(W124) Aux fans: blown fuse in K9/1 relay, fans won't come on? Trouble diagnosing.

[krshultz]

Like probably half the world's W124 owners come summertime, I'm trying to sort out some warm running condition problems. I went ahead and ordered a fan clutch, which isn't here yet. Along the way, I noticed that the aux fans are not coming on at low speed. While troubleshooting, I noticed that the fuse on the K9/1 relay, which is the blue relay nearest the firewall, had blown.

This is a 1994 E320 sedan, by the way. I'm sorry for having not included that.

Is there a typical situation that causes this fuse to blow? The fan resistor down by the headlight seems to be okay. I tested it with 12V, and it does pass current successfully.

The various forum posts and FSM pages, which I've spent the last three days reading, state that I need to jumper the pressure switch in the AC system. The problem I'm having is that the pictures I see show what I assume to be an early car. The shop manual diagram doesn't look like what I have. I'll post a picture in a second - would anyone mind helping me figure out how to test this pressure switch, and what my next steps are?

[krshultz]

In this picture, the front of the car is to the right:

IMG_3206 by krshultz, on Flickr

My sense is that the red capped switch is the pressure switch. It has two wires coming straight out of it, a brown/white and a brown/black. These wires connect to the rest of the car at the two, large, white plastic connectors. Here is what I've observed:

1. When I apply my test light to the brown/white wire coming off of the red-capped switch, the test light glows, and the blue K9/1 relay engages. The aux fans do NOT come on.

2. Removing the test light causes the relay to disengage.

3. Bridging the brown/black and brown/white wires does nothing, although the test light still comes on and the relay clicks.

4. I've tried two different blue K9/1 relays, the behavior is the same for both.

5. The fuse in the K9/1 relay has remained intact throughout this testing.

6. Disconnecting the coolant temp sensor on the thermostat housing causes the aux fans to come on at full speed.

Disconnecting the other switch (two black connectors with blue/red wiring) appears to cause the compressor to engage or disengage.

Am I on the right track here trying to troubleshoot the aux fans? I feel a bit out of my element.

[nulu]

If you bridge the large brown with brown /white the fans should come on

[krshultz]

(Edit: I went out and replicated the behavior. Found an error in my post. I have corrected it now.)

It feels like I'm moving backwards. Here's what I know now:

Originally Posted by nulu

If you bridge the large brown with brown /white the fans should come on

OK, here is what I've tried.

Key on, engine off:
Test light to (brown/white) / (brown blue) connection lights up the test light, and the relay clicks. Connecting to both wires at once makes the light go off, and the relay does not click.
Volt meter, with black lead grounded to the shock tower, bridging the two connections reads 0V. The relay clicks.
Volt meter, with black lead grounded to the shock tower, but the read lead touching ONLY the brown/white wire: 12V. Relay does not click.

Engine running:
Test light no longer lights up. No relay click.
Volt meter no longer reads anything. No relay click.

Turn engine off, key on, and repeat the "engine off" sequence:
Test light no longer lights up. No relay click.
Volt meter no longer reads anything. No relay click.
Fuse in the relay is fine.
Reseated the relay, no change.

It's probably pretty clear by now that I'm not an expert in automotive electrics. Still, though, it seems impossible that starting the car would make all that stuff change.

Assuming the pressure switch is bad, doesn't bridging its two wires mimic the behavior of the switch activating? Once the wires are bridged, closed is closed, regardless of what the actual switch is doing.

Do I have any of this right?

I've learned a whole lot about this car in the 1.5 years I've had it. I've replaced wiring, fuel filters, trans fluid, the accessory belt, lots of stuff. But this problem completely escapes me. These fans used to work, as recently as last summer.

[krshultz]

Minor update: after having been in here typing for a few minutes, I went back out to re-run the tests. Everything was back to where it was before I started the engine.

[compress ignite]

Please see reply #3 for help.

Do an "Advanced Search" on this site using Arthur Dalton's name:

He's got so many posts on this system and the "Fixes",might as well have
designed and built it!

One exemplar is:

PeachParts Mercedes ShopForum - Search Results

[compress ignite]

PeachParts Mercedes ShopForum - Search Results

[krshultz]

I made some progress tonight, but still haven't got everything fixed up.

First of all, I think the two wires at the pressure switch may have been crossed. I switched them, and now, when I bridge between the two of them, the relay clicks.

I also applied battery power directly to both ends of the fan resistor. When applied to the side nearest the firewall, the fans come on slow. When applied at the end closest to the front of the car, the fans came on high.

The cables at either end of the resistor check out ok, best i can tell. I've tested several inches up them for continuity, having stripped aside just enough insulation to see copper in there.

My only guess at this point is that somewhere, there is a broken connection between the k9 low speed fan relay, and the fan resistor. My understanding is that the relay sends power to to that resistor, and if thats true, the power isnt making the trip. I took the headlight out of the car to start chasing the problem, but ran out of time.

[pmckechnie]

The 124 was made from 1986 to 1995. I have a 124, so do a lot of other people. However the electrical systems change over the years. I have a 91. If yours is a 91 I may be able to help you, other wise, I would be afraid I would give you bad information if your car is some other year.

Paul

[krshultz]

Is a little embarrassing to see that I've left out the model and year:

1994 E320 sedan.

Sorry gang, I know better than that. I'm going to add it to my forum signature so I don't forget it in the future.

[krshultz]

The wiring diagram I found here:

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-606.pdf

Leads me to believe that there is a single wire going from the K9 relay, to the R15 resistor. My first thought is to pull the relay assembly out of the box, and look for connection problems up there. That seems much less invasive and time consuming the tracing the wire all the way from R15 to K9, because that wire is in a fabric wrapped bundle. The parts of that wire that I can see, near R15, look and test find.

I should mention that this project is pretty much the first time I've tried to read a wiring diagram, so I'm mo than happy to be told that I'm doing it wrong.

What's a little frustrating is that I was at a junkyard just yesterday going through a 93 300E with an M104 engine. I didn't think at the time to pull out the relay box and and maybe that entire bundle of wires. It started to storm, and I went home.

[pmckechnie]

The Post that was here was completely off base. I misread the diagrams in the previous post which is for a different car I think.

Paul

[krshultz]

With a huge, impossible-to-relate-on-the-internet Thanks to Arthur Dalton for walking me through some diagnosis, I've got the fans working.

The diagnosis?

Corrosion on some connection, somewhere.

Essentially, he had me perform a series of tests, things like jumpering relay pins in the relay box, or running a cable from battery POS to a relay socket. I had the fan resistor out, checked its connections, checked the wiring to it on both ends. We worked our way towards establishing that the wiring between the relay and the fan was okay, and it was. Everything kept passing tests.

Then I popped the relay back in, and everything was fine.

Part of me wishes we'd discovered some obvious thing - a broken wire, or maybe a freak occurrence of having TWO bad relays in hand. But we think that all the testing and taking apart must have cleaned up some connections.

The only collateral damage came while putting the driver side headlight back in. There is a plastic ball and socket inside of the headlight, which is part of the aiming mechanism. The socket part broke. So, in true SCCA racer fashion, I conjured up a bracket made of a couple of zip ties, and got it cinched down and adjusted. But that headlight probably needs to be replaced.

A great learning experience, to be sure. I'd actually gone through some of the grounds on this car as I thought to, while chasing a cruise control glitch. Maybe sitting in the Florida salt air much of its life caused more corrosion than I think. But in any case, it's all better now.

Had Arthur not helped me out, I'd have spent several hundred dollars having someone else do this. I'd even told my wife that I was giving up on it last night.

[Zulfiqar]

Arthur is a real nice guy and knows about these problems inside out.

[pbuchmueller66]

I am asking to contact with Arthur. Just wrote a new Post about likely problems.
please help