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Car Forum / Mercedes-Benz Cars / March 2005Yet another heater problem on a 1980-123 chassis
Hey all, I got my rebuilt servo installed last fall with great success. HOWEVER, here it is 5 months later and the fan speed is messed up. Temperature control hasn't been great for about a month. The fan starts out OK (not full speed) then reduces to zero after several minutes. DEF position works for a while, but the fan still dies out. If I shut off the car and restart, I get heat again. No burning smell (i.e. I don't think it's the fan) but could it be the amp? As far as I know it's original 1980. It wouldn't be the sensor under the right front speaker, would it? AJ The amp is what you need; its behind the glove box which you must remove to get to it. Thanks TG -- it's sorta what I expected. You wouldn't have any leads would you?? AJ There are a couple of shops that sell the amplifier as rebuilt - about US $100. I believe on-line parts houses i.e Benzbin.com, Silverstar.net etc. also sell these. My experience is that bizarre behavior occurs when the amplifier is kaput, and the system is dead when the servo is kaput although my servos usually cracked their plastic bases. Unfortunately one is never sure which part is broken so I always start with the cheaper part - the amplifier. One seller told me that 20% of the rebuilt servos were defective and it's best to keep the old core for a few weeks to be sure the rebuilt servo works before sending the core for credit. Too bad you just replaced the servo for Performance Products sells a digitally controlled kit that replaces all this crap - not cheap but all new state of the art parts, about $700. I intend to buy one of these upon the next "event" with the OEM system in my '80 116 (its long overdue for one). Looks like +$100 will get a rebuilt amp. Bizarre behaviour is right!! Even on "DEF" the fan dies off to nothing. I wonder if trouble shooting might help, like put a V-meter on the power supply to the fan when it's doing this weird stuff. 12v means the amp is OK, and the fan is the problem, right? 1v means the amp is screwed (or possibly the "new" servo). Will source an amp first thing Monday. AJ "Looks like +$100 will get a rebuilt amp. Bizarre behaviour is right!! Even on "DEF" the fan dies off to nothing. I wonder if trouble shooting might help, like put a V-meter on the power supply to the fan when it's doing this weird stuff. 12v means the amp is OK, and the fan is the problem, right? 1v means the amp is screwed (or possibly the "new" servo). Will source an amp first thing Monday. AJ " You can easily rule out the fan motor without testing it. As TG pointed out, erratic behavior is often the amp or servo which drives the whole system. When you move the temp wheel from min to max with the cabin at about 70, the system should cycle from max cooling to max heat. Among the things that change while doing this are the flaps to divert air and the blower speeds. The air should be all coming out the dash vents for max cooling and from the footwells, with some bleed to the two side dash vents, for max heat. When you move the wheel from min to max, the flaps/air should change over and at the same time the blower should go from max cooling speed to max heating speed. And of course, hot air should show up at the footwells too. It does this changeover in steps over a min or so.T he max heat blower speed is one speed less than max cooling speed. If it's the servo/amp, you will typically not get the correct flap/air movement, nor the blower speed changes, heat, etc. If it's the blower motor, then just the blower will be erattic and will warrant further investigation of the blower/circuitry. If you are convinced it's the servo, before you buy a new one, there is one last test you can do. That is to drive the servo directly. There is a small motor and gear train in the servo that in one direction winds over to max heat, in the other to max cooling. You can disconnect one harness from it that has the wires that the amp uses to drive this motor. With jumpers, you can momentarily connect a 9V household battery to the two terminals on the servo that drive the motor. With one polarity it goes to max heat, the other maz cool. You can hear it run. The terminals are shown in the wiring diagram in the service manual. If you do this make sure you don't leave the battery connected for more than a couple seconds without hearing the servo motor running There is no cutoff, so when it reaches the end of travel, if left on too long, you will burn the motor out. By moving it to max cooling, then starting the car with the one harness still off, the system should be at full cooling. Vice/versa for heating. If that works, it's most likely not the main part of the servo that's a problem. However, there is one part inside the servo that could still be a problem. That's the feedback potentiometer. This is part of the temp chain of resistors that together with the feedback amp drive the servo. The chain is an outside temp sensor, the dash thumbwheel, and the feedback resistor in the servo. If any one of those is intermittent, you get erratic performance. The pot in the servo, like all pots, is subject to getting bad spots, like the old pot type volume control on radios. If you take the top of the servo off, the pot is sitting there, driven by the gear train. Some electronic contact cleaner sprayed into it has been known to rejuvinate them. It's also a good idea to always cycle the system from max heat to max cool at least once a month. That moves the gear train, pot etc through its full range and keeps it lubricated. hello, Great information you provided, i too have a servo problem on my 300sd 1980. Would you know which terminals for the motor on the servo are they 1 and 5, green and brown? i have taken the servo top off and there are 4 screws holding it down inside. if these are removed does this give access to the unit for viewing? please advise brian According to my diagram, the servo motor is driven by terminals 4 black and 5 green which are on the servo connector nearest the front of the car. Terminal 1 is brown. If you try driving it with jumpers, use a 9V household battery and make sure you only keep it connected for a couple seconds unless you hear the motor running. When it reaches the end of either direction, there is no cuttoff and you could burn it out. As far as taking the servo apart, it's been quite awhile since I took the top off mine. I recall taking out the 4 screws from the top plastic cover, but don't recall exactly what's under there. I think that was all it took to gain access to the gear driven potentiometer which can be cleaned if it's dirty and making poor electrical contact. Beyond access to that, there is no point in going any farther. There are more gizmos and widgets inside that thing than a german koo koo clock and you'll never get it apart, much less back together again. hello, Thanks for the input. Unfortunately i dont have the workshop book on this car, so i have been using the one that's available on the net. The schematic diagram is very hard to read though, but good for other stuff. see............. http://skinnerbox.steaky.org/Service/W123/Index/110index.html I have just bypassed the servo, only on the input side, this gives me heat now, but all the time. Do you know the purpose of the other pipes, nearest the fire wall. I have left them connected. They are interconnected inside the servo, i guess a return feed back into the heater again. i blocked off the two pipes that i bypassed with a plug/clamp. For those people who don't want to spend the $400++ for a new/rebuilt one this an option. I live in california, and if i cannot get the servo to work i will probrably just install in the engine compartment an on/off valve in the hot water feed side. For those interested, is the pipe that comes from the back of the engine block and goes the Aux pump. Most older cars had this on/off switch feature before modern auto HVAC systems. Again many thanks for you patient. brian hello, Thanks for the input. Unfortunately i dont have the workshop book on this car, so i have been using the one that's available on the net. The schematic diagram is very hard to read though, but good for other stuff. see............. http://skinnerbox.steaky.org/Service/W123/Index/110index.html I have just bypassed the servo, only on the input side, this gives me heat now, but all the time. Do you know the purpose of the other pipes, nearest the fire wall. I have left them connected. They are interconnected inside the servo, i guess a return feed back into the heater again. i blocked off the two pipes that i bypassed with a plug/clamp. For those people who don't want to spend the $400++ for a new/rebuilt one this an option. I live in california, and if i cannot get the servo to work i will probrably just install in the engine compartment an on/off valve in the hot water feed side. For those interested, is the pipe that comes from the back of the engine block and goes the Aux pump. Most older cars had this on/off switch feature before modern auto HVAC systems. Again many thanks for you patient. brian The coolant flow through the heater core is basicly a loop from the engine through the heater core and back. The coolant valve in the servo basicly is a shunt across the middle of this loop. In the full heat position, it sends all the coolant through the full loop. In cooling mode, it shunts all the water across the loop, so it only goes from engine to servo and back to engine. At servo positions in between, it only shunts some of the water. I don't see bypassing the servo with a valve under the hood as a viable solution. Doing that, you have no control over the temp, amount of heat/cooling, which vents it comes out of, etc. That leaves you with no fresh air coming in and cycling the system on and off to try to maintain temp is going to get tedious real quick. It also won't work when you have days where you need some heat in the am, maybe some cooling in the pm, etc. hello, Yes, i agree it will be tedious. Other than buying a kit which uses a cable from under the dash to control the on/off, do you have any suggestions or work arounds. I don't see paying $400 for a device that is problematic and may or may not fix my entire problem. Some of the vacuum units under the dash may also have failed including the amp. my 1980/300sd has 235k miles, but still going strong mechanically. i may just have to live with it. i also have 82/240d with manual HVAC, works great! what do you think? brian hello again, Just came across a great site discussing the Acc servo, but its listed under Chrysler and not mercedes. I assume its the same unit. http://www.duricy.com/gallery/view_album.php?set_albumName=The-AutoTemp-II- Servo What it shows is the internal problems and working of the unit. brian "hello again, Just came across a great site discussing the Acc servo, but its listed under Chrysler and not mercedes. I assume its the same unit. http://www.duricy.com/gallery/view_album.php?set_albumName=The-AutoTe... What it shows is the internal problems and working of the unit. brian " Yep, that's it. Must be photos from the museum of worst engineered products ever. It was designed by Chrysler and used on their Imperials. Great idea, put elec motor, pot, contacts, vacuum valves, gear drive, and 200 deg coolant all in the same small unit. There are only two real solutions I see to your problem. Either replace the servo or go with a conversion kit that replaces the servo/amp with a whole new design that's microprocessor controlled. For the servo route, I'd recommend Performance Analysis in Tenn. For about $500 you get a rebuilt servo that has a bottom section made of aluminum instead of plastic. That eliminates one major source of problems. PA also guarantees it for 1 yr and after that if it fails, they will fix it for $100 for life. I've had one for over a year now and it works fine. The conversion solution I think is around $700. As for the vacuum actuators and the rest of the system, with a shop diagram, a vacuum tester, and a VOM, you can test a lot of it to verify that it works by applying vacuum to the various lines, measuring resistance, etc. But you're right, you can't test it all. In my case, I was sure it was the servo. Hello again, Thanks a million for you assistance, i have a plan now. Understanding the workings is what its all about. Brian |
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