floor heat/vacuum - PeachParts Mercedes-Benz Forum

peterhardie · #1 12-21-2006, 07:59 AM

Note, I have the 85 California wagon, YMMV.

This is just a caution to folks who assume the ACC unit is always the first culprit. I had two problems with my climate control: the fan would only start in defrost and high defrost mode. Also, the floor vents were not opening. Curiously (you'll understand why later) swapping out the unit for another (not mine unfortunately, my spares were no better) seemed to make the vent problem go away. I took my ACC control apart and resoldered all the connections, which did fix the fan problem. Still no floor vent opening.

I tested the vacuum pod, found it not holding vacuum, replaced it, vents worked for all of about 10 minutes. Checked the vacuum coming into the climate system, driver's footwell, 15". Checked it at at the new pod, 4". Bypassed the vacuum switch behind the control unit (all vacuum all the time to the floor vent pod), vents worked. Till I took it out for a drive. Cold feet.

Tested the vacuum under the hood, 23+" steady at the main junction, at the next junction jumping all over the place, and dropping hard under throttle. By squeezing off lines, I traced the problem to a switch on the driver's fender near the glow relay box. Unplugging that switch steadied the vacuum at all rpms. And fixed my floor vent problem.

I did not get a chance yesterday to figure out whether that switch is faulty. The trans may be shifting a bit differently, but not badly, and may even have fixed the hard 1-2 shift There are vacuum lines at the switch that lead to the outlet vent and another I haven't traced. Time to hit the manual.

Any ideas?

my123ca · #2 12-21-2006, 09:55 AM

Could be the switchover valve to the ARV. Most common problem is the rubber vacuum tube to the ARV. It deteriorates due to heat from the turbo. Some have disabled that valve permanently together with the EGR.

toomany MBZ · #3 12-21-2006, 04:49 PM

I have a similar problem on my '83 300SD, no floor heat to driver floor, passenger, yes, heat also blows out of cowl. A vent vac problem, there is not a vac connection on drivers fender. Anyone know where the ARV? is, what is it, it's function? How to bypass EGR? Thanks.

85 DSEL · #4 12-21-2006, 05:06 PM

Quote:

Originally Posted by toomany MBZ

I have a similar problem on my '83 300SD, no floor heat to driver floor, passenger, yes, heat also blows out of cowl. A vent vac problem, there is not a vac connection on drivers fender. Anyone know where the ARV? is, what is it, it's function? How to bypass EGR? Thanks.

A search on the subject "ARV" will reveal alot of information...

BTW, the 83 MAY not have an ARV, someone more knowledgable than me might be around to help out.

toomany MBZ · #5 12-22-2006, 04:49 AM

Thanks. 85 DESL.

Jeremy5848 · #6 12-23-2006, 06:48 PM

In most parts of California, it doesn't get cold enough to need a lot of heat, but when we left for Utah yesterday morning, a few hours in the cold and snow driving I-80 across Nevada left us with an uncomfortable feeling in our feet. A cold feeling, if you will. No heat was coming from either footwell vent.

We survived by donning our "moon boots" and today I did a little reading, first in my CD-ROM manual and then on this forum. Even without a Mityvac, I was able to determine that the footwell vent pod was getting vacuum but wasn't holding the pod open. Thanks to the posts of Kerry Edwards and others, I was able to disconnect the pod from the doors and restore footwell heat.

Whether I have a bad pod or insufficient vacuum is a question I will leave for when we return home. I noticed that the vacuum level in the engine compartment is normal, about 15"Hg at idle. Vacuum in the trunk, at the "egg carton" reservoir, is low, about 10"Hg, should be at least 15. (I have gauges permanently installed in both locations.)

This is a "chicken and egg" question -- is the vacuum low because the pod is leaking or is the pod not working because of a leak elsewhere? I'll work on that question next week. Stay tuned!

Incidentally, I had a problem getting the ACC system to put out enough heat until I discovered that warm air was being channeled from the defrost ducts, under the fabric cover of my terribly cracked dash, to the air temp sensor. Since the sensor was nice and warm, it told the ACC to cool things off (at least I know it's working!). Removing the pad restored full heat. Maybe I can use the problem to justify buying one of those neat dash caps.

No starting problems as it hasn't been cold enough to even plug in the block heater. I did make the sacrifice of switching to dino diesel for the duration.

Jeremy

my123ca · #7 12-23-2006, 07:30 PM

Isnt the vacuum in the engine compartment supposed to be 20+ Hg? I would think that the pods would actuate even with low steady vacuum. It would just do it slowly. The vacuum reservior is only for the doors.

Jeremy5848 · #8 12-24-2006, 12:25 PM

Quote:

Originally Posted by my123ca

Isnt the vacuum in the engine compartment supposed to be 20+ Hg?

I suppose it would depend on where you connect and how worn the pump is. My gauge is in one of the tees off of the main line from the pump to the brake booster. My '85 has 21700 miles and I have no idea if the pump has ever been rebuilt.

Quote:

Originally Posted by my123ca

I would think that the pods would actuate even with low steady vacuum. It would just do it slowly.

That suggests I have a damaged pod.

Quote:

Originally Posted by my123ca

The vacuum reservior is only for the doors.

My CD-ROM copy of the FSM (83-604a.pdf) shows the reservoir in the climate control circuit also.

dmorrison · #9 12-24-2006, 01:18 PM

Since your asking for heat out of the AC/Heat system your using more than one vacuum element. Asking for heat CLOSES the center vents. Your fresh air vent is open, your floor vents are open, If that button is selected. so you need to check all your elements for a leak. and the vacuum switches behind the Ac control panel for leaks.

See the attahed diagram for the flap logic when using the Ac/heat system.

Dave

Edited for key to letters.

peterhardie · #10 12-24-2006, 01:53 PM

Jeremy,

Permanent guages sounds like a fix I want, might save a few steps in diagnosing a leak, help isolate a leak till I can fix it.

How expensive would some cheap vac guages be? Any recomemndations, or should I just pick whatever is on the shelf down at Auto-Moan?

JimmyL · #11 12-24-2006, 03:06 PM

Quote:

Originally Posted by dmorrison

Since your asking for heat out of the AC/Heat system your using more than one vacuum element. Asking for heat CLOSES the center vents. Your fresh air vent is open, your floor vents are open, If that button is selected. so you need to check all your elements for a leak. and the vacuum switches behind the Ac control panel for leaks.

See the attahed diagram for the flap logic when using the Ac/heat system.

Dave

Hey Dave,
Do you have a pic of the "key" that tells us what is selected at the buttons for A, B, C, D, E, etc.........
I have no idea about the main air flap and the recirculate flap situations.....

dmorrison · #12 12-24-2006, 03:32 PM

Quote:

Originally Posted by JimmyL

Hey Dave,
Do you have a pic of the "key" that tells us what is selected at the buttons for A, B, C, D, E, etc.........
I have no idea about the main air flap and the recirculate flap situations.....

Edited the post with the key.

Dave

Jeremy5848 · #13 12-24-2006, 05:32 PM

Quote:

Originally Posted by peterhardie

Jeremy,

Permanent guages sounds like a fix I want, might save a few steps in diagnosing a leak, help isolate a leak till I can fix it.

How expensive would some cheap vac guages be? Any recomemndations, or should I just pick whatever is on the shelf down at Auto-Moan?

Peter, you can use just about any kind of vacuum gauge. Accuracy isn't really important as the primary benefit is the ability to "see" the vacuum level regularly, get used to it, and use the changes to help diagnose problems.

I bought two gauges from eBay. They were surplus from someone who had been building carburetor adjustment sets for motorcycles. See my thread "vacuum gauge fun" at http://http://www.peachparts.com/shopforum/showthread.php?t=162069&highlight=vacuum

My '85 with its California emissions crap has two vacuum takeoffs from the main line, one with a 3-way splitter and one with a 4-way splitter. I removed the 3-way and replaced it with a 4-way, ran the extra leg to the gauge. This port is connected to the transmission, I think, because the vacuum level goes down as engine speed increases, in simulation of a gasser's intake manifold vacuum. That gauge indicates vacuum only when the engine is running.

The other gauge is in the trunk, tee'd into the line to the reservoir. It indicates vacuum all the time, slowly decreasing each time you use the door locks. There are many other places where a gauge could be put but I don't know enough about the system yet to make a good judgment.

Jeremy

Jeremy5848 · #14 12-24-2006, 05:36 PM

Quote:

Originally Posted by dmorrison

Since your asking for heat out of the AC/Heat system your using more than one vacuum element. Asking for heat CLOSES the center vents. Your fresh air vent is open, your floor vents are open, If that button is selected. so you need to check all your elements for a leak. and the vacuum switches behind the Ac control panel for leaks.
Dave

The system works correctly now that I have the footwell pod mechanically disconnected from the flaps. When I get home, I'll test the pod for leaks.

It has also occurred to me that my vacuum levels will read low here at elevation 4500 feet compared to 200 feet at home.

JimmyL · #15 12-25-2006, 12:47 AM

Dave, or I guess anybody for that matter, could you explain to me the location and function of the main air flap and the recirculate air flap?
I'm interested this summer in doing what some folks have done as far as using 100% recirculated air for the AC. I simply don't understand where the outside air comes from, where it draws from in recirculate, etc.....
I know they are the 2 flaps I think near the blower fan.
Thanks!