HELP W124 Evaporator

[flanso]

I am in the midst of replacing the evaporator and 6 vacuum servos on my 1993 400E. The dash and cross brace have been removed. The clips holding the top and bottom halves of the ac/heater box together have been removed. Although the two halves of the ac/heater box have separated on the cabin side of the box, the firewall halves still seem firmly joined. How does the box come apart? Does the heater core need to be removed for the evaporator replacement. Thanks.

[jcyuhn]

The procedure is to remove the entire HVAC box from the car, including both the heater core and the a/c evaporator, then disassemble the box. You will have to drain the engine coolant and remove the heater hoses from the heater core inlet/outlet pipes. Also disconnect the expansion valve.

The box itself is retained by five nuts. Four are on the firewall, one is underneath the box (kinda in front of the radio). Much prying is required to break the sealant used between box and firewall.

Once everything is disconnected and out of the way, the entire box pulls out and is serviced on your workbench. This includes the evaporator, heater core, and all vacuum elements.

- JimY

[Lenny Lipscomb]

I was onced faced with replacing the evap on my 320E. Instead I sold it. I had the MB CD ROM shop manuals at one time. These are exremely helpful in cases like this. Try to attact the attention of one of our fellow members who has a manual. You can purchase this software from an MB dealer for around $120.

[flanso]

Thank you, Jcyuhn, your advice got me back on track and the heater box is out! I had removed the sides of the box while it was still in the car, the sides that connect to the ducts going to the outer dash vents. Should the entire box have been removed with the sides still attached?

MB used foam around several of the flaps and around the sides of the heater core. On my car that foam is badly deteriorated. Should I blow away the flaking remnants or do I need to take some other remedial action?

I very much appreciate your taking the time to answer my SOS yesterday.

Frank

[jcyuhn]

Yeah, the box comes out first, then gets disassembled. Suggest you do all the evap/heater core/pods work on the bench. Get a small hand vacuum pump (Mityvac) and bench test the pods.

By removing the sides first, you may have been able to pull the box without removing the ignition cylinder and lock. It blocks the left hand side of the box just enough. I don't have my shop manual handy - ran out of disk space on my office computer - but I think all you do to pull the ignition is insert the key, turn it to position 2, and remove a single allen head bolt. The whole ignition assembly then detaches from the steering column. My memory is a bit fuzzy on it, that's probably close but not exactly right.

Re: the foam, take it all off. I went after it with a shop vacuum. No need to attempt a replacement, the HVAC system works fine without it. You need it to get that perfect, German seal on the doors, but really, you won't miss it. Actually, you're wife or girlfriend (possibly both ) will appreciate not having little black specs of foam deposited on her clothing while riding in your car.

- JimY

[flanso]

Jim, Your memory is just fine; the ignition switch removes pretty much as you described. Even with the sides of the box off, I had to take out the ignition switch to get it to clear. I couldn't find many threads about W124 heater core failures so I am not planning to replace mine. Did you replace yours? What did you do about the disintegrated foam around the heater core? I am wondering if there is a possibility of the core rattling in the box. Thus far, I have spent about 12 hours over 5 days and am going to resume Monday. In the meantime, I occasionally walk out to the garage and gaze at that heater box contraption and pile of interior parts laying on the floor.
Frank

[jcyuhn]

I didn't replace the heater core. Have heard of only one leak - Larry Bible did his within the last year. So I wouldn't worry about it.

I don't remember any foam around the heater core. Guessing that it was completely gone on my car! Sure didn't miss it. Nothing rattled.

Took me about 20 hours total to replace the evap and vacuum elements. Sounds like you are on track for the same.

- JimY

Yes indeed there is lots of foam around the heater core. A new heater core comes with all the foam strips necessary to seal it up and keep it from rattling.

I would get some weather stripping from the home center and seal it and pad it with that.

Good luck,

[flanso]

I accidentally pulled off a couple of my masking tape labels from the pod vacuum lines while trying to reconnect them. I am left with two unlabeled lines, one is red with a white tacer coming from manifold connection 5. The other is red with a blue tracer coming from manifold connection 7. They go to the pod that mounts on the right side of the heater box. But I do not know which of the line goes to the lever side of the pod and which goes to the nipple on the top of the pod. Anyone know? Thanks for the help, both past and future!

[jcyuhn]

OK, here's your answer. The red/blue line goes to the side nipple on the vacuum element. This is the "small lift" portion of the vacuum element. This line is connected to spot #7 on the vacuum valve. The red/white line goes to the bottom nipple on the vacuum element. This is the "large lift" portion of the vacuum element. It is connected to spot #5 on the vacuum valve.

You'll find the system operates as follows. When the car is first started and the aircon switched on, #5 will have vacuum, #7 will not. This will close the defroster flaps about 85% of the way, leaving just a bit of refrigerated air directed to the windshield. It will remain in that mode for ~30 seconds, then #7 will be activated, completely closing the defroster flaps. Both #5 and #7 must have vacuum to completely shut off flow to the defroster nozzles.

A common failure is for the small lift portion of the vacuum element to fail. When this happens, the defroster flaps don't quite close all the way. Enough cold air is directed to the windshield that condensation forms on the outside of the windshield on warm, humid mornings. I've seen bunches of older MBs driving around like that. Replacement of the vacuum element is the cure.

My $.02 worth of rambling commentary,

- JimY

[flanso]

Jim, I got all of the vacuum lines hooked up, thanks to your advice. I checked the pods with a Vacuala brake bleeder. I had a difficult time (about 90 minutes worth) getting the underhood lines to connect to the expansion valve; apparently the replacement ACM evaporators's lines need just a little bending. There isn't much room to do that once the evaporator is in! Then there was one of the three 5 mm screws that holds the metal mount for the two upper servos that wouldn't line up with the threaded, sleeved hole in the upper flange on the heater box. I finally gave up on that one but I think it is secure with the other two screws plus the two 8 mm studs that attach at both corners. I was going to replace the two foam drain lines with 3/4" heater hose. I tried Advance Auto but they did not have the heater hose. Plenty of pine tree deoderizers to hang from the rearview mirror but no heater hose. I miss the real parts stores of yesteryear. Tomorrow is another day, and I think I'll finish. I have about 18 hours in thus far.
Thank you again for your help, Frank

[jcyuhn]

Sounds like you are doing fine. I used an ACM evaporator (with expansion valve pre-installed) and had no difficulty with the underhood lines. But I suppose every job is different.

Can't picture the screws you are having problems with. I replaced all the vacuum pods, but don't they just rotate to release the tabs, and then pull out of the mounting brackets? Or am I thinking of some other car...

Try Lowes or Home Depot for heater hose. My local Lowes has a huge variety of rubber & plastic tubing in the plumbing department.

- JimY

[flanso]

Jim, On my car, the top of the box is held in place by a metal bar that spans the width of the heater box and has two holes on either end that go over the two 8 mm studs that protrude from the firewall. Onto that bar fastens the two top pods. The bar has three additional holes through which are screwed 5mm screws into the top flange of the box. I think my box sagged a bit and I could not get one of the 5 mm holes to line up. Oh, well.

Ace Hardware had some .75 inch ID vinyl that made very nice condensate drains, see-through at that. I got enough of the car reassembled today to start it and move it; a very good sign. There are still some interior trim pieces to reinstall but it is all down hill. The going price here in Naples for recharging an air conditioner is $40 per pound for 134A, plus labor. That seems high for the refrigerant so I thought I would recharge it myself. Robinair and others make venturi-powered vacuum pumps, about $16 at Harbor freight, and I thought I’d buy one and use it to suck out the system. I don’t have AC gauges so I thought I’d buy 134A refrigerant and the PAG46 oil in can sizes that total up to the systems capacity. Is it essential that I use gauges to first see if the system will hold vacuum? Do 134A cans have sufficient pressure to trip the low refrigerant switch so the compressor clutch will engage to enable me to continue filling? Also should I assume that all oil is out of the system and reinstall the total specified or does some oil remain in the system even after a good vacuuming? Thanks, Frank

[jcyuhn]

I think there are better approaches that won't cost any additional money.

For starters, the venturi vacuum pump won't do the job. You need to pull a deep enough vacuum that water boils inside the system - difficult or impossible to do with a venturi. And you need a set of guages to insure the system doesn't leak down and also to verify proper operation when you get it running again.

Autozone is pretty good about loaning vacuum pumps, and possibly guages. You basically buy the pump ($200), they refund your purchase when you return. I'm not positive they loan guages, but I've read (here) that Harbor Freight has them for ~$39. Seems worth the investment. Might be able to find guages at a rent-all type of place.

Because the system has been sitting open for several days, you need to replace the receiver/dryer. It's an easy task, just be sure to get all the o-rings for the inlet/outlet and switches on the darn thing.

You'll need to add oil for certain. Replacing the evap and dryer will require - I'm guessing here - four ounces. Best bet is to purchase a bottle of PAG46. On a 124 it is very simple to open the low side line just in front of the forward firewall - there is a threaded connection there. The oil can just be poured in. You'll of course need to replace that o-ring as well. Don't forget to lube the o-rings. Refrigerant oil is good, Nylog is better (if you can find it).

Vacuuming does not remove any oil. You just need to make up for what is lost when the old evap and dryer are changed.

I agree that $40/lb for R-134a is too steep. Even with the price increases this year, single cans are going for $11 in my part of the woods.

Your system likely calls for 1.0 kilogram of refrigerant. Conveniently, this is almost exactly 3 12oz cans. You'll be undercharged an ounce or two, but that's better than overcharged.

With the system under vacuum, it will draw in almost the entire first can of refrigerant. Before starting the engine, add the first can as liquid to the high side port. When the flow of refrigerant slows, close the manifold valves, start the engine, switch on the a/c, and continue adding through the low side port (as vapor, of course). It doesn't take all that much refrigerant to trigger the low pressure switch.

And don't forget to purge the air out of the manifold hoses before adding refrigerant!

I'm going to be offline for about 10 days. Best of luck. I expect cold air when I return!

- JimY

[flanso]

Welcome back, Jim. I could not find a pump or gauges for rent or sale in Naples or the nearby, larger community, Fort Myers. So I ended up ordering inexpensive ones from thetoolwarehouse.net. and they arrived yesterday. I like tools so I am happy to have these two new ones. I used your recommended technique and ended up with 53psi/395psi and 62° outlet air on high fan with 85° ambient and 80+% humidity. You were right, the first partial can as liquid into the high side was enough to trip the low refrigerant switch after the engine was started. Anyway it's great to be cool again!

I had a few leftovers as you can see. I am going to look around for any obvious holes that need screws. Those two clips hold the 8mm bolts in the dash bottom that attach to the console. I left them out intentionally. Any ideas on the two small screws? They match the two that hold the temperature dial panel together with the rocker switch panel. I'm not going to go back in; I'm curious though.

Thank you again for your guidance; I would still be struggling to get that box apart while it was fastened to the firewall!
Regards, Frank