Overheating due to aux fan flaky-ness
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Junior Member
Joined: Jun 2006
Posts: 52
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From: Durham, North Carolina
1995 e320 Wagon
Overheating due to aux fan flaky-ness
My 1995 E320 is overheating because the aux fans in front of the radiator seem to be turning off when they should be staying on. Most of the time they stay off and in this 100+ degree weather in Durham, North Carolina that is making me run way too hot. I have had the issue before where the blade fuse in the relay blows and they just stop working period. This time the fuse is good and I was very surprised to see the fans both turning on while I had it in the driveway idling. Then they turned off which it was pretty clear they should not of. Is there a thermostat or switch somewhere that I can test? When the fans are running they appear to be running fine and they spin freely when not running. Is there a way I can jump to the fan(s) to make sure that they are ok and start looking at switches or relays?
Thanks very much,
Jim Lee
1995 E320 wagon
Thanks very much,
Jim Lee
1995 E320 wagon
Super Member
Joined: Oct 2008
Posts: 837
Likes: 25
From: seattle
1990 300ce supercharged and intercooled
You need a new fan clutch. The aux fans run on low speed as a response to the a/c pressures. High speed fan comes on as a last resort from engine temp at 112-115 c. Even at the hottest temps outside your fan clutch should be able to keep this from getting up there. The fan clutch is commonly overlooked on these cars and almost all of them I see out there could use a new one.
Through a new thermostat at it also while you are at it.
Through a new thermostat at it also while you are at it.
Member
Joined: Aug 2007
Posts: 166
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From: USR, NJ, United States of Americaland
1994 C124 E320 Coupe - 1975 W116 280S
My 1995 E320 is overheating because the aux fans in front of the radiator seem to be turning off when they should be staying on. Most of the time they stay off and in this 100+ degree weather in Durham, North Carolina that is making me run way too hot. I have had the issue before where the blade fuse in the relay blows and they just stop working period. This time the fuse is good and I was very surprised to see the fans both turning on while I had it in the driveway idling. Then they turned off which it was pretty clear they should not of. Is there a thermostat or switch somewhere that I can test? When the fans are running they appear to be running fine and they spin freely when not running. Is there a way I can jump to the fan(s) to make sure that they are ok and start looking at switches or relays?
Thanks very much,
Jim Lee
1995 E320 wagon
Thanks very much,
Jim Lee
1995 E320 wagon
I had this exact issue. Turned out I had to replace the entire fan assembly.
Here are my threads at Benzworld. Good info in both of them
http://www.benzworld.org/forums/w124...-fan-help.html
http://www.benzworld.org/forums/w124...erheating.html
MBWorld Fanatic!
Joined: Nov 2002
Posts: 1,028
Likes: 133
From: Massachusetts
2018 GLE350, 2014 G550, 2000 SL500, 1980 TR8, 1995 E320 Cabriolet
I had the old temp gauge hitting close to 120 as well last night. I discovered that the aux fans we not coming on and that one of them seemed to have an obstruction that was impeding its rotation. After reading the various forums, looking at the 1995 Cabriolet schematics and doing a few simple tests, I was able to quickly diagnose the issue as both the 15 Amp and 30 Amp on-relay blade styles fuses inside the relay box were blown (not sure why MB used fused relays as there is an additional corresponding 15 and 30 Amp fuse directly in-series in each circuit). After making sure that both fans could rotate freely, I replaced the blade fuses and all is well. Temp is rock solid now. But in my reading, I noted a few descriptions of the aux fan operation that were not quite correct (at least for a '95 Cabrio).
So for what it's worth, here is what I learned:
- FD
* Many auto A/C systems use a technique called superheat to increase system efficiency. What they do is reduce airflow over the condenser while the compressor is running thus causing it to "superheat". Once the pressure in the system gets high enough (indicating the the condenser is hot enough), high air flow via fans is added. The superheated refrigerant then rapidly gives up its heat, causing significant extra cooling at the evaporator end.
So for what it's worth, here is what I learned:
- The blue low speed relay has the 15 Amp fuse and is controlled by the A/C controller. Low speed is achieved via a dropping power resistor just behind the left front headlight (British cars favor a series-parallel relay design). The A/C controller measures coolant temperature to determine if the A/C loads casuing a temp rise in the engine. If the temp is high, the LOW speed aux fans are activated via this relay to help alleviate the high temp condition.
- The green high speed relay has the 30 Amp fuse and is controlled directly by the A/C "superheat" or pressure switch.* This means that ONLY refrigerant pressure, not engine temperature, can activate the HIGH speed aux fans.
- With a properly functioning fan clutch, the correct thermostat and the A/C off, the cooling system is capable of maintaining normal engine operating temperature without the use of the aux fans. They exist solely to deal with the A/C heat load.
- On a late production Cabriolet with the square wiring harness, the thermistor sensor for the fuel injection and the cooling sense for the aux fans are now located in one part with four leads (e.g., the square connector).
- I discovered a cool test to quickly determine if the fans themselves are working. With the car off, spin one fan as fast as you can (they kind of resemble breasts so men will have no trouble with this) and then look for the other fan to start turning. Now do it with the other fan. You should get the same. This works because the fans are permanent magnet motors wired in parallel so the one you are spinning is acting as a generator which feeds the other fan causing it to turn. If the fans don't behave this way, there is something wrong with the fan motors or the wiring between them. It's a very sensitive assessment of the condition of the bearings and brushes too because even small problems will prevent the slave motor from turning.
- FD
* Many auto A/C systems use a technique called superheat to increase system efficiency. What they do is reduce airflow over the condenser while the compressor is running thus causing it to "superheat". Once the pressure in the system gets high enough (indicating the the condenser is hot enough), high air flow via fans is added. The superheated refrigerant then rapidly gives up its heat, causing significant extra cooling at the evaporator end.
Last edited by Floobydust; Sep 24, 2010 at 12:17 AM. Reason: fixed typo
