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Why not just bypass the ac pressure switch at the dryer to test the auxiliary fan motor and at the same time you’ll know if the switch is faulty.
Because what I suggested is easier than having to wrestle with the drier pressure switch. Most times thing are dirty or the connectors are stuck together, and it’s a bitch to separate them.
The plug slides off the tstat sender right away, and all you need is a metal object that fits inside the plug. Presto, you can test the aux fan.
That leaves the pressure switch out of the equation no? If a blade fuse is used to jump the two wires going to P/Switch that also checks the circuit integrity from there.
Also do what you suggest and the whole circuit gets tested and there are no unknowns.
 

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Discussion Starter #22
So now tested.

Direct 12V from some coupling into fan-coupling, fan running like a dream. Tried el-spray and clean all couplings, all connections etc.

Electric fan never start AUTOMATICALLY. Never. Not at 100C , 110C 118C. Not when AC on. Not when air cirulation in car is on.

Checked and cleand alle fuses. OK
Checkes and cleaned all rele. Looks fine. (Not tested functionality though)

So the plug in the picture ( plug female from fuse box name 9622/1 ) which leads down to the fan plug female NEVER have 12V. Measeure constant with fluke during 1hr testing.

so here is what i think.
1) Caput rele. But not sure which.
2) Some sensor not gove the trigger point for this 12V to start at 105C or when AC is running.
3) other?

appriciated. See pics as well
 

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so here is what i think.
Some sensor not gove the trigger point for this 12V to start at 105C or when AC is running.
other?
/QUOTE]
I had a 500sel back many years ago on that engine if you disconnected the two wire coolant temperature sensor on the thermostat housing the auxiliary fan would stay on, that’s how you knew the temp. sensor was bad, yours is not coming on from high engine temp. or high ac system pressure. I think you should locate and swap the auxiliary fan relay with one of the others in the fuse box. Some of them are the same.
Check for 12v at the ac pressure switch. Assuming you have power to the pressure sensor, I think if you disconnect the two wires at the red pressure switch that is on the receiver dryer, and bridge them you may find the fan comes on. If it does come on you will know the that switch/sensor is faulty. There is also a resistor in the circuit somewhere under the master cylinder for that fan.
You need member MLB560SEC for a wiring diagram for that circuit.
 

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Discussion Starter #24
so here is what i think.
Some sensor not gove the trigger point for this 12V to start at 105C or when AC is running.
other?
/QUOTE]

Assuming you have power to the pressure sensor, I think if you disconnect the two wires at the red pressure switch/sensor that is on the receiver dryer, and bridge them you may find the fan comes on. If it does come on you will know the that switch/sensor is faulty. There is also a resistor in the circuit somewhere under the master cylinder for that fan.
You need member MLB560SEC for a wiring diagram for that circuit.

1) The only thing i know i that fan is running when it is energized with 12V. Fan has no noice or any other issues i see, feel og hear. Period.
2) So now we can assume fan is OK so far.
3) So why do i not have the 12Voltage at any stage presented to the fan wire. If i had a diagram i would read it (el engineer) but right now i dont. If anyone can provice appriciated a lot.
4) gogi, thanks for reply. But i dont know location of what you are talking of. Which pressure sensor and what other sensor ? And show location of resistor and i will check it quickly. Please, send pics or dwg and i will understand asap. Thanks again.

HIGHLY APPRICIATED
Best
 

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Discussion Starter #26
Here is a pic. I believe it’s on the left side under the brake master cylinder.

so here is what i think.
Some sensor not gove the trigger point for this 12V to start at 105C or when AC is running.
other?
/QUOTE]


AGREE - A SENSOR ISSUE, BUT WHICH? HELP !

Thaks gogi, and where are you other good guys, i dont wanna buy a bunch of sensor incl work for nothing... lol

Where is this one you reffere to located mate ?

Best
 

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Discussion Starter #27
OTHER GOOD TIPS or other sensor or problems to adress from experts here? , exept gogi sensor (thanks gogi)

Best
 

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Wire the fan to run then repeat your overheat test. I think you'll find it still overheats. If not, it should actually be pretty easy to fix the electric fan. Unlikely all the sensors are bad, so I'd first look at the power side of the circuit.
 

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That leaves the pressure switch out of the equation no? If a blade fuse is used to jump the two wires going to P/Switch that also checks the circuit integrity from there.
Also do what you suggest and the whole circuit gets tested and there are no unknowns.
Geez. What I suggested is simply a way to test if the fan works, that takes a few seconds, as the OP was asking.
 

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One mod I’ve done in the past on a car that was running hot, is I’ve replaced the 110*C sender with a temp gauge sender. I also tapped into the dryer pressure switch and hooked it to this sender.
What this does, is the fan comes on low speed at about 60-is Celsius, and stays running wether the AC is on or not.
One can of course just ground the wire on the 110*C sender, but that would make the fan go full speed as soon as the ignition is on. It’s noisy, and consumes a lot of power.
 

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OK, the only point in fooling around with this electric fan is to see if it matters. Can we assume that you wired the fan to be on all the time, found a nice hot day and let it idle for a while and it did not overheat? Did it?
 

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Bottom line is, that w the AC on and the electric fan inop, the car will overheat when not moving even at idle on a hot day.
Agreed! The water sensor on top of the block, (There are two, I forget which one controls the fan, but I can look if you like, the other controls the A/C cutoff) needs to be working. I got a lower temp one for my coupe and it works great when your system is working properly.

Another thing you need to check for is proper flow of coolant through the engine. The water passages can get blocked over the years if improper coolant types are used, muck like my arteries, and need to be cleaned out with a citric acid solution. Available from MB. Be careful how you use it and follow the direction to the letter or your freeze plus will melt out!

Ask me how I know!
 

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To the OP: Do you have the service and electrical manuals for the 126?

It is CRITICAL that you have these if you are going to do any serious troubleshooting.

The ETM (Electrical Troubleshooting Manual) is THE single most critical manual you need to have. Because every system is basically electrical in nature and will be covered in that manual. You need to have it and use it, and you need to really learn how it's written so you understand the electrical system of your car. I have them all and can send them to you and I can teach you how to read them, but there is a primer in the front of the ETM that teaches you how the manual is written, and how the wired are coded and colored and located, along with all of the components in the car.

So if you are having an issue with the cooling system, you go to that section of the manual for your year and model and there you can see how the whole system is laid out and wired.

Going to the overheating issue at idle, the electric fan(s) MUST be working in all but the coldest ambient temps. As I said before, there are two temp sensors on the of the block in front.

Click this link: http://http://www.michaelclare.com/Mercedes/89-126%20Electrical%20Manual.pdf

And you will get the ETM and you can download it for future reference. PM me for all of the others.

I'm assuming it's the '88? I uploaded the page in question.

Here is an over view of the Aux Fan System: The Temp Switch: (S25/5 a 105/115 C temp sensor) Has the temp sensor attached to it. When that temp is reached it closes and sends power to relay F1. That closes and triggers the fan. The other (S32, a pressure switch) is triggered at 20 bar. When it closes, it too applies power to the F1 relay forcing it to close and apply power to the fan.

You can have a brand new fan clutch and at idle it will not be enough volume of air to cool the engine alone. It HAS TO HAVE the electric fan. Trust me on this. I completely rebuilt my SEC as is well documented here and that relay was bad and the car overheated. Once the fans were sorted, it kept 90C consistently.

If you have any questions, do not hesitate to reach out to me.
 

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Bottom line is, that w the AC on and the electric fan inop, the car will overheat when not moving even at idle on a hot day.
Yours might. Mine doesn't. But, then, I have a new radiator. Electric fan doesn't even come on unless I roll up to a traffic jam after highway travel at high speed. I've never even seen 100C on the gauge.
 

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Disagree.
The reason why the aux fan comes on at idle is not necessarily to cool the engine down, but mostly because the AC pressure switch has turned it on. This triggers the low speed of the fan. This in turn lowers the pressure in the AC system, and makes for cooler inside temps.
If your fan doesn’t come on w the AC on, your refrigerant may be low, or just the outside ambient temps low enough to maintain low enough pressure in the AC system.
Now, if the aux fan does not come on at idle on a hot day with the AC on, it will bring up the pressure in the AC system like crazy, reducing its efficiency, and also condenser will transfer so much heat to overheat the engine.

When the engine overheats, (hotter than 110*C), the aux fan should come on on high speed, wether the AC is on or not. Completely independent of the AC pressure switch on the dryer.
 

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The car should not overheat even with the electric fan inop, at least at reasonable ambient temps. Unless I misunderstand the OP's info, he's in Norway, not Saudi Arabia. The electric fan is primarily to make the ac more efficient at low road speeds. The mechanical fan with locked clutch will move more air than the electric fan can ever touch, in fact, I think the reason for the "high" setting on the fan is to prevent it from interfering with the mechanical fan.

Wire the fan on all the time and see how the car does then. My money says it will still overheat. (Hope I'm wrong...)
 
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