87a.

So I do a test and I get all circuits open except 87/30, which is closed. If I power 85/86 in any polarity, NOTHING HAPPENS.

So I thought, bad relay right? Went on ebay and bought a set of two relays (used) and they BOTH TEST THE EXACT SAME as my old relay.

What in the hell is going on here, did I pick up two more bad relays? Something tells me this relay is special, I even found another thread on it on another site complaining about the same problem. No resolution in that thread.

Man, this is driving me crazy. I even pulled the old relay part to look inside, looks brand new, no corrosion. The armature flips over manually just find. The flipping the armature over manually seems to activate 87/87a fine.

It's rated at 12v but does it need some kind of high amperage? This circuit controls the air pump clutch, the A/C compressor, and the "kickdown solenoid valve / au transmisson". Have had problems with the A/C compressor- throws a clutch obstructed code, and I do have a transmission problem but I could not verify if it affected kickdown.

So confusing.

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#2 ·
your relay coil winding is blown (open circuit). The idea of pins 87 and 87A is that it switches between them. When the relay is not energized 30 and 87 are connected. When the relay is energized 30 and 87A are connected. 85 and 86 are the coil winding connections. You have 3 dud relays. Have a look on the web as to how a relay works. It really is very simple.
Peter.
 
#6 ·
My understanding is 30 to 87 is normally open, 30 to 87a is normally closed. And it reverses when power is applied to 85/86?

Not sure if this helps. I did find with my 96 that Mercedes changed the way they controlled the AC system sometime in or before 1996. As compared to the R107 the 1996 R129 switches the AC grounds to turn things on and off instead of switching on the hot side. This lead to a whole other host of issues that had to be dealt with. One of the things I found was this:

DIN standard is to applying +12 to terminal 86 and ground to terminal 85.
Mercedes Benz Is using the K9 and K10 relays in reverse of this DIN standard.
As far as whether you apply power to terminal 85 or 86 should only make a difference of you have noise reduction diodes in the relay.

It looked to me like the R129 used a new AC controller that switched ground and adapted it with older circuitry that was set up to switch on the hot side. I know this because my car is a hybrid. Half 107 half 129. It uses the 96 129 relay pack to control the AC with 107 controls. The 107 was fully DIN compliant the 129 was not. I couldn't figure out why my aux fan was turning backwards until I looked at the wire diagrams and harnesses and found that on the R129 there is a little short pigtail of wire going to the aux fan that just reverses the polarity.

Just some things to think about.
 
#7 ·
roncallo said:
... The 107 was fully DIN compliant the 129 was not. I couldn't figure out why my aux fan was turning backwards until I looked at the wire diagrams and harnesses and found that on the R129 there is a little short pigtail of wire going to the aux fan that just reverses the polarity.

Just some things to think about.
Click to expand...
@sushipaste , maybe this might explain why your blower controls were working in reverse a while back?

Good info Ron (y)
 
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