[MBL87560SEC]

Hi there all,

My '91 500SEC temp gauge reads 100C when fully warmed up.. This condition is seemingly independent of the ambient air temperatures which I've observed as varying between a low of 60F this morning to the mid 90's last week.

This condition is also independent of road speed and or sitting in traffic idling.. The temp may vary just a couple of degrees higher idling .. and reduces if the engine is revved to say 1000 - 1500 RPMs.. normal.

This means the auxiliary fan will run darn near all the time, something I do not care for, or feel needed when cruising on the open road at any speed.

I first got the car in the spring I found it had a disproportionate antifreeze to water ratio… Way too much antifreeze… That has been sorted to 50/50 mix now of the MB blue coolant. No change on the gauge,,,

It has a new Meyle 80C thermostat.

I changed out both coolant temp sensors at the thermostat housing.. (gauge & fan speed).

I found the radiator matrix weeping so it now sports a new BEHR radiator.. at the same time I blew out the A/C condenser from the backside with compressed air to get rid of any dead bugs & other schmutz that could block airflow..

Next week I'll be installing a new A/C compressor as the original is leaking… At that time both the temp & pressure switches on the drier will be renewed as well..

Today I disconnected the aux fan electrical feed. There was no change at traffic lights, and stop & go .. or on the highway for miles on end at 75 - 80 mph.. 100C. The viscous fan appears to be doing its job nicely.

Last week I drove my US spec 560SEC and it warms to 85 - 90ish only.. Like the euro 500, its indicated coolant temp never varies once warmed up.. The aux fan seldom engages… That has been my SEC 'norm' over the last 14 years of ownership..

I am curious as to why this happens .. and delving further, is it possible to have the 500 act same as the 560SEC?

The only clue I might offer is the US spec M117.968 engine has 9:1 compression ratio.

The German M117.965 5 Liter engine has 10:1 compression… and is pretty lively since the try-y install & cat removal..

Could the compression be all the difference?

Thanks,

MBL

 

[mrboca]

Could it be a tired water pump ?

 

[281lxm]

I had experienced similar last year. Fresh off a complete new system from waterpump to radiator including fan clutch. The ac dryer compressor and switches were all just 3 years old as well. I had also changed the temp sensor so decided after changing out tstat and no change in results to get new fan clutch. I didn't have usual fan clutch issues but that was it.

Spend what you have to, it is important.

 

[Richter Rox]

When you say you changed your thermostat did you do that before these issues arose or did you change the stat trying to fix the high running temps ?

Sometimes older low mile cars sit a lot and may need a good coolant flush , sediments form more from sitting than driving.

That said I have also had ( not on a sec) a fan clutch that looked and felt fine but just wasn't engaging enough to keep the engine cool,

Also I wouldn't trust any t-stat but a Mercedes one,

Saw your car on german day looked fresh ,tight and brite. Lucky man!

 

[mclare]

I had the same problem in my coupe after my rebuild and I replaced the Radiator and condenser because the fins get clogged with shit over the years. This is assuming all is well mechanically.

It can't get enough air passing through the fins. Also the tubes could be blocked too.

 

[jzchen]

I suspect the T-stat. Reason is no change during highway driving, which usually does wonders on big engines to cool them...

 

[John350]

Have you verified the aux fan is running, or are you just assuming it is? If it's not, could be the gauge just reads a little high. If you have an IR temperature sensor, aim it at the thermostat housing and take a reading.

 

[mclare]

Are you sure the AUX fan is running at BOTH speeds? Hi and Low?

 

[MBL87560SEC]

Thanks all for your ideas & combined W126 wisdom...

Radiator: new BEHR

Coolant: New MB blue 50/50 mix

Aux fan: When the temp sensor is disconnected the fan spools up to full speed, connected it went back to 1/2 speed. It runs nearly all the time when engine is fully warmed. Loud as hell on full speed.

Thermostat: This was the first thing I changed in my quest for a lower coolant temperature. It made no difference from the original MB t/stat.

Water pump Today, in the 4th of July parade, I revved the engine to 1000 . 1200 rpm and this immediately brought the temp down a couple of degrees to the 100 mark. I have seen this over and over whilst sitting stationary with the A/C on for ten > fifteen mins the temps climbs a couple degrees only and can be brought down by revving engine ... It just goes up a small amount and stays there...

At road speed the temp will go down a drop when decelerating down a hill .. as I might expect...

MBL

 

[mclare]

This was the exact same thing that was happening in my coupe and when I changed out my condenser, that fixed it. The fins get clogged with shit over the years and restricts the airflow. I know its a bitch to change and you'll need a re-charge, but if my opinion means anything...

 

[MBL87560SEC]

This was the exact same thing that was happening in my coupe and when I changed out my condenser, that fixed it. The fins get clogged with shit over the years and restricts the airflow. I know its a bitch to change and you'll need a re-charge, but if my opinion means anything...

I follow your logic completely Mike..

I had attempted to clear the condenser airflow using compressed air.. To take it to the next level, I shall try out some CRC [or like product] foaming condenser cleaner... See if that makes any difference at all... Funny thing is the fact I can see the radiator matrix whilst sighting through the condenser fins.. it really 'looks' pretty clear..

If I can find some of that stuff, I'll give the full report yay or nay later today..

Home Depot here I come! http://www.homedepot.com/p/AC-Safe-...PIPHorizontal1_rr-_-100664957-_-206740351-_-N

Mike, your W126 knowledge & opinions are valuable.. both to me & others who may come upon this thread in the future..

Thanks

MBL

 

[jzchen]

Thermostat. Rated 80C means it starts opening at 80C. System is operating fine. If you want it to run cooler get a lower one...

 

[John350]

I disagree. With an 80 C thermostat he shouldn't see temps higher than 85 or maybe 90.

 

[mclare]

I follow your logic completely Mike..

I had attempted to clear the condenser airflow using compressed air.. To take it to the next level, I shall try out some CRC [or like product] foaming condenser cleaner... See if that makes any difference at all... Funny thing is the fact I can see the radiator matrix whilst sighting through the condenser fins.. it really 'looks' pretty clear..

If I can find some of that stuff, I'll give the full report yay or nay later today..

Home Depot here I come! AC-Safe Air Conditioner Foaming Coil Cleaner-AC-921 - The Home Depot

Mike, your W126 knowledge & opinions are valuable.. both to me & others who may come upon this thread in the future..

Thanks

MBL

Thanks MBL!

So listen, I had the same thing. I could look through the fins and see no visible blockage and I did the exact same cleaning route you are going through. Then I bit the bullet and tried replacing the rad and condenser. Viola! And you know I lived in the Antelope Valley where outside temps in the summer top 120F and from then on she would never climb over 90C at idle with the A/C on full.

As you know, the engine had just been rebuilt so all the water passages were perfect, all new rubber, new everything, except the condenser. Remember I had even done the Dual fan conversion? That was a no no on a coupe putting a SEL fan set on there. Look, I know it's truly a pain in the ass switching it out, and is really a matter of last resort, but if everything else is sorted as you say, then IDK????

That's my .02 cents.

PS - I just put all of the pics back in my "Trek Up North" thread as you can see, and I know you followed it live at the time because I saw you in there over this weekend, but I go through this EXACT issue in it towards the end. IDK, maybe page 35 or so. Read up on it.

 

[MBL87560SEC]

Hi Interested All,


Sooo... I got da stuff at Home Depot this morning, got lost on the way to my time sensitive meeting, nearly got stopped for my custom europlate on the front.., and used the elixir thisafter when I got home...

No change

I am admittedly TOTALLY resistant to just chucking the SEC condenser & replacement of it.

So.. Mike... I am replacing the A/C compressor in the very near future... Part ordered etc.

At that time, I shall remove the condenser from the car & drive for a bit WITHOUT A/C !!!! to test your synopsis.

If different OK ... I spring for new one and like overnight ship..

Keep on truckin' man, ..... & thanks.

MBL

 

[tusabes]

It does sound like it could be the condensor

Let us know what your test with it removed does

 

[MBL87560SEC]

My '87 USA version 560 never had any of this...........

Such a price Glamor!

MBL

 

[mclare]

I understand your logic, (Money) and I'm with ya, but that test does not really test the problem because the engine will not be under full load conditions, will it? But okay, do what you gotta do. It could give you an indicator.

Also, replace all of your belts at the same time. Ensure they are properly tensioned too.

Make sure the two driving the WP are matched too.

 

[Richter Rox]

You say you can cause the temperature to drop by slightly by reving the motor, that makes me think clutch fan ,weak water pump or plugged passages in the block or heads.

 

[mclare]

You say you can cause the temperature to drop by slightly by reving the motor, that makes me think clutch fan ,weak water pump or plugged passages in the block or heads.

You're kind of all over the place here pal. Are you guessing? Or do you know? You said T-stat, he has a new one. You said 80 degree, that makes zero difference. Now all this.

He said when he revs it the temp goes down. Says to me airflow. Says to me WP and fan clutch working. Says to me, old car and not enough air flow through the front end.

He's got a new Behr so we'll see when he does his test. I could be wrong? Have been before, but I went through this exact same issue with a brand new motor, and told him what fixed it.

And MBL, it prolly wouldn't kill you to do a CA flush either. Just don't leave it in too long. Read my thread on the WP install. I'll fix the pics in that one shortly.