More no heat diagnostic..
 

Sorry to start another thread but this thing keeps biting me in the rear. (1985 300SD)

Car is up to operating temp. Set ACC to just under max heat. Cool air from vents. If I put to max heat same thing. It seems like more heat from vents below than from dash vents. Now I hit the Economy button and keep heat set at same place. Much warmer now.

I rebuilt monovalve, flushed core and checked aux pump. All seems OK.

Something funny I noticed. When I set the thumbwheel for heat and hit any of the ACC buttons other than economy, I can hear the AC compressor clutch kicking on. When I hit the economy button with the heat on the air gets noticeably warmer and the compressor kicks off.

Is my air conditioning kicking on the same time as my heat? I am having difficulty with this. I was going to try and unplug my compressor and see if my heat works correctly then.

Everything that I have read says that the A/C on these cars is only completely off in economy mode. Otherwise it is set to run a little or a lot to decrease humidity. The lack of heat to overcome the A/C can be related to bad thermostat, plugged heater core, broken/ripped monovalve diaphram, or auxiliary heater pump problems.

Someone had told me that before but I wasn't sure they were 100% accurate. I checked everything I can think of but the heat is still really weak. I was thinking it may be the push button unit but all the buttons seem to work as far as duct control and blower speed. I don't think it is the sensor either because the heat is still weak with the thumbwheel set to max heat.

How quickly does everyone else's SD warm up? Maybe my car is slow to warm up. New thermostat?

If you only want heat, don't select anything but the economy mode. The other selections are supposed to engage the compressor.

You say up to operating temp - what is that? My SD stays around 82C fully warmed.

If it's not that warm, try a new t-stat.

If it is, check or replace monovalve again (unplug to get flow) and check aux pump again. Otherwise, you might have a clogged heater core.

The ACC turns on the compressor in all modes except econ. Sounds like your A/C works better than your heater!

It was like 45-50 degrees F when I left work this morning. I started the car and let it idle for about 3-4 minutes. I then did some around town driving for the next 10-15 minutes or so. I had heat but it wasn't really hot like it should be. When I got home the temp gauge was still below the normal operating temp. (80)

Firstly, the aforementioned posts are correct..........the a/c is engaged in all modes but economy.

Second, the 617 does not put out much heat until the gauge gets over 60°C. and even then, the temperature is not all that hot.

Third, if you drove it around town.....at low speeds.......it does take awhile to warm up. A properly working auxiliary pump might assist in getting heat through the core faster. IIRC, you've verified this already, correct?

Fourthly, remember that the temperature inside the cabin is 50°F. before any heat is produced. Accordingly, the system will restrict the heat from the monovalve because the temperature in the cabin is approaching the setpoint (70°F.). Move the dial all the way to the stop to evaluate the capability for heat........independent of the dial setting.

And finally.........do we really need another thread on this.........you've already got one going IIRC? I might just merge this with the old one.

Your third point= I thought it strange that after 20+ minutes of around town driving that my car was still not warm. yes I tested the aux pump. (I stated this earlier in the thread)

Your Fourth point= I stated earlier in this post that I moved the thumwheel all the way to the stop. It should throw heat no matter what the temp inside or out.

Your Final point=There is no old thread. There WAS some redundant info so I took your advice and deleted it. I feel this is a valid thread because I am having strange, unresolved issues.

I don't know if I am just in a bad mood or not but I am getting a little pissy. If this thread is bothering anyone else feel free to delete it or lock it. Or I will do it myself.

My apologies ahead of time, I am on the rag.

You seem to have all bases covered. I also seem to remember that you flushed the heater core........but........it was not fully clear??? Maybe you need to use the citric acid in the system for a hundred miles or so.

If the old thread is deleted, then it's fine to leave this one alone. But, it's always best to start a problem and continue it to the end on the same thread.........even if many months elapse during the diagnosis and repair.

When I flushed it didn't look bad at all. BUT the water just smelled really, really stagnant. I will give it more time and see what happens. That is what I don't understand, I seem to have tested just about everything and my heat is still weak. I just ordered a new thermostat to be on the safe side. I will see what happens when I install that. If there is any breakthrough I will post back here.

Thanks everyone.

Stagnant water could be telling you that the internal surfaces of the fins are rusted. If so, they won't transfer heat from the coolant. I had this occur with my tankless coil on the house furnace. After 20 years, the rust buildup on the copper fins prevents heat transfer. Finally had to replace the coil. Problem solved.

In your case, you've ruled out everything else. It's time for a good citric acid treatment. I'd leave it in there for at least 100 miles and drive the hell out of it, in an attempt to clear out the rust.

Get a Behr thermostat... I purchased 2 others , a motorad and another brand and both failed in the stove test.
The Behr worked great and great heat now. My gauge read 80*c after putting in both bad stats and NO heat. Both slightly failed open.
Behr all the way for me.

I called around a few local parts stores and nobody carried the Behr. I did a search and read about the others failing so I know I wanted the Behr unit. i also ordered new fuel filters. I figured I'd better change them out since I started running the wvo/diesel mix. I looked in my service records and I haven't replaced them in about 2 years. Pretty sad though, I only put on about 8,000 miles in those 2 years.

Brian,
How do I go about the citirc acid thing? Where do I get it? Was this covered previously in another thread?

Thanks.

found it.

The point to note, if you do it, is the need to thoroughly flush it out. Probably takes at least three fills and subsequent drain to be sure of removing all of it.

Maybe the thermostat will be the culprit and you won't have to do all that work. But, if the engine gets up to 90°C. on the highway, I doubt that it's the culprit unless it's opening too early.

In real cold weather, the SD won't get over 81°C. around town. It takes a 3000 rpm run to move it up to 87°C. or so. Maximum continuous power for 30 seconds or more will push it up.........sometimes slightly over.........90°C.