Heater Control Unit - Fried

northcave

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300sl/1992/3l
When i had my alarm fitted there was a clicking behind dash in-front of the passengers seat. This went on for about 2 weeks before suddenly a lot of smoke appear from the heater control unit. The clicking stopped at that point and ever since the control for the heater has been broken.

Part number is: 129 83004 85
Car: 129 - 300sl (1992)

A side effect is that the auxiliary fan in the engine bay turned on constantly. I am not sure if that was because the wiring through the heater control was no longer working and so the engine assuming the water temp was high.

Anyway attached is a picture of the fried unit. I am hoping that it was simply a loose connection and that replacing the unit will fix the problem.

1. can these be fixed? You can see in the picture where is has fried.

2. if not where can i get a replacement?

3. based on the info above do you think it is actually a problem with the unit?

Thanks in advance for you help.

control_unit.jpg


control_unit2.jpg
 
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J

jetjockey

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Purely from an electronics point of view it looks easily repairable after a clean up of the circuit board.

Unfortunately, you are going to need another fully working unit for comparison to get the component values. Unlike the row of four identical resistors on the right of the photograph there doesn't appear to be a double of the one that is burnt.

The next question then has to be............ Has anthing external to the control unit caused the trouble. You will have to do some checking at the connector with a multimeter I guess. Difficult though without knowing what the control unit triggers on what output.

It could well be that the burnt resistor is in the supply line to the output switching control circuit. Could also be possible that it has been dry joint for a while hence the clicking. Eventually any intermittent connection goes high resistance and can easily get hot and burn.

This is all supposition of course without seeing the unit.
Gerald.
 
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television

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There look to be 4 components that have to be replaced, but as jetjockey what are the values.

I would like to look at the circuits to comment on the damage,, one dare not just plug in another or that can go the same way
 
J

jetjockey

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Yes

The components either side of the resistor are Diodes. One of them looks to be a Zener. One of them (next to the resistor) also looks to be dry jointed. There is a bit of smoke damage to the diodes but they look as if they will clean up using some Iso Propyl or even some Acetone thereby revealing the ident codes. The resistor though is fried. I suspect the only remedy is to compare with a new unit unfortunately.

I wonder if somebody on this forum could help with a photograph?

Gerald.
 
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northcave

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Thanks for the advice guys. If anyone wants a stab at then let me know and I'll post it over. Failing that I'll see if i can fine one at the breakers. Might be lucky though.

Tim
 

television

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I will have a look over the week end to try and see a connection between the fan and this control box.

It is sometimes times possible to work out the value of a burnt resistor by measuring ½ of it,,and you can get quite near to the value
 
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northcave

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I will have a look over the week end to try and see a connection between the fan and this control box.

It is sometimes times possible to work out the value of a burnt resistor by measuring ½ of it,,and you can get quite near to the value

Thanks. I removed the old resistor and looks like the board below is damaged, or at least the small strips that ran underneath are. Not sure if that makes a big difference.

Tim
 

television

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Thanks. I removed the old resistor and looks like the board below is damaged, or at least the small strips that ran underneath are. Not sure if that makes a big difference.

Tim

Printed circuit boards can be repaired and the tracts bridged

I can have a go on it for you if you wish
 

Wirral_guy

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I would be worried about what caused the resistor to fry in the first place before fitting a new unit or re-fitting yours after repair. Interesting point about the clicking noise after the alarm was fitted - it's possible that a fan was disturbed\blocked or jammed during the fitting or maybe a wire shorting on an earth somewhere or, worse, the alarm wired up in/near/on the heating circuit.

It's unusual for a resistor to burn out of it's own accord unlike, say, a capacitor as it is purely passive. It normally points to a failure elsewhere rather than itself being the failure point - either a surge, short or over-voltage.

Impossible to trace from your pics but I'm guessing that resistor is not far away from an input pin (via the tracks on the board not actual distance ;) ) and an external influence has caused this.

Dave
 
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northcave

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Yeah I agree Dave but I don't have much in the way of options unfortunatly. My beat bet is to put a working unit back in and try it. If I hear the clicking again I'll be certain to remove it immediatly for sure.

What are my options other than taking it to a garage if I can't solve the problem. I'm pretty sure any Mercedes specialist will go round in circles for days with this problem and hit me with a huge bill for not fixing it.

Thoughts?
 

television

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My thoughts were the same as Wirrel guy,, this unit does control up to a point the air con rad fans,,and you must back track on what you have done on the car, or if someone else fitted the alarm, then you must have a go at them.

If you get another control unit you can use one from a 129 from the following cars,,your unit is the last in the updates

129.067 sl500
129.058 sl280
129.060 sl300
129.061 sl300
129.063 sl320
129.066 sl500
129.067 sl500



Not
129.059
129.064
129.068
129.076

If you get another one, then some good photo's of the inside before you plug it in.

I would like to know the pin number the damaged parts go out or in on
 


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