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

ps2cho · #1 04-10-2012, 07:55 PM

This relay was just removed from my 260E...I would LIKE to put it on my 300TE.

300TE stock: Currently the A/C aux fan does NOT turn on at low pressures, only medium to high pressures...

260E /w this relay: Aux fans kick on medium immediately when A/C is on.
I believe this was done by dealership...there is an MB sticker on radiator support (writing rubbed off), showing R134a retrofit (although I BACK-converted it to R12)...but nonetheless, I did not put this relay in myself and this isn't something that came from the factory, so SOMEONE put it in...

Since I drive the 300TE daily, I want to get this swapped over so it will be better for cooling for both myself and the engine.

I am stuck though because the two A/C systems are different...there is an extra wire on my 260E that I cannot see on the 300TE:
NOTE: the blue wire on the 260E comes from the A/C compressor harness, but the wire does not exist on the 300TE

Photos:

Looked up the above relay, and amazon states:
BOSCH STARTER RELAY 80-92 0-332-019150

I hope the above photo's make sense...because I don't have a clue what that blue wire is...its missing from my 300TE...I thought maybe its a wire that came off the main harness that goes into the A/C compressor, but nope, both cars have 3 wires going in, so what's that wire? What could it be for?

Yak · #2 04-10-2012, 08:33 PM

It looks like they bypassed the thermo-switch on the R/D with an simple switchover relay. When it sees +12v across pins 85 and 86, it closes the connection from pin 30 to pin 87.

I assume you mean temperature and not pressure on the 300TE fan operation.

In normal operation, the temp switch for the aux fan would be open, then the temps would rise and the switch would close and allow electricity to pass through to turn the fan on. With the relay, as soon as the compressor comes on, the relay closes and turns the fan on.

If yellow went to ground, that's probably 85 or 86 on the relay.
I'll guess and suggest the black wire goes to the upstream side of the R/D and normally sees +12 when the system is on. It will probably go to the 85 or 86 (whichever the yellow isn't).
If that's correct, when black gets +12, it'll close the relay and connect across from red to blue. Red would go to the "downstream" side of the R/D and tell your fan to turn on (I'll guess pin 87). But it needs something to say, so it probably gets +12 from the blue on pin 30. The blue on pin 30 was connected to the 260E. Is that wire in the 260E constant hot connected to the battery, or switched +12v with the ignition?

ps2cho · #3 04-11-2012, 11:55 AM

So you think that wire is either constant 12v or ignition 12v then?

Is it safe just to run an extra wire to battery or ignition and see if that works? I see no way of finding out where that blue wire goes since its part of the harness and I'd have to tear the intake manifold apart to see where it went...

compu_85 · #4 04-11-2012, 12:28 PM

That odd wire probably goes up the compressor control relay. When it gets +12 (to turn the compressor on) it closes the relay and connects the two wires, turning the fan onto low speed.

-J

ps2cho · #5 04-11-2012, 12:44 PM

Since I am no electrician, if I pop open the relay and take a photo, could we reverse-engineer it and find out for sure?

compu_85 · #6 04-11-2012, 12:55 PM

Just get a multi meter or test light. You could go down to the compressor too, just find the wire which gets +12 when the compressor is running.

One thing to double check... The wires which were going to the pressure switch go to pinns 87 and 30 on the relay, 86 is hooked to ground and the "Mystery wire" goes to 85?

That setup looks like it was probably from a fog light harness or something. The fuse can be eliminated (or changed for the lowest amp one you can find).

-J

ah-kay · #7 04-11-2012, 01:47 PM

It may be difficult for you to retrofit it into the 300TE unless you know relay and auto electric well. This is what I suspect has been done on the 260E.

1) The 'odd blue' wire is tapped off the compressor harness. It is energized with 12V whenever the compressor is turned on.

2) The 'odd blue' wire is fed into the relay pin 86, 85 to chassis ground to turn on the relay to 'jump' the high pressure switch.

3) pin 30 is tied to the one side of the high pressure switch, pin 87 is the other side.

Others can chime in. Good luck.

Yak · #8 04-11-2012, 06:29 PM

Quote:

Originally Posted by ps2cho

So you think that wire is either constant 12v or ignition 12v then?

Is it safe just to run an extra wire to battery or ignition and see if that works? I see no way of finding out where that blue wire goes since its part of the harness and I'd have to tear the intake manifold apart to see where it went...

That would be my supposition, yes. The blue wire is probably either +12 from the battery, or +12 from some switched source.

It is possible that it could be run from the compressor, by I'd speculate that whoever wired it was cautious about overloading circuits and would be less likely to tap off the compressor pull-in wire.

As far as safely tracing it back, I'd put a meter on it to figure out how it's powered (steady +12, switched + 12, compressor switched +12) and then check those places. I'd look at the battery terminals, and near the fuse box and see if there's a blue wire.

Yak · #9 04-11-2012, 06:40 PM

Quote:

Originally Posted by ps2cho

Since I am no electrician, if I pop open the relay and take a photo, could we reverse-engineer it and find out for sure?

There's no real need to open it up. Does it have a small diagram on the side? Or a set of numbers, eg; 85, 86, 87a, 87 and 30? If so, it's a basic relay. I did a search and came up with a replacement that says it supersedes yours, and it shows up as a basic relay so odds are high that it's nothing complex.

Look on the bottom of the relay and match up the numbers with the colors of the wire in the pics. ah-kay has suggested one possible wiring combo and I suggested a different one.

About the only benefit I could see to ah-kay's suggested wiring would be that the aux fan would also shut off under WOT, or whenever the compressor shut off.

compu_85 · #10 04-11-2012, 07:21 PM

A relay is not going to overload the compressor circuit. The blue line can't go to constant power... The fans would run all the time.

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Yak · #11 04-11-2012, 07:46 PM

According to the schematic, they changed the aux fan wiring from 87 to 88. In '87 it looks like the aux fan was slow speed at medium pressure and high speed at temps above 105. In 88 they re-routed hi-speed control to the aux fan controller.

Post the connections on the installed-relay and it should be easier to figure out.

Yak · #12 04-11-2012, 08:08 PM

Quote:

Originally Posted by compu_85

A relay is not going to overload the compressor circuit. The blue line can't go to constant power... The fans would run all the time.

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If the trigger to close the relay is on the compressor circuit, I agree it wouldn't overload, but if the fan is on the compressor circuit it might. I can't really see a need for a separate fuse since I think everything is fused elsewhere. That was one other factor in thinking it was a separate circuit.

Having looked at the schematic, it looks like the installer is using this relay to trip the pressure relay. I didn't realize the pressure switch had a separate relay (low speed) in addition the temp relay (high speed)

If blue is off the compressor (he said compressor harness, so it might be the compressor or it might just be wrapped in the bundle) and goes to 85 or 86, and yellow (ground) goes to the other; then it'll close red and black whenever the compressor is switched on, and the fan will run at low speed.

ps2cho · #13 04-12-2012, 01:13 AM

So if that is the case between the model year changes, is there a different way that I can get the aux fans to turn on low as soon as I turn the A/C on?

compu_85 · #14 04-12-2012, 11:23 AM

You want to connect it up like this:

You can get the feed from the compressor on either at the Klima relay, or right at the compressor. The output is on pin 7 of the Kilma relay.

-J

ps2cho · #15 04-12-2012, 01:19 PM

Just thinking about it, would there be any consequences to running it on a switched +12v line and have it run all the time anyway?

I mean being in AZ, its pretty much hot all the time? Then during the winter I could simply remove that wire for the 2-3 months where it is actually cold?

That way I don't have to mess around with gutting klima wires etc. I could hook it up to the blower motor resistor on the driver strut mount since its ignition 12v...maybe throw a small toggle switch inline so no need to even disconnect it...just flick the switch and its back to stock settings for the winter?