420SEL Running Hot Hotter Hottest

To save a little gas Mercedes designed this system to rise in traffic. However when it gets to 105degrees the electric radiator fan(s) will come on. If that doesn't stop the rise at about 110degrees the visco clutch engages. Also with the A/C on when the receiver/drier temp rises that switch also turns on the radiator fan(s). On my 1980 this is the slow speed thru a resistor - the radiator switch overrides it to high speed if both click on..
On the open road the needle should be horizontal, after a hot rise it will take some time to go down but it should go down. If at 70MPH the needle keeps going up you have lost a lot of system capacity. 70MPH on the Autobahn is for someone pulling a trailer with a clapped out Panda. Your 420 should cruise over 100MPH all day with no problem whatsoever. or it did when it was new

I did some troubleshooting. As I said before the fan comes on with the AC so I don't think the fan itself is bad. Next, I tried swapping in and out some some known good ice cube relays to the fan hi/low. Nothing.

Next I pulled the two connectors from the top off the thermostat/water pump stack. Nothing. One post I found said if you pull the two prong, the fan should come on. I am not sure I am buying that because the next thing I did was jumper the two prong. That caused one or both relays to click. It seems that if I disconnected the single prong, both relays would click whereas if I left it connected, only one would click (honestly, I may have that backwards, now I can't remember). Either way, no fan. This is with the car on or just on accessory.

I have heard there is a fan resistor somewhere. Maybe the culprit? Anything else in the fan circuit I need to check?

Anybody got any ideas for me? Am I at least troubleshooting correctly? Thanks!

Progress

One problem solved. Turns out my temp sensor fan switch was bad after all. It also turns out, that the design of the how the auxiliary fan temp sensor switch works is different on MY86,87 vs. 88 and up. Not sure about Gen I but my guess is it is similar to my 86 420SEL.

By the way, I am not saying this information is not available already in this forum. It's just not easy to piece together given the differences between years, engines, etc.

Basically, on Gen II 88 and up, if you unplug the temp sensor switch, the fan will come on high as a default. I assume this is a "safer" setting so if/when the switch fails, it is better to have the fan on and cooling than off and overheating. Plus, you will hear it running all the time and possibly be clued in to the fact that there may be a problem. Prior to 88, if the switch fails, you just don't get any fan action no matter how high the temp goes.

This information is available by looking at the circuit diagram. You can see that the diagrams are different for 86/87 and 88 and up.

I also was finding conflicting information about how the temp sensor, auxiliary fan, AC, and high/low relays all interact. Here is what I found:

1. When you turn on the AC, the low speed fan relay should click passing the current through the fan speed resistor. The AC itself will never turn on the high speed fan.
2. Whether the AC is on or not, the temp sensor switch monitors the coolant temperature. If the temp reaches 105C, the switch kicks on the fan at high speed. The fan will stay on until the gauge in cluster reads somewhere below 90C. I am not saying this is tied to the cluster gauge, in fact it is not, I just used that to monitor the temperature myself.
3. If the high speed fan still cannot cool the engine, and the temp reaches 115C, the temp sensor switch will turn off the AC compressor and you will lose your AC. I believe that is what the third pin/single connector on the temp switch is for. Of course, this is done as a last measure in the event the cooling system cannot keep up.

I replaced my switch yesterday and now it kicks on at 105C.

Of course, this leaves the issue of my engine getting to 105C in the first place. This is not from hard driving but just from at least 20 minutes or so of stop and go traffic. I *think* this is because my main engine fan clutch is bad. Over the past couple weeks with me testing these various components of the fans, I did notice that when my car is heating up in my driveway (not moving) the fan clutch never seems to kick on. Also, when the engine is hot, sometimes at almost 115C (before I fixed the temp sensor) and I rev the engine, the main fan seems to still be in "coast" mode i.e. it does not speed up and roar like I understand should happen. Now, I could be wrong here, but, I am going to go ahead and replace it anyway as it seems like the next easiest/cheapest thing to do. I still have a new t-stat I have not put on yet but that will happen soon enough as well. We'll see what happens after this before I move on to replacing the water pump or rad next. The fan clutch should be here today so I should know by tonight. The temps here are already in the 90's so it won't be hard to test.

Picture below is of the three prong temp sensor switch. Know that if you ever need to order one of these, there are two part numbers because they work differently depending on MY86/87 and 88 and up. This is noted in the EPC. My part number was 0065454524.

Well, it seems like I fixed my overheating issue. Well, maybe overheating is a little bit of an exaggeration but I bet it would have overheated if I was not careful. As I mentioned in the previous post, my fan temp sensor switch was bad. However, that had nothing to do with the overheating itself. I replaced the fan clutch last night. I had noticed that it always seems to be in "coast" mode. Even with the engine sitting at idle for 20-30 minutes with the water temp approaching 120C and me revving the engine with the throttle linkage, I could not see it pick up speed.

So, I replaced it with a Behr unit I got from O'Reilly yesterday. Since the temps here were approaching 100F this week I just wanted to get it done. Unit was made in China and I paid $200. Found them online cheaper but I wanted it done right away. Worst case, it has a lifetime warranty through O'Reilly. Anyway, after installing it, I *think* it is functioning correctly. I definitely have that "roar" back when I rev the engine upon startup. Also, if I am cruising at speed and punch it, I don't always here it spin up unless I have been sitting in traffic so it must be doing its thing. Biggest test of course was on my way home from work today. At least 15 minutes in stop and go traffic and my temp gauge barely gets to maybe 84C at most. Soon at I am moving, it is quickly back to its historically normal spot at just a tick above 80.

This install was pretty easy. Four 10mm bolts to take the fan off the water pump, four 10mm bolts to take the clutch off the fan. No need to remove the shroud or anything. Probably took 30 minutes. Would have only taken 15, but, the new clutch had a thicker shaft and I could not get my 10mm gearwrench on the bolts putting it back together so I had to tighten everything with the open end.