Relay usage, Break ground or break power

Thread Starter

MB107

Joined Jul 24, 2016
122
Hello people, Once again I could use your help. The attached diagram is for the AC compressor circuit for a 1986 Mercedes 560SL. It uses the N6 compressor control unit to turn the compressor on or off by opening the ground circuit to the compressor. In my custom application I only needed one feature of this controller and that was the S25/5 temperature switch to shut the compressor off in the event of high engine temp. I did this by using a 5 pin relay wired to be normally closed and it would break power on the positive side of the compressor instead of the negative side. It ran fine for several months but then I damaged my N22/2 AC push button controller. The N22/2 is not the mechanical switch as shown in the wire diagram but it uses the push buttons to actuate relays within it. It is a very problematic unit and it is recommended that when it is replaced the N6 compressor control be replaced with it because a compromised N6 controller has been known to damage the N22/2 push button control. So the system is not too robust to begin with. My question is, would switching on the negative side be a more robust solution. And is there any other steps I could use to improve the longevity of the N22/2 AC controller.

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Thread Starter

MB107

Joined Jul 24, 2016
122
For reference I added a picture of the N22/2 Push Button Assembly and the N6 Compressor Control Module. The second picture is of the push button assembly opened up. Note this is spare definitely not the one from the car I'm working on, This one was from a car that was under water and probably salt water.
 

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Thread Starter

MB107

Joined Jul 24, 2016
122
Does anyone have any comments on protecting this circuit. With 3 contacts down stream of the N22/2 controller. I believe there may be an opportunity to put some kind of filter between the N22/2 push button control and the B10/6 Evaporator temperature switch. I assume each time any of these contacts make or break there may be arcing going on that the N22/2 push button control is being subjected to and possibly damaging it.
 

Thread Starter

MB107

Joined Jul 24, 2016
122
I just realized I mis-spoke in the first post. The original circuit switches power, see the wire diagram. My new circuit switches ground.
 

dl324

Joined Mar 30, 2015
11,833
I just realized I mis-spoke in the first post. The original circuit switches power, see the wire diagram. My new circuit switches ground.
A disadvantage of switching ground is that the load will have voltage present even when it's off; making it a potential safety issue. When you switch power, there's no voltage to the load until the relay contact is closed.
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
If you look an any N.A. automotive service manual, you will find many electrical devices that are live and switched on the ground side.
As per the example on the OP.
Max.
 

Thread Starter

MB107

Joined Jul 24, 2016
122
If you look an any N.A. automotive service manual, you will find many electrical devices that are live and switched on the ground side.
As per the example on the OP.
Max.
Apparently at least for this application Mercedes went from switching power in the 80's to switching ground in the 90's. I'm now switching ground because I'm using a 1996 control module. It may have something to do with reducing wire count or something. The new module called the Base modal does the job of many individual modules in the past.

Not only did they do that but I also found that many of the relays in the newer 1996 car were wired to non standard convention, where they would use pin 87 for power and pin 30 for load. They actually relabeled the relays to say pins 1 to 5 but the wire diagrams still use the older 30, 85, 86, 87 convention.
 
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