ac fans not engaging - PeachParts Mercedes-Benz Forum

tower · #1 05-30-2003, 08:41 PM

I am trying to remedy a problem with the a/c on my 95E320. I am to understand that the two fans should go on when the a/c is on and shorting the pressure switch. This is not happening.

I cannot seem to establish a ground at the aux relay. If I manually close the relay, the fans do turn on. I have checked for voltage from the a/c circuit (pin #86 on the relay) and it is there (12v). But ground is open. Even with S32/1 shorted out of the circuit. In fact, I get an open circuit between pin #85 of the relay and either pin of S32/1.

Am I infact checking S32/1. Is it the pressure switch adjacent a/c compressor switch that has two blue with red strip wires coming from it. The reason for the question....the schematic says that the wire between the relay and the pressure switch is a brown with grey strip. Mine has a brown with grey strip exiting the relay, but going to the pressure switch it is a brown with blue strip. Very confusing! Do I have the correct part? If not, where the hell is the d..m thing?

Arthur Dalton · #2 05-30-2003, 11:43 PM

s32/1 is on the drier with a short pigtail set of leads ..
The top switch is not the one you want for a/c aux fan, it is the lower one..
If you have manually closed the relay and have fan, make sure you have the correct relay... there are two [one for high fan-Engine temp sensor activated--- and one for low fan- a/c , pressure activated].
You are looking for continuity from pin 85 to s32, but make sure you are on the correct relay...

Slot B is K10 -low fan pre resistor relay [ a/c]

Slot C is K9- high fan [ engine temp ECT sensed]

These are interchangeable, so you may be on the wrong relay when looking for ground from 85 to s32....
As you already know, 85/86 are relay primary coil circuit....

tower · #3 05-31-2003, 08:26 AM

Thanks for your reply arthur.

You have confirmed my observations. If both pressure switches are on the drier, then I am definitely missing continuity between pin 85 and S32/1. I can identify S31/1 from S32/1 because pulling a lead from S32/1 with the air operating immediately shuts down the compressor. So my problem is a lack of continuity between the relay and S32/1. The only missing piece of information is where or why the leads from the relay (br/gy wire) changes to a br/bl wire. Can anyone say previous butchery? The harness in question does not match the other harnesses surrounding it as it appears to be taped with a cloth based tape, whereas the balance of the harnesses appear to have a pvc tape covering. Cut and splice time.

Was just wondering if someone out there with the same vehicle can confirm whether this is a non stock environment..ie does the wiring harness leading to the fan pressure switch exit at the pressure switch with brown/blue trace wires or brown/grey?

Regards

Tower

Arthur Dalton · #4 05-31-2003, 10:19 AM

<<>>

S32 will not stop the comp clutch by pulling a lead ... So, I think you are on s31.
S32 has brn/gy and brn/bk leads

...or you have a typo???

tower · #5 05-31-2003, 04:26 PM

New fingers are on order. Typo it is.

Rephrase. Pull a wire on S31/1, compressor quits.

I just went out and measured resistance and voltage on the k9/1 s32/1 leg and got a whopping 148ohms between ends. There is something going on here.

Another request. Could someone measure the resistance between k9/1 pin #85 and the br/gy lead to S32/1. I think I have it isolated but I just want to make sure.

Thanks in advance.

Tower

Arthur Dalton · #6 05-31-2003, 07:09 PM

You should have trace R on that leg... that is direct wiring [ according to schematic]

Ground that circuit at relay and see if coil pulls in

tower · #7 05-31-2003, 11:20 PM

Just prior to you suggesting this, I ground out the leg of the relay (actually penetrated the lead wire about 6" from the relay) and as expected, the fans came on. Ground out the br/bl lead at the pressure switch, no fans. When I measure the resistance between the two points (relay and pressure switch removed from the circuit) I get 148 ohms.
So...if I'm getting some resistance, this must be the two ends of the wire even though the colours are different. That I must confess still bothers me. As the schematic says there is no other junction between these two components, I believe I'm going to patch in a new wire and hope for the best.

By the way Arthur, I am using the green 30amp relay.

Regards and thanks to all.

Tower

Arthur Dalton · #8 06-01-2003, 08:48 AM

I think you are reading the wrong relay
The 30 amp relay is a/c low fan..
High is the 15 [ which Benz has allowed upping to 25]

See what relays are in which sockets ["B" & "C"]
I have seen guys change them to the wrong slots..

On your car Low fan is in slot B. high fan is in C

*edited for typo

tower · #9 06-02-2003, 07:39 AM

Arthur
Again thanks for your reply, but the reality is I am addressing the a/c - fan area. The vehicle appears to be running too warm at low rpms with the air on. The wire that I am confused on is the ground leg of the relay that connects to the pressure sensor of the a/c system. My schematics show it as a direct lead with no branches.

I will attempt to clarify. Yesterday I retested the lead (removed the relay, disconnected the pressure switch and checked continuity between the ends) with an ohm meter and got different results. It almost appears to have a capacitor between the circuit. I get an initial surge of current (resistance peaks low at 200+ ohms) and then goes to open in less than a second. What is still bothering me is that the wire colours do not match the schematic. The ground leg of the green relay uses a brown/grey leg, but the terminating end at the pressure switch uses a brown/blue lead. The lead from the pressure switch is brown/grey (connected to the br/bl stated earlier). I'm wondering if there is another branch from this wire to another circuit, but even that doesn't make sense as the wire is isolated at the two ends I'm testing and therefore there is no route for the current I'm applying with the voltmeter to "leak out".

Its getting more complicated than it should for just a single wire lead!

Again thanks for all your help. You have been most helpful in the past and now.

Tower

PS. I re-read your message and glimmered that both relays use a br-gy ground lead from the relay and maybe the relays are reversed....dah! Re-investigating now. Thanks

tower · #10 06-02-2003, 08:18 AM

Arthur

If you aren't a benz genius, you should be. You were exactly correct. Relays were reversed, 15 amp fuse blown. Trace on relay ground to pressure switch 0 ohms. Short the pressure switch fans turn on. Fan-Da*n-tastic!

Thank you so much for the insight.

Tower

Arthur Dalton · #11 06-02-2003, 09:57 AM

Cool..
You can safely replace the 15A with a 25.......[Benz recommendation]