Persistent high temps in 1994 E420
 

My 1994 E420 has a strange problem with its engine temperature. In everyday driving, the temperature usually hovers between 82 and 100 on the gauge. However, if I have been driving for a while at 75-80, and then hit bumper to bumper traffic, the temp can go as high as 110. At that point, the high speed fans come on, and the temp drops to just below 100, but this can fluctuate back and forth. My mechanic does not believe that our mild climate (San Diego) should cause the temp to rise above 100 in just about any driving condition.

I suppose that my question is really two-fold:

1. What is the "normal" operating temperature range for this car, ie., is an occasional spike to 110 considered normal?

2. If this is not normal, why is it happening?

About one year ago, I replaced the radiator due to the plastic neck issue. At that time, I had the radiator filled with Mercedes red antifreeze at a 50/50 mix. This has probably shifted a little to the water side as I have topped off the reservoir a couple of times, only using water. After this temperature problem started, the mechanic replaced the thermostat, hoping that it would take care of the problem. Just a couple of days ago, he replaced the fan relay, since he believes that the fan clutch is operating normally. He does not know of any other variables which could cause the car to behave this way.

I greatly appreciate any assistance that the knowledgeable members of this forum may provide. FYI, I also started another thread concerning the proper way to maintain the throttle valve actuator, again seeking some good advice. Have a look if you might be able to help.

I have a '93 400E which is essentially the same vehicle. My temps sound about the same as yours. With ambient temps now in the 60's, it runs about 82 at cruising speed, but then like you I hit a stretch of bumper-to-bumper traffic and it will then get up to about 100 (and definitely would hit 110 in the summer months). Seems to take a while once back to cruise speed to come down, and never comes down to the low 80's once it gets hot.

I too have a new radiator and thermostat, and a relatively new water pump, so I THINK this is normal behavior of these V8s crammed into the W124 engine bay.

It will be interesting to hear from others. I have heard many different opinions on this subject. Some say that any temp below the red line is OK, though that's hard to accept. It is interesting, though, that the engineers set the aux fans to come on at 105 or 107 degrees coolant temp, so apparently they weren't too worried about 100 degree temps!

Mercedes tech bulletin July 22, 1994
Message #322

Paraphased for brevity:

The temp in MB vehicles may rise substantially above 100C under various conditions. Not a problem as the boiling point of properly mixed coolant is well above this.

See Service information 20/9 for coolant mix ratio.

Good Luck.

Is idling after a prolonged stint on the highway at speeds of 75-80 one of these "various conditions" described in the bulletin? Is it fair to assume that as long as the temp does not go into the red, that there is nothing to worry about?

The rest ofthe bulletin:

...when vehicle is
a) stopped
b) in city "stop and go" traffic
c) when driving uphill
d) When driving at very high speed (with high engine load)

In individual cases the temperature may approach the red field of the temperature guage.

Provided the cooling system is well-maintained and in good working order, such temperature levels do not create any danger for teh engine and therefore give no cause for concern.

You now have seen everything in that technical bulletin. It was brief, but I an no typist...

Thanks, Steve! That's reassuring.

By the way, how do you access the tech bulletins? Sounds handy to be able to see them.

You can access them through the dealer, or through a Popular Mechanics model-specific repair CD from an auto Parts store. You do have to pay for the privilege. I think tech bulletin access was an additional $10.

Regards.

The one thing I haven't seen mentioned is the low speed aux fans. When all of this is going on, the low speed fans should be on...especially with the A.C. on. I have seen the resistor for the low speed fan circuit fail many, many, many times.

Donnie,
How does one test the resistor? Where is it located?

AHA!
 

While working on the car this week, I noticed a ceramic resistor block on the front left "frame member" of the car. It is tucked up under the headlight.

The resistor was broken and the wires were attached to the SAME end of the resistor. I expect that the PO did this when the resistor failed.

Never knew or cared what that part was until now!

Thanks for the tip!

Benzmac,

I have noticed that the fan doesn't kick in when I turn on the A/C and have not observed it do so although the A/C works fine. Is the resistor you mention the one that Steve found? Was the resistor used to drop the fan speed? I take it that Steve's fan was always running at one speed then if it was jumpered. I will have to check that fuse as well.

I have observed this temp behaviour in my 2.8 and in other cars as well and I would tend to agree it is normal. One other thing I haven't seen is "Guido's cooling mod" - or is that too controversial? I am planning on getting one of his add-ons as it appears to be an easy install / deinstall. I get anxious when the temps start to hover around 110 - anything to protect my new hg!

Cheers

Regarding the aux fan coming on: Isn't there a temperature switch, mounted in the coolant outlet neck of the engine, that can be switched to a different p/n, to come on at a lower temperature? I seem to recall something about that, when researching this for my 560SEL. I did not turn up anything on the search though. For some reason, I thought the standard temperature was 105C or so, and the new p/n would let it come on at 85C. The temperatures are from memory, so could be off. Maybe someone could post some information on this, or tell me I am all messed up?

For the record, my 1989 560SEL, (little more cubic cc's, same engine roughly, I think), hits 100C in traffic here in Sunny Florida, sometimes exceeds 105C or so. This is without the a/c on. When I notice, I kick the heat on, and it cools right down to about 85-90C, where it runs all the time. So far, no bad things have happened to mine. But I would prefer to have it run cooler.

As Benzmac rightly points out, Mercedes must have sold a zillion of these resistors. My dealer told me they have a habit of busting when the car goes through the car wash as the ceramic cover and/or the exposed coil, once soaked with car wash water, shatters as they are really not in the best of locations, low on the frame next to the headlight. He has suggested that they are covered with a rag when either hand washing the vehicle or sending it through the washer ! Good advice I think anyway;)

whew, i just read through the "300e auxilliary fans and a/c" thread and got a thorough explanation of how everything is supposed to work.

can't seem to find this info on the cd - can someone tell me where these might be located on a 300e:

- the engine temp sw that should turn on the aux fans at high speed at approx. 110 deg. (on the thermostat housing - i take it this is on the engine block and not on the radiator).

- the drier switch that will activate the low speed fan.

screedchan, have you
 

put in the 'resistor' mod to turn on the aux fans earlier?

If not, give it a try. Link: http://pages.prodigy.net/jforgione/MB_CTS.html

hi Jim,

BTW, thanks to the other thread I found why my low speed wasn't working w/ A/C - the fuse was blown! Replaced it - et voila!

Now on to this cooling mod - did that ever get resolved as to whether or not it works on e series?

I think the
 

key is the fact that it's a V8 not the 'E'. So it has the same compliment of sensors as shown in the picture on my page.

All you need is a 1.1k ohm resistor and a little solder, so have at it!

Just wanted to add, I observed the same thing as decribed by screedchan today during my trip from Vancouver to Bellingham...while waiting at the Canadian/American Peace Arch border crossing to come back into Canada for around 40minutes, I noticed how the coolant temperature would shoot right up to 110C, then the fan comes on to take it down to 100C...happened 3 or 4 times during the 40 minute wait...A/C was off...temperature reading on dash said 20.5C...after I got up to speed on the highway with the A/C on again, the coolant temperature quickly dropped down to a little above 80C...wow...this is one HOT operating car...

I’ve got the "resistor mod" on my 94 E420 and now shes now cool as a cucumber....well not that cool because you do want to reach normal operating temps. I think I may do this to my 300E before I go to Florida. If I recall, I used a 1.1kOhm resistor.

Andy, since I'm still having problems I'd like to verify if this picture I've attached is the resistor you and benzmac have been referring too. A coil in the middle of this? At what points do I measure the resistance, and what values am I looking for when the car is off? If this resistor is bad, would that cause my cooling problems?
thx...

forgot the pix

Benzmac,

Can you describe where the low speed fan resistor is located on a 92 500E? I'd like to check mine for good measure. TIA


Steve,

The temps on my car can see up to 106 after hard driving and or altitude. I've also experienced 110F at plus 4500Ft altitude, 98F and hard driving.

Try changing your coolant ratio to 40% Coolant/60% Water. We don't have problems with freezing in San Diego.:) so this will benefit the cooling system. I've found a good synthetic oil also helps keep my car running better.......I know it sounds weird but it's worked.

In my experience with this complaint here is what I discovered.

The placement of the Right side auxiliary fan sits directly in front of the fan clutch assembly on the 124 and the 129 chassis cars with the 119 engine. Both of these models have had excessive engine temperature issues from day one. Traditional MB owners were use to seeing their cars run at 80 solid and not wavering from that unless it had a problem.

My thought is this. The placement of the aux fan blocks a good percentage of airflow directly to the fan clutch. Without direct airflow to heat the bi-metal strip than how is the fan going to engage? Heat transfer through the water pump? Heat transfer around the blades of the fan? The bi-metal strip should get direct influence form a reliable air stream as it was designed.

My solution to bring the temp into an acceptable range for stop and go conditions was this. A 15 turn 2k potentiometer, dialed it in to 1405 ohms, than soldered it parallel to the blue temp switch which controls the fan(s). The temp stays below 95 and the oil pressure stays above 1 bar now.

I am going to experiment next summer with removing the right side fan to see if my suspicion is factual or preposterous.

I spent many hours with this problem mainly because the oil pressure was below 1/2 bar on really hot days with the a/c running.

FYI: Voltage measured across the blue switch 1.74 (approx) turns the fans on and
1.55 (approx) turns them off.

These cars REALLY do run HOT!!! I've been experiencing the same temperature range of my car even with 45F weather. I thought highway driving would bring the temp down a bit (hovering at 82C) as the cool air would pass through the radiator but it didn't make any difference.

This was one of the reasons I replaced the foam hoodpad because of the deterioration from the excessive heat. I would also be interested in what "MrCjames" discovered after removing the right side (passenger) aux fan. It does seem to be a weird design as the fan clutch is off center.