I had my A/C worked on today. I had the switch (used the red one as suggested by GSXR) and the other part replaced that are behind the driver's side head light. He recharged the system and could not get the aux. fan to come on with the A/C on. He let the high side pressure get up to 350 and it still didn't come on. I can jump the fan and make it work, so I know it isn't the fan. What am I missing? What high side pressure activates the fan with the red switch?

I'm sure this has been discussed before, but I couldn't find my situation using the Google search.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

The red switch turns the fan on low speed at 16 bar (232 psi), and turns the fan off at 12 bar (174 psi). Pressures above 300psi should have been way more than enough to trigger the fan (that's even high enough to trigger the old green switch, which needed 290 psi). If you can short the leads of the red switch and make the fan turn on low speed, it sure sounds like a bad switch... bad news is, the system has to be discharged to replace it, then vacuum pulled, and re-charged...




Note: If you are jumping the fan on HIGH speed, using the switch on top of the engine... then check the aux fan resistor. If that's bad, the fan will work on high speed, but not low.



Best regards,

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Dave
Boise, ID
Check out my website photos, documents, and movies!

Thanks Dave. The fan would only come on high when I jumped it at the 100/110 switch. When jumped at the new A/C switch it wouldn't do anything. So I'll keep my fingers crossed that it is the resistor.

Where is the aux fan resistor located? Do I check it for power or......?

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Whew! That's good news - most likely just a bad resistor. It's located near the receiver / drier behind the headlight, below & behind the ABS pump, towards the outside of the car... a real bugger to access. The ABS pump has a black plastic cover, the resistor is pretty much directly below the "S" in "ABS". It's roughly the size of your thumb, it's (dirty) white ceramic with a black metal bracket wrapped around it that may say "Behr" on it.

With the red switch jumped, if you get +12V on one side and nothing on the other, the resistor is bad. Or measure resistance across it, should measure something quite low (say, under 10 ohms)... if it's infinite or very high, it's bad. If you jumper the resistor to bypass it, the fan will run on high when the AC pressure is above 16 bar. This may improve AC performance a bit at low vehicle speeds, the drawback is more noise, a bit more wear on the fan, and increased load on the wimpy stock alternator (especially at night with headlights on).

Can I replace it without moving the ABS unit? Any chance you have a part number handy?

Fortunately it's back to 75 for a high during the day and 45 for the lows at night. So no A/C at night or during the day for that matter.

When I get the 150A alternator I'm getting the 500E headlights so as Sixto put it (my car can serve as a backup lighthouse lamp.) Yes I remember that I'll have to give up my fogs for driving lights.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

Pretty sure it can be done by removing the driver's headlight. I wouldn't mess with the ABS unit. Looks like p/n 000-158-25-45, roughly $25 or so.



Ooooo, nice! Those are kinda spendy now though, plan on ~$750 for a set at current (lousy) exchange rates. Apply Lamin-X film immediately to protect the lenses (the early style lens to 1993 are no longer available, and the 94-95 style lense are ~$300/pair shipped across the pond). With good bulbs, the light output is amazing. Even more amazing with relays & overwatt bulbs...

Check fuses 7 and D. Spin them in their holders to ensure good contact.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Both good suggestions.
There are a couple of simple tricks for these systems.

On the fuse suggestion, a good tip, but in this case , you know it is neither fuse b/c they are common feed for both high and low fan..and you state the high fan works when you jumper the 105/115sw.

On the resistor [ R15] , it is usually the wire has burnt off the terminal.

So, here are two quicky test:

When jumping the drier pressure sw, listen real close to the relay compartment behind the fuse box. If the relay clicks w/jumper test, you know the problem is on the Load side of the circuit..meaning resistor or wires.

On the resistor, a simple test is to bring a jumper wire from the bat+ terminal over to each side of R15..the two wire side of R5 will give you high fan, the single wire side will get you low.

If YES to high , but No to low , then R15 is bad.
If yes to both, then the problem is upstream from R15.

This is always the second test, with the first being the pressure sw jumper.
And this test does not require key On , as it is simple a direct battery feed to verify both R15 and the fan motors themselves.

A word of caution on jumping the R15 to get a modified high fan for additional air-flow:
The wiring on that chassis to the low fan relay is only 1.5mm wire, so guys that do that mod always wind up with burnt wires. If one does want to do that mod, it is suggested to up the wiring on the load side of the low fan relay to 2.5mm [ from 1.5mm]
You will notice that the high fan load circuit uses a 2.5mmmm wire to carry the increased load demand of high fan.
On the dual fans models , we recommend a seperate relay tiggered by the R16 low fan feed for that mod.
And finally, many guys run a cabin sw so they can control the aux fan at will..that is done by simply bridging the a/c aux fan pressure sw.[ as in the jumper test] ..but with a toggle sw. and some leads to go into the cabin ..nice to have for towing, traffic jams, etc...this also does not change any of the factory circuits..it simply over-rides them on command, with the advantage of still using the factory circuits relay and fuses , so you do not have to contend with any of that with this mod..just a $3 toggle sw and some 2 wire zip cord [ lamp cord works fine]

Also, when one suspects the high pressure sw , instead of jumping it for low fan as the test, the correct sw test would be guages on the high side and an OHM/Continuity test at the sw leads..when your guy saw 350PSI at high side , that is the sw test he should have made.
The jumping of the sw is a test for the low fan ciruits integrity, not a sw test.

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A Dalton

Thanks Cal and Arthur.

I ruled out the fuses as the fan worked when jumping the 100/110 switch as Arthur said.

I just put my ohm meter on it and it read infinite. It is stamped .2, I'm not sure if that is the ohms, but it could be. As Dave said if it reads infinite, it is bad. So until I get a new one, I hooked the 2 wires together so the fan will run on high when the switch calls for the fan.

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

They are .2 ohm....just enough to slow the fan down...and being so little ohms is why they get real hot.

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A Dalton

So that would also explain why they burn out. How long do they typically last? I should probably just eliminate it and let the fan run on high. Of course then I'll probably have to replace the fan. I just ordered one at the dealer today, so I'll see how long it lasts.

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

See post #8.

The stock wiring is not adequate for high fan on the low fan circuit.

But it can be made to be ....

The resistors have a long life and the new ones are open coil, so they last longer than the originals.

What taxes the resistor is the fan motors start to get dirty/rustted bearings and worn brushes, thereby drawing over spec Amps..the R15 does not like that ....the other is the connections start to conduct poorly b/c of heat fatique...so, again, tha R15 gets a shortened life.

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A Dalton

Woops, my bad, I breezed over that part of your post the first time. OK, no A/C until the resistor comes in on Tuesday.

A partially related question. Does the A/C run in my car when the defrost is selected? I know on the new modern cars it does.

__________________
1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss