79 450sl- No Heat!!! - PeachParts Mercedes-Benz Forum

jjewell · #1 11-29-2006, 05:46 PM

I've gone over the entire system with a fine tooth comb; still stumped. When I got the car it had a bad servo, amplifier and aux. pump. Chg'd all parts. I've checked all the sensors and the adj. dial pot. All within proper ranges. I even tried hooking up other amplifiers & servos. When ign. switch is turned on the servo drives what sounds like fully in "some" direction. The adjustment dial has NO impact in the servo positioning. Pretty cold here, sure could use some advice on this problem!!
PS- Yes I did search & didn't find anything that helps beyond what I've already tried. I'm running out of hair for pulling it out!!!

page62 · #2 11-29-2006, 06:55 PM

**************.com sells a "cheap and dirty" heater control fix for 75 bucks. It's better than freezing to death...

http://www.**************.com/node/606

Gilly · #3 11-29-2006, 07:02 PM

Asa temporary fix I'd try running the servo to full or near-full heat, just to make sure the system will in fact heat the interior.
Gilly

jjewell · #4 11-29-2006, 08:05 PM

When I hooked up the spare servo it also drove immediately to what must be full cool because the water path thru it is blocked. The fan speeds aren't the same (only high speed, no slow to high ramp up) I just am not seeing what is causing it to fully close & keep it there regardless of thumbwheel adjustment. It would be great to drive the servo open to get heat, but with this going on how can I get it to drive in the other direction?

Gilly · #5 11-29-2006, 09:06 PM

Figure out which 2 run the motor open and closed and open it, then I think the electrical plug can be kept off, pretty sure you'll still have an operating fan but not sure.
Gilly

ctaylor738 · #6 11-29-2006, 09:48 PM

What happens when you hit DEF? You should get full heat to the windshield. This is the 'default' position. If you still have no heat, I would suspect a plugged heater core. Is the hose on the exit side of the heater warm?

560SELGold · #7 11-29-2006, 11:30 PM

jjewell,

here's an idea -if you don't ger it figured out come spend the winter down here in Florida! Then you can take your time and figure it out between trips to the beach..........Seriously though - good luck with it, I've spent some time in some cold places with no heat - try Norway in Nov-Dec!!!

t walgamuth · #8 11-30-2006, 04:49 AM

another thing not yet mentioned is possibly air in the system. (you may have already checked for this). but sometimes it is easy to overlook the simple problems.

tom w

jjewell · #9 11-30-2006, 07:37 AM

Quote:

Originally Posted by ctaylor738

What happens when you hit DEF? You should get full heat to the windshield. This is the 'default' position. If you still have no heat, I would suspect a plugged heater core. Is the hose on the exit side of the heater warm?

The fan does come on with all air to the windshield but no heat & no position change from the servo. Before replacing the previously inop servo I took the top of the servo off & manually opened the water valve. This of course gave me plenty of heat but no control so I'm sure there is no air in the line problem or plugged up core/line(s).

Does anyone know the function of the diode on the pushbutton control? I know what a diode does in a circuit but could it cause this apparent "full cool" positioning of the servo? Yes, I'm grasping.........

ctaylor738 · #10 11-30-2006, 08:44 AM

Have you checked the change in the resistance values when you move the temp wheel from cold to hot? What do you get?

Also, for $5, George Murphy at Performance Analysis will sell you a very good troubleshooting guide with a lot of additional information about the system. He is also good on the phone, 865-482-9175.

Doesn't a diode act like a one way gate in current flow? If it goes bad, no signal to the servo to move to heat.

I feel your pain, I had the same thing in my miserable '81. It took a servo, amp, pushbutton unit, and a flush of the heater core to get it going.

BTW - your fuses are good, aren't they? Sorry, I had to ask.

Gilly · #11 11-30-2006, 08:35 PM

When that system first came out, I think in 78, there was a good diagnostic routing written in the Intro book for that year.
Gilly

jjewell · #12 12-01-2006, 07:57 AM

Quote:

Originally Posted by ctaylor738

Have you checked the change in the resistance values when you move the temp wheel from cold to hot? What do you get?

Also, for $5, George Murphy at Performance Analysis will sell you a very good troubleshooting guide with a lot of additional information about the system. He is also good on the phone, 865-482-9175.

Doesn't a diode act like a one way gate in current flow? If it goes bad, no signal to the servo to move to heat.

I feel your pain, I had the same thing in my miserable '81. It took a servo, amp, pushbutton unit, and a flush of the heater core to get it going.

BTW - your fuses are good, aren't they? Sorry, I had to ask.

The wheel & the other sensors all have resistance well within the ranges spec'd by the mbz cd info. I think I'll give George a call later today; thanks for that tip. I'm very curious to see what he has to say about the diode & if these symptoms can be caused by it. It acts like a wiring problem almost, since even with the pushbutton switch in off when you turn the key the servo immediately drive to the extreme (which I assume is full cool) & won't budge regardless of thumbwheel adjustment OR temps @ other sensors.

And yea, the fuses are okay. You can't assume anything.

ctaylor738 · #13 12-01-2006, 09:01 AM

Wiring Diagram 10 on the CD shows the current path for DEF, which is the only setting my simple mind can comprehend. That path sends 12V to the amplifier and the amplifier I think puts power to pin 4 and ground to pin 5 on the servo, which is full heat according to George. This path does go through the diode.

If in DEF or full heat on any other setting, you are not getting this power/ground to the servo, then I would suspect the push-button unit, diode, or wiring given that you have replaced all the other components.

You could get emergency heat by rigging up this power arrangement along with an on/off switch.

Richard Wooldridge · #14 12-02-2006, 02:22 AM

Hi there,
I also have a '79 450SL, which I converted to the digital servo unit, the way to go, in my opinion! However... if your fan only comes on in the defrost position and not the other main switch positions, you most likely don't have enough vacuum at vacuum switch 19, the "MAIN" vacuum switch, to operate the electricals for the heater. That switch needs 175 mbar of vacuum to operate, and in my car there were too many vacuum leaks to allow the switch to close. Get a Mity-Vac, or go to Harbor Freight and get their vacuum unit, which is similar but costs about half the price, and check for leaks. I'm willing to bet you will find massive leaks from at least two pod diaghrams, and also probably from the vacuum line connector on the servo unit. You must fix these before the heater will heat, regardless of whether you have the original unit or the digital servo replacement.
Also, just to be sure, check fuse 2, 6, 9, 10, and 12 - fuse 12 is an auxiliary fuse (2 amps) for the amplifier.
When you turn off the ignition, the motor gets a signal to drive the servo to the park position, to shut off the water. It sounds like that is what's happening all the time in your car.
From what I can figure out on the wiring diagram, that diode on the switch pulls the amplifier line number 2 to +12 volts in the defrost position, which should cause the servo motor to drive the arm to max heat and blower, provided fuse 12 is good. Also in the defrost position, relay 18 is energized through the main switch from fuse 6, through preresistance 10, (in from fuse 6 on pin 2, out on pin 4) to switch connector pin 1, through switch to pin 4, to relay 18 coil and to amplifier pin 2.
Good luck with it - it's a complicated heater unit. The digital servo replacement is worth the money, I think!

jjewell · #15 12-02-2006, 11:27 AM

Well, I called George yesterday & he's sending me his $5 info. He seems to think the wiring somewhere is the problem since the amp & servo have been replaced.

Looks like I'm tearing into it today after I print out some of the diagrams. If anyone else out there has had problems similar to mine please reply to this. Meanwhile I'll keep you all posted on my progress & thanks for all the responses!