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  #1  
Old 01-08-2012, 06:34 PM
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Modified Aux Fan Circuit

I have modified the Aux Fan circuit in my '73 450 SL to add a 3rd condition for cooling. In addition to the 2 design conditions that will energize the aux fan, my fan will run with the ignition turned off when the temp switch grounds at 100C.

I think this modification would suit only a very few early SL owners - later cars likely had the feature as part of the design.

If there is interest in this topic, I will explain the modification and how I became compelled to do it.

Cush

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Modified Aux Fan Circuit-10-design-circuit-mounted-option-circuit-right-3.jpg   Modified Aux Fan Circuit-9-designed-circuit-assembled-2.jpg  
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  #2  
Old 01-10-2012, 04:00 PM
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No coolant flow with the engine off, What is the reason behind doing this?
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  #3  
Old 01-11-2012, 12:58 AM
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I think it is the law of entropy in science that says matter in a high energy state will seek a low energy state. It has been a long time since I was in a science class, but I believe this law because I can see it working in my 73 450 SL.

Like many owners, my cooling system does not perform well with the added load of the AC that MB bootstrapped to satisfy the American market. My AC is marginal but at least it decreases my mileage and makes my engine run hot. After shutdown, my aux fan did not run like modern cars do. If I looked at the temperature gauge with the ignition at position I, it approached 240F (red zone). This led me to the wiring diagram for my AC system and the rest of the electrical system.

MB designed the aux fan to run under two conditions:
1. >100C coolant temp at the aux fan switch on the water neck
2. >67C refrigerant temp at the reciever drier intake.
Both conditions require ignition accessory power at terminal 15 (ingition switch postion I or II).

I did not enjoy sitting in my hot car controlling the aux fan with the ignition switch so I wanted to add a third condition:
3. >100C coolant temp at the aux fan switch with igintion power off

This is done with a decision relay and a switching relay (pictured outer right and left in thumbnails of original post). The decision relay issues the ground signal from the 100C switch to one of two places depending on the state of terminal 15 power (fuse 9 in my car). If terminal 15 is powered, the ground signal is sent to terminal 85 of relay 18 as MB designed it. If terminal 15 is unpowered, the ground signal is sent to terminal 85 of the switching relay which is powered by terminal 30 (fuse 1 in my car).

I am happy with the results and safety of this modification. Temps are controlled near 200F after shutdown and the relays are MB spare parts mounted in an MB relay rack. All switching is done on the low signal side (ground switching). Assembly required one solder reassignment at terminal 85 of relay 18, the rest was pin movement and additional wiring using MB wire, pins and color conventions.

I created color wiring diagrams of this modification and the entire electrical system of my car using MS Visio (reader is a free download). I am happy to share these rather large files via PM upon request.

Cush
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  #4  
Old 01-11-2012, 02:21 AM
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Oops, 2 mistakes in previous post.

2d MB design condition for aux fan operation is 62C vice 67C.

Option circuit is center relay in handheld series of second thumbnail picture
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  #5  
Old 01-11-2012, 07:43 AM
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Any possibility of modifying the aux fan circuit of a 380sl or 560sl so the fan turns on at a lower temperature? The Cool Harness is a simple device that will do this on a number of other models, but it won't work on a 107.

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  #6  
Old 01-12-2012, 05:40 PM
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I saw the Cool Harness on your webpage. It looks like a 4 pin adapter is used to interface with the switch. The switch I'm dealing with on the 107 is a single pole grounding switch. If we could find out what the 4 pins are talking to (wiring diagram), and the threads match on the switch, we might be able to build a solution.

Another possibility is to find out if Bosch or another vendor made aux fan switches with different temperature ratings. Volvos and Porsches also used Bosch D-Jet, maybe they have cooler trigger points.
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Old 01-12-2012, 07:06 PM
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I did some searching to see if vendors make different temperature ratings for the fan switches. If you have a 560 SL with an aux fan switch that has 3 pins to control a two speed fan, EPC says the part number is 006 545 45 24. The plastic base of the switch should be green in color. The threads should be 14mm x 1.5 and the trigger points of the switch are 100C for the first stage and 105C for the second stage.

Part 006 545 42 24 also has three pins, is red in color, same threads with trigger points at 95C and 97C.

As far as I could tell, all other 107s employ part 006 545 14 24 which EPC says has been changed to 006 545 39 24. It is a single pin grounding switch with a trigger point at 100C.

I found some two pin switches with common threads and cooler trigger points. 006 545 15 24 and 006 545 40 24 with black base triggers at 80-85C. I found a little schematic that indicates one of the pins is wired to ground instead of grounded by the switch body. If this is correct, it is easy to adapt. I will look for these at U Pull & Pay to see if I can make them work.
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  #8  
Old 01-13-2012, 08:04 AM
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Nice job. Brad. But wouldn't the 80-85 C switch be a problem because that's normal operating temp. So the aux fan would be on all the time?
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  #9  
Old 01-13-2012, 09:23 AM
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Quote:
Originally Posted by 79Mercy View Post
No coolant flow with the engine off, What is the reason behind doing this?
Many CIS VW / Audi products had this feature. They used a temp switch mounted on a little bracket on the cylinder head connected to a 10 minute timer relay. It would run the fan(s) on low speed until the temp switch got below 85*C or until 10 minutes went by. Cooling down the engine bay would help prevent hot re-start problems. Plus I'd think lower under hood temps would tend to make various bits and pieces last longer.

-Jason
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  #10  
Old 12-10-2012, 07:44 PM
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What's the likelihood that this mod would solve a vapor lock problem on a 1975 450SLC?
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  #11  
Old 12-10-2012, 09:05 PM
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I'll answer that inquiry here, there and everywhere: I don't think it will help a vapor lock or a hot start sputter. In post #3 of your thread on BW, you asked about making the aux fan run on a timer after you shut down. I made mine run on the aux fan switch temperature trigger of 100C.

Even though the aux fan is lame, I think it helps normalize the temps in the engine bay after shutdown. I am happy with what my circuit does.

Vapor lock in the fuel loop would make me suspect a leaking injector or start valve. I would have to do some reading before I broke out the wrenches though.
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Last edited by cushjbc; 12-10-2012 at 09:06 PM. Reason: 2 forum confusion
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  #12  
Old 12-10-2012, 10:16 PM
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Quote:
Originally Posted by cushjbc View Post
I'll answer that inquiry here, there and everywhere: I don't think it will help a vapor lock or a hot start sputter. In post #3 of your thread on BW, you asked about making the aux fan run on a timer after you shut down. I made mine run on the aux fan switch temperature trigger of 100C.

Even though the aux fan is lame, I think it helps normalize the temps in the engine bay after shutdown. I am happy with what my circuit does.

Vapor lock in the fuel loop would make me suspect a leaking injector or start valve. I would have to do some reading before I broke out the wrenches though.
Wikipedia says that 75 and 76 450's made for the USA market are known for vapor locking due to their under-hood catalytic converters. Since the mid-1970s, gas formulations have changed (especially in Calif.) and my online research points to the extra ethanol in the winter gas formulations being more prone to vapor locking the car's fuel rail. I bought this car only a few months ago, and when I measured the fuel pressure on the fuel rail it was 32psi. I had a competent local indy adjust it to spec -- 28.5psi -- and that's when I first noticed the hot start sputter that I attributed to vapor lock. I should mention that more of my online research revealed that boosting the fuel rail pressure was a common remedy to vapor lock on these cars despite the excessive fule consumption it caused, which I attributed my car's initial high fuel rail pressure to. Ideally, I'd like it to run at 28.5psi on CA winter gas with no vapor lock.
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  #13  
Old 08-12-2016, 06:49 AM
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Share modification details

cushjbc; Can you share the modification details?
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  #14  
Old 10-09-2016, 02:14 PM
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Modified wiring for aux fan

The trick is to reroute the aux fan switch lead (blue) from where it is in your circuit (pin bushing 5 from the relay separating AC from ignition changeover in my car) to an added relay (option relay). A second relay is used to provide battery bus power to the aux fan when the start/run bus is powered and a ground signal is present at the aux fan switch (over 100C). The option relay prevents feedback from the battery bus to the start/run bus.

See attached wiring diagram; zoom to 300% and hover a cursor over the 4 comments at the lower right.

I used wire, connectors and relays from a salvaged 1980 450 SL main harness and installed the circuit in vacant spots on my relay rack.

It is interesting that after 4 years I have had two inquiries about this all of a sudden; built this circuit for a guy last month.
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File Type: pdf Visio-MB 107 Wiring Diagram 014105.pdf (550.4 KB, 123 views)
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  #15  
Old 10-10-2016, 08:47 AM
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The temperature of my 380sl used to approach 100 in traffic when the outside temp was over 95, and the AC was on.

But I solved the problem by draining the radiator, replacing the 50/50 mixture with distilled water, and adding a bottle of Hyperlube Supercoolant.

My engine temp didn't go over 90 once this summer.

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