A little help with this A/C relay for aux fan operation? - Page 3 - PeachParts Mercedes-Benz Forum

compu_85 · #1 04-14-2012, 12:55 PM

Quote:

Originally Posted by Yak

In Arizona, I'd definitely look at an install that is ignition switched, eg, when the underhood temp is 140 F AND the ignition is on, then the Aux fan runs at least at low speed. I suspect that is easily and cheaply do-able with one of the Summit Racing kits. This type of mod should not affect any of the safety cut offs or function of the OE climate control.

This is an excellent idea... in fact to hook it up all you need to do is parallel the switch with the pressure sensor. Have something like this hooked up on my car:

I built an adapter harness which plugs between the sensor and the car harness, so no mods were needed for the car harness. If I ever decide to remove it I just unplug it.

Note the sensor with the 2 spades on the back is the too low / too high pressure cutout switch, which is wired in parallel with the compressor clutch. The red switch with the wires coming off of it with spades on the end is the 250PSI switch for the aux fan(s).

-J

ah-kay · #2 04-12-2012, 09:33 PM

The OP asked how to wire up the relay which we posted. He was not asking how to re-design the A/C system or replace the switch. Some MBZ switches at the rec/drier work on temp, some work on pressure. Some need to discharge the freon, some doesn't. I do not know what he has. He just wants a simple hack or instructions. Why make life complicated for OP?

The hack is simple and does the job. Keep It Simple Stupid.

RANDY P · #3 04-13-2012, 12:34 AM

This thread reinforces what I'm saying - read Arthur Dalton's and the last post by SteveFL:

http://www.peachparts.com/shopforum/tech-help/66625-1991-300e-c-question.html

cypress · #4 04-13-2012, 05:05 PM

The cool harness mod would be ideal for your car...just one 1.1k ohm resistor involved.

compu_85 · #5 04-13-2012, 08:15 PM

Jumping the high pressure switch does not hurt anything. The overpressure cutoff is done by the low pressure switch.

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compu_85 · #6 04-14-2012, 12:05 PM

Just connect the two wires going to the pressure sensor together... the fan will run on low all the time.

-J

ps2cho · #7 04-18-2012, 03:15 PM

Figured it out...Blue was as you guys said +12v source that went to Klima somewhere..

Ac relay working.MPG - YouTube

Now I will just bring the blue wire up into the dash somewhere discreet so not to ruin the dash or any interior by making horrible holes and voila!

Would you guys say its ok to use the blower motor resistor as my +12v source? I'd prefer not running another +12v wire across the engine.

compu_85 · #8 04-18-2012, 03:34 PM

You could use that, but it's on a really big fuse. I'd try to tie into something with a smaller fuse on it, like the radio power or dome light (which should be available at the buzzer relay)

-J

ps2cho · #9 04-18-2012, 03:36 PM

I'm not electrically savvy...what's the issue with having a big fuse? Is it in case of overload the fuse won't blow to protect the motor?

compu_85 · #10 04-18-2012, 03:52 PM

So the blower is on a 30 amp fuse. To blow the fuse you need to draw more than 30 amps. If the small wire going up to your relay shorted out it would probably melt / catch fire before it drew 30 amps of power. That's why you need to put the 12v source for your relay on a lower amp fuse, like the one for the dome light or radio, both of which are 8 amps in my car.

In your case you might just want to jumper the two wires going to the high pressure switch and have your fan run whenever the key is on. When it cools down in the fall you can simply return the wiring to stock.

-J

ah-kay · #11 04-18-2012, 05:19 PM

Quote:

Originally Posted by compu_85

In your case you might just want to jumper the two wires going to the high pressure switch and have your fan run whenever the key is on. When it cools down in the fall you can simply return the wiring to stock.

-J

x2 This is the easier way to do it.

Since you have got the relay/fan going then why not go all the way and do it properly.

1) The blue wire is best go to a switched +12V at the compressor. You only want to turn the fan on when AC is called for. It may be difficult to splice into the compressor harness but it is not that difficult if you try.

2) Alternately, the relay only draws 10s of milli-amp. So it is possible to tap it off Pin 7 of the Kilma relay instead. Remove the kilma relay, insert a small gauge stranded wire into the Pin 7 hole and then push the Kilma back in. Then connect the small wire to the blue wire. You should be all set.

ps2cho · #12 04-19-2012, 02:00 AM

Now that I am this far, I may as well give myself full control over the fan.

Can I still use the blower motor resistor as my switched +12v source since the relay already has a fuse in it?

ah-kay · #13 04-19-2012, 02:56 AM

Quote:

Originally Posted by ps2cho

Can I still use the blower motor resistor as my switched +12v source since the relay already has a fuse in it?

Do not understand what you meant by the blower motor resister as I am not familiar with W124 car. There should be no resister in the blower motor circuit as the speed is controlled by solid state electronics. Even if you could then the aux fan will run even when you only call for air, not A/C. I think we are beating a dead horse here as you know what to do to get it right.

ps2cho · #14 04-19-2012, 03:23 AM

Sorry, I mean blower strip fuse.

For those with a W124, what's the best way to route a wire from the klima relay to the driver side? Am I going to have to pull off all that engine shield foam to route the wire?

I struggle with soldering and cutting wires in tight spaces, so I really don't want to touch any of the factory wiring near the compressor for my own sake and make a real mess of it. Its nice and it works, I don't want any future issues because of my incompetence with splicing and cutting wires.

ps2cho · #15 04-19-2012, 03:42 PM

Here is my temporary solution until I can find a good way to access +12v compressor source. Runs all the time this way as soon as engine starts, but since I run the A/C all the time anyway in the summer, its no different in my case and if it cools down, all I gotta do is pop hood, flick switch and its back to stock settings.
I like the fact this solution has ZERO factory wires cut or spliced...nothing. If I take the relay out, its back to stock with no consequences

Its mid 90's today so will be interesting to see any difference in A/C performance or engine cooling at idle. In 90F in traffic A/C on, my 79C thermostat will go up to about 85C, so I wonder if this extra airflow will keep it close to T-stat opening point now?

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