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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53043
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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1987 Mercedes 300D Turbo. No heat, fans a/c nothing until

Customer Question

1987 Mercedes 300D Turbo. No heat, fans a/c nothing until the car fully warms up. Then I have to turn off and turn on car only after hot and everything works normal. I already replaced mono valve problem continues. In others words no power to mono valve or climate control until car is fully warm usually about 20 min. On a hot day everything works fine and immediately without any wait time. Is this a climate control issue or something else.
Submitted: 17 days ago.
Category: Mercedes
Expert:  Marty replied 17 days ago.

Hi Peter, my name is***** 7 powers the mono valve and the climate control. If you do not have 12V to the mono valve or the climate control with the key on you must have a problem with the fuse circuit. Have you cleaned or replaced fuse 7 yet? A bad fuse connection could cause this problem. If the fuse connections are good you will need to test for power to fuse 7 when nothing is working. If you have no power to the fuse you may have a bad ignition switch.

Let me know what you find.

Customer: replied 16 days ago.
Ok I will try to do this tomorrow. Remember I have no power to the mono valve if the car is cold or on a cold day. If the car is hot or its a hot day not a problem power is supplied and all works perfectly. On a cold day soon as I start to drive i get hot air and no control of the climate control unit. After 20 to 30 min of driving if I pull over turn off the car and restart everything works perfect and now i can control hot or cold air and all the varying speeds of the fan hi / med / low. Also when the car is cold I can not turn on the fans at any speed, nothing happens when I push any of the buttons of the climate control. But once The car warms up power is supplied to mono valve and climate control unit. So are you sure this is a fuse or ignition problem? Its just really weird and has me stumped. Thanks Marty
Expert:  Marty replied 15 days ago.

If you have actually tested the wires to the mono valve and neither wire has 12V then the problem has to be the fuse circuit which comes from the ignition switch then to the fuse and then to the valve and the climate control unit.

Customer: replied 15 days ago.
I've tested and again no power until the car is fully warmed up then I get 12v from the mono valve plug, but when the car is cold no power to the plug or climate control unit. The car has to be hot or a hot day otherwise no power until that happens.
Expert:  Marty replied 15 days ago.

Okay, so now you need to go to the fuse and do the same test. If there is no power to the fuse then I suspect the ignition switch is faulty. When testing for power make sure you are not testing across the wires. Connect your voltmeter or test light to ground and probe the terminals with the other lead.

Customer: replied 14 days ago.
Hi Marty,
Ok so I tested with both test light and multimeter. When I turn on Ignition fuse 7 is energised whether car is hot or cold doesn't matter soon as I turn ignition I got power at that fuse. There is no power to mono valve or climate control unit. Now if I let the car get to operating temp usually about 20 min or if it's a hot day power is supplied i.e. I get 12 volts from the plug to supply the mono valve and the climate control is also energised and working perfect. So it's not the ignition or fuse number 7 they have power. I am back from my road trip so I can check and respond much faster now sorry about the delays. Thanks again Peter
Expert:  Marty replied 13 days ago.

Hi Peter, the valve and the control get power directly from the fuse. It sounds like you have no power leaving the fuse box. If you have replaced the fuse and cleaned the contacts I would suspect a bad fuse box. I'd lift up the box and test for power on the wires leaving fuse 7. It's a straight shot from the fuse to the valve and control unit. This has to be a bad connection at the bottom of the fuse box.

Customer: replied 13 days ago.
Ok I'll lift the box and check that. But why would power supply the wires to the climate control and the valve once the engine is warmed up everytime without fail? When cold no power being supplied to climate control or valve without fail. Even bad connection and all should temperature make any difference besides expansion and contraction on the wires I guess?
Expert:  Marty replied 13 days ago.

It doesn't make a lot of sense but if you are sure of your tests there has to be a fault with the circuit leaving the fuse box. I've attached the wiring diagram below. Notice how the wire from fuse 7 goes directly to the components. If you have 12V at the fuse you must not be getting 12V from the fuse box.

Customer: replied 12 days ago.
Ok took apart the fuse box found the pink wire with white stripe going to fuse 7 and disconnected from the fuse box. With the wire separated from the fuse and the box the wire still has full power with ignition on. Power is still not getting to climate control or mono valve until the car gets hot then climate control and mono valve are energised as normal. So power is coming from the fuse and out of the fuse box unless there is another power box or source that I should be checking. Thanks Peter
Expert:  Marty replied 12 days ago.

How are you testing for power at the mono valve?

Customer: replied 12 days ago.
I use a multimeter and also a 12V positive and negative test light
Customer: replied 12 days ago.
Mono valve gets just shy of 13V when warm and .50v when cold using multimeter
Expert:  Marty replied 12 days ago.

Are you connecting the meter across the two terminals or are you connecting one lead to ground?

Customer: replied 12 days ago.
Hi both terminals...
Expert:  Marty replied 12 days ago.

You can't test it that way. You need to test for 12V to the individual terminals. Connect the meter to ground and probe each terminal for 12V.

Customer: replied 12 days ago.
Ok I'll do that in the am, but remember when it is warm and showing power in at least on of the terminals it's working fine so I'm assuming tomorrow when I try both terminals seperatly and the other to ground are going to register 12v anyways and if so then what would be my next step after this? Thanks
Expert:  Marty replied 12 days ago.

If you are testing across the two terminals you are testing for both power and ground. If you test the individual terminals and one has 12V then we are lacking the ground from the climate control unit. If you have 12V on one wire then our fuse circuit is fine. If that is the case we are lacking a ground from the climate control unit.

Customer: replied 12 days ago.
That makes sense will check for this in the am thanks again
Expert:  Marty replied 12 days ago.

You're welcome

Customer: replied 10 days ago.
Here is a thread and fix with my exact problem, apparently it is pretty common. I replaced my climate control unit and presto. Pelican parts has the forum and fix on this. Share this with your other benz drivers, I was able to find a fix within 3 minutes of searching pelican parts site. Another customer wrote "
I have an '87 300D whose climate control system has a weird latch-up problem:
- start car cold from overnight rest ( outside temp less than approx 65 F )
-user has no control over system operation..system delivers full-hot air via outboard dash vents, blower does not run. Air movement from vents is due only to movement of car..stop car at traffic light, air movement ceases.
-after driving approx 5 minutes, turn off engine and restart, system now normally controllable, blower runs.
-after the restart, sporadic relay cycling can be heard in dash cavity..diminishes with time as car temperature increases.If the overnight outside air temp is above approx 65 F, problem does not occur.
Anyone seen this problem?"There is a 5 volt regulator on the lower printed wiring board within the control assy. This regulator probably supplies the 5 volts for all the digital logic ICs in the control assy. A 33 uF capacitor on the output side of the regulator had gone bad (capacitance much too small) due to age, temperature extremes, etc. The regulator needs a minimum of 22 uF to work properly, per the manufacturer's data sheet for the regulator. I replaced the capacitor and another one just like it that was found on the upper printed wiring board...they both had the same bad appearance on visual inspection. My control assy part number is ***** 830 18 85. Assy works fine after repair. I was having the exact same symptoms as described in the first post. I found the 5 volt regulator and googled up a data sheet for it so I would know which leg was the output. Then followed the output to the capacitor. It was labeled 33/16 along with some other markings and was a brownish plastic cylinder. I replaced it with an aluminum electrolytic capacitor of the same rating although the physical size was a little bit smaller. It seems to be working correctly now.
Expert:  Marty replied 10 days ago.

Hi Peter, glad you got it squared away. I mentioned that the climate control unit was the likely problem if you had 12V at the mono valve. I was going on your statement that you did not have 12V at the mono valve. I asked you twice during our exchange how you were testing for 12V. The problem was that you were testing across the terminals instead of testing the individual leads.