500SEC Boiling when at idling (stand-by) app 600rpm for more than 6-8min - Page 5 - Mercedes-Benz Forum
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post #41 of 92 (permalink) Old 07-10-2019, 03:40 AM Thread Starter
GT1
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Hi all,
sorry late reply. Im out sailing for 1month. Terrible net.

Lets get straight to it, i like facts and clear view.

1) This is the 1983 German 500SEC. (Not saudi arb. car)

2) Aux fan runs when energized from an external 12V source for testing purpose. Ergo, fan is OK.

3) Aux Fan never starts. Not at temp 105C (or was it 110C) trigger point.

4) Aux Fan never start when i put the AC on.

5) Where car is used its never above 30C. (Normally 20-30C only.) period.


- SO to conclude:

Either a sensor or an electrical part is not working.

Can anyone please (and no guess, answer from those good guys who KNOWS please. I Have seen you here. thanks)


- Which sensor or sensors (if two) that trigger Aux fan start at 105C + Aux fan start when AC in on ? + The location of these ones.
- Post me the electrical diagram for a merceded 1983 500SEC
- Tell me which rele/ or electrical component that i schould look closer at?


(For information cables are perfect. Fuses and all fuse contact is good under this hood..)

Thanks for answer on above.

Appriciated a lot.

Thanks
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post #42 of 92 (permalink) Old 07-10-2019, 07:18 AM
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In my opinion, as I said a month ago above, you will find, after all of this fooling around, that you need a new radiator.
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post #43 of 92 (permalink) Old 07-10-2019, 07:37 AM
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Attached is the schematic for your aux fan. It turns out that it is not as fancy as the later models and comes on only in response to refrigerant temperature. That said, the set point is pretty low, and on a hot day I would expect it to be on at idle pretty much all the time when the AC is on.
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post #44 of 92 (permalink) Old 07-11-2019, 02:33 AM Thread Starter
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Originally Posted by John350 View Post
Attached is the schematic for your aux fan. It turns out that it is not as fancy as the later models and comes on only in response to refrigerant temperature. That said, the set point is pretty low, and on a hot day I would expect it to be on at idle pretty much all the time when the AC is on.
Might be the rele then. But from another view the AC works, and blows hell of cold when at lowest .... Any idea ?

Thanks a lot
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post #45 of 92 (permalink) Old 07-11-2019, 03:20 AM
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Originally Posted by roadhouse View Post
Geez. What I suggested is simply a way to test if the fan works, that takes a few seconds, as the OP was asking.
If the electric fan doesn't come on when you shorten out the 2 connectors with a screwdriver etc, the temperature sensor is the likely culprit (see post #18)

Shortening out the A/C pressure switch should do the same, but I would check the temperature sensor first.
If neither of the 2 methods work, there is also a fusible link for the electric fans power supply if I remember correctly.....

If you have a remote infra red thermometer, its fairly easy to check the radiator for cold spots. If a new one is needed, I think Denmark's Nissens makes the radiator with the reinforced neck. On a BEHR, the little plastic pipe on the side of the neck breaks quite easy (don't ask how I know).

A new thermostat is cheap, and if the old tests ok in boiling water, you'll have a spare.
I don't think anyone here has mentioned to replace the overflow containers pressure cap.
Its an even cheaper item, an old/weak spring can reduce the function = reduce system pressure with corresponding increase in coolant temperature.

BTW, If you ever go through your whole cooling system with new water pump, hoses (with quality clamps), thermostat etc, just before installing is the perfect time for a de-oiling flush (can be done anytime), followed by a citric acid flush to remove calcification/scale. If you don't know the age of your water pump, forget the citric acid, it might finish it off.

Edit: Just read John350 post #43. Disregard the A/C pressure switch procedure.
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post #46 of 92 (permalink) Old 07-11-2019, 03:36 AM
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Might be the rele then. But from another view the AC works, and blows hell of cold when at lowest .... Any idea ?

Thanks a lot
The A/C pressure switch (should be near the dryer, if as you say the A/C works, the electric fan should automatically engage, or at least at idle.
An earlier post explains how the 2 blade connectors at the a/c pressure switch can be shortened out (directly connected) to see whether the electric fan starts.

Second electric fan test: After carefully pulling off the plug, using a screwdriver blade to connect / shortening out the 2 poles of the temperature sensor should start the electric fan as well (actually, see post #18).
Both are easy to check.

Edit: Just read John350 post #43. Disregard the A/C pressure switch procedure.
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post #47 of 92 (permalink) Old 07-12-2019, 05:34 PM Thread Starter
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Hi, im out sailing, poor net and poor time. Thanks for all those with good reply.

• shorten out the 2 connectors with a screwdriver - Please send location on picture or diagram ?

• temperature sensor - - Please send location on picture or diagram ?

• If you have a remote infra red thermometer, its fairly easy to check the radiator for cold spots – Have checked. All fine , no spots.

• A/C pressure switch - Please send location on picture or diagram ?


Thanks a lot, best
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post #48 of 92 (permalink) Old 07-21-2019, 09:15 PM Thread Starter
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Originally Posted by GT1 View Post
Hi, im out sailing, poor net and poor time. Thanks for all those with good reply.

• shorten out the 2 connectors with a screwdriver - Please send location on picture or diagram ?

• temperature sensor - - Please send location on picture or diagram ?


• If you have a remote infra red thermometer, its fairly easy to check the radiator for cold spots – Have checked. All fine , no spots.

• A/C pressure switch - Please send location on picture or diagram ?


Thanks a lot, best

BUMP, PLEASE GOOD WELL APPROVED MB specialist can anyone please take picture of the above 3 sensors for me. Appriciated a lot - just got home and will use two days for troubleshoot from tuesday. Pictures for location, please.

Much thanks
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post #49 of 92 (permalink) Old 07-22-2019, 05:52 AM
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There is only one temperature sensor (a switch, actually) associated with the fan. It is on the receiver/dryer. There should be another switch there, too, that prevents the compressor from running if the pressure in the system is not within specs. To tell which one is which, engage the compressor, then pull the connector off the switch. If the compressor stops, that is the compressor switch; the other one must be the fan. Pull that connector off the switch and jump the two contacts with a paper clip. The fan relay should click and fan should run

To find the receiver/dryer, follow the hoses from the condenser (the radiator for the AC.) One hose goes to the compressor and the other to the receiver/dryer then on to the passenger compartment. It is usually somewhere behind the left headlight. Here's a picture of one with a single switch.
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Last edited by John350; 07-22-2019 at 06:02 AM.
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post #50 of 92 (permalink) Old 07-22-2019, 05:57 AM
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An aside: this fan is to increase the efficiency for the AC, not to prevent the engine from overheating. Have you confirmed the mechanical fan clutch is working correctly? One thing that can happen is debris builds up on and between the condenser and radiator and reduces the airflow through the radiator. Without airflow, the fan clutch doesn't sense the heat from the radiator and never engages.
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