Auxillary Fan On All The Time - Mercedes-Benz Forum
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#1 (permalink) Old 06-01-2010, 11:59 AM
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Date registered: Apr 2005
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Auxillary Fan On All The Time

Hello,
The auxiliary fan on my 1987 560 SL is on ALL THE TIME whether the AC is on or the AC is off. I can stop the fan two ways:

1. Pull out the relay behind the glove compartment.
Replacing the pulled-out relay with the relay next to it causes the fan to go back on. This swap/check makes me conclude that the relay is OK.

2. Pull out the two-prong connector to the Engine Temperature Sensor.
I do not have a replacement sensor to do a swap test. (Pulling out the one-prong connector has no effect.)

How do I diagnose/confirm that the Engine Temperature Sensor is bad and needs to be replaced?

Thanks a lot in advance.

Richard
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#2 (permalink) Old 06-01-2010, 05:15 PM
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The fact that it stopped when you pull out the two-prong connector to the engine temperature sensor should tell you that is the bad one. what it does is go to ground when it reaches the temp that it is progrm to do. Now in this case it is grounded all the time which mean it is bad. If you place both end of the two-prong connector to ground on the motor the fan will start. Leave the two prong connector off and pull off the one prong connector and put a wire in it and the other end to ground and the fan will come on. That will tell you that the one prong connector is fine. Good luck Jerry

Jerry Cox
1985 380SL 66,000 Miles
Ocala FL
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#3 (permalink) Old 06-02-2010, 08:47 AM
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Originally Posted by Ocala450SL View Post
The fact that it stopped when you pull out the two-prong connector to the engine temperature sensor should tell you that is the bad one. what it does is go to ground when it reaches the temp that it is program to do. Now in this case it is grounded all the time which mean it is bad. If you place both end of the two-prong connector to ground on the motor the fan will start. Leave the two prong connector off and pull off the one prong connector and put a wire in it and the other end to ground and the fan will come on. That will tell you that the one prong connector is fine. Good luck Jerry
Hello Jerry,
Thank you for your input. I grounded both leads of the two prong wiring and the aux fan motor started as you expected. Grounding the lead of the one prong wiring did not cause the fan to start. Does this outcome indicate that there may be other problems elsewhere? If I go ahead and replace the sensor will the aux fan function as intended, even if the fan would not start with the one prong lead grounding test?
Thanks!
Richard
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#4 (permalink) Old 06-02-2010, 12:06 PM
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I have the exact opposite problem. Mine will never run no matter how hot my car gets. My fan motor runs when the leads are grounded. I have a new relay but have not located where it goes. Does anyone have a photo showing the location of the relay?
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#5 (permalink) Old 06-02-2010, 12:33 PM
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Here. You need to remove the glovebox liner.
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#6 (permalink) Old 06-02-2010, 12:37 PM
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Originally Posted by Chemtj View Post
I have the exact opposite problem. Mine will never run no matter how hot my car gets. My fan motor runs when the leads are grounded. I have a new relay but have not located where it goes. Does anyone have a photo showing the location of the relay?
In my 1987 560 SL, the aux fan relay is not to hard to locate. You need to remove the glove box housing (4 pressure pins) and the bottom panel below the glove box (2 bolts and 2 screws). You will see some relays looking into the glove box. The aux fan relay is the one in the right hand side corner. Replace it with the one you just bought and see if the fan turns on. You may also check the old relay to see if it is salvageable (clean up corrosion, etc.)
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#7 (permalink) Old 06-02-2010, 01:13 PM
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Thanks Rowdie and Coupe. Looks like 2 relays related to the fan I need to check out.
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#8 (permalink) Old 06-02-2010, 01:31 PM
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Richard, if the one prong did not make the fan come on then you have a problem either with the wire going to the relay or the relay. Sence it works with the two wire I would think that the relay is working from there. I think that you have to check the relay and the wire coming from the one prong sensor to the relay. Grounding either one of those sensor should make the fan come on. If the wire check out good then you will need a new relay. Now just to make sure we are all on the same page: the one prong sensor is on the intake manifold and the two prong sensor is on the drier for the AC. The one prong is program to come on at a set temp and the two prong is set to come on with the pressior on the high side get to a preset pressior. I looked at the wiring diagram and both of the leads go to the same pin on the relay, which tell me that you have a open wire from the one prong sensor. Good luck Jerry

Jerry Cox
1985 380SL 66,000 Miles
Ocala FL

Last edited by Ocala450SL; 06-02-2010 at 01:36 PM.
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#9 (permalink) Old 06-02-2010, 02:13 PM
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Originally Posted by Ocala450SL View Post
Richard, if the one prong did not make the fan come on then you have a problem either with the wire going to the relay or the relay. Sence it works with the two wire I would think that the relay is working from there. I think that you have to check the relay and the wire coming from the one prong sensor to the relay. Grounding either one of those sensor should make the fan come on. If the wire check out good then you will need a new relay. Now just to make sure we are all on the same page: the one prong sensor is on the intake manifold and the two prong sensor is on the drier for the AC. The one prong is program to come on at a set temp and the two prong is set to come on with the pressior on the high side get to a preset pressior. I looked at the wiring diagram and both of the leads go to the same pin on the relay, which tell me that you have a open wire from the one prong sensor. Good luck Jerry
Hi Jerry,
Thank you for the response. The unit I am referring to is a "Behr Engine Temperature Sensor" made of a brass base with a green plastic top. One side has two prongs and the other side has one prong. It is mounted on the engine block coolant port that connects to the top of the radiator by way of a rubber hose. I'm not with you on the intake manifold or the drier. See link to part below:

PartsTrain.com - Hard to Find Auto Parts and Truck Parts - Engine Temperature Sensor For 1987 Mercedes Benz 560SL Base 8 Cyl 5.6L

Thanks.
Richard
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#10 (permalink) Old 06-02-2010, 02:30 PM
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I certainly would not buy it there. Shop around.
Your Parts Search Returned 0 Part(s), Sensors and Switches&gid=2188@Coolant Temperature Sensor

enter part number 0065454524 in the search by part number
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