What happened here ?? fuseholder burned 88 300CE - PeachParts Mercedes-Benz Forum

JOE49 · #1 10-07-2012, 02:31 AM

Fuse "D" melted plastic on fusebox. '88 300ce. 1st. Benz for me (112,505 miles). All fuses are new, copper (Flosser), OVP is new too (per forum advise), AC was converted to R134a at 85,000 miles by orig. owner at Beck Benz of N. Carolina. Was evacuated & re-charged at 112,982 miles due to AC blowing cold when first turned on, but after parking & re-starting no more cold. Now has 113,072 miles & fall is here, so tried heat & no hot. Looked under fuse cover yesterday & found melted plastic at fuse "D" . Removed screws of cover to inspect beneath & ran out of daylight. Am going over wiring diagrams found at Benzworld. What could have caused this ?? Where to start ???

sixto · #2 10-07-2012, 03:01 AM

First, how about using the default font? That eye chart is difficult to read. In fact for the benefit of others, here's your post is a more legible form -

Quote:

Originally Posted by JOE49

Fuse "D" melted plastic on fusebox. '88 300ce. 1st. Benz for me (112,505 miles). All fuses are new, copper (Flosser), OVP is new too (per forum advise), AC was converted to R134a at 85,000 miles by orig. owner at Beck Benz of N. Carolina. Was evacuated & re-charged at 112,982 miles due to AC blowing cold when first turned on, but after parking & re-starting no more cold. Now has 113,072 miles & fall is here, so tried heat & no hot. Looked under fuse cover yesterday & found melted plastic at fuse "D" . Removed screws of cover to inspect beneath & ran out of daylight. Am going over wiring diagrams found at Benzworld. What could have caused this ?? Where to start ???

Fuse D is for the aux fan. I think there's only one fan in an '88 CE. I don't know why it didn't blow in a peaceful manner. It looks intact in your pictures.

While the aux fan usually comes on to cool the condenser, the aux fan circuit is isolated from the ACC ciruit. The only common wiring is a shared ground for the refrigerant low pressure switch and aux fan. Logic suggests you can't short circuit a ground wire in a manner that will overload the circuit but I'll defer to an expert.

The first thing I'd check is the aux fan. Does it spin freely by hand? Is there axial or radial play in the motor? Disconnect the fan from the harness and momentarily hook it directly to the battery. It will spin backwards if you get the polarity wrong but it won't harm the fan or battery for this diagnosis. Increased electrical load from a stuck or binding fan overloads the circuit and should blow the fuse. But if the fan passes the basic tests, the fault likely lies elsewhere. Maybe relay B (K9 = high speed) or C (K10 = low speed) is bad. Or there's a short in the power wires to the fan.

Quote:

Was evacuated & re-charged at 112,982 miles due to AC blowing cold when first turned on, but after parking & re-starting no more cold.

Sounds like a Klima relay issue, not a charge issue.

Might the aux fan have been modified to run continuously when the compressor is engaged? Otherwise reasonable techs have done sillier things.

Sixto
87 300D

vstech · #3 10-07-2012, 12:21 PM

Welcome to the forum, this is the Diesel discussion, but your question is on a gasser.
since it's electrical in nature, we can likely help, but it's more correct to post in the tech help section in the future.
actually, I think I'll move your question over there, and leave a redirect here to keep us in the loop on the fix for you.

JOE49 · #4 10-07-2012, 01:15 PM

Not too great with messaging,hope this is the default font,Sixto. Aux. fan spins freely by hand. Where to disconnect to test fan? But for head lights & radio,wiring appears unmodified & original. Motor is quiet & steady, no unusual vibrations. Looking over wiring diagrams now. Pulled out relay D & C yesterday(pn.0015420219 on both) & relay B & C look identical (did not pull them as yet). <> Vstech,appreciate the re-direct & greeting. Will eventually figure out the forum protocols. Thank you both!

pmckechnie · #5 10-07-2012, 02:35 PM

If it has the correct fuse installed, then the problem is a bad connection at the point of the melting plastic. It could be just a bad connection at the end of the fuse or where the wire hooks to the fuse holder. If the problem was anywhere else, the fuse would just blow. A bad connection causes resistance. Resistance + amperage = heat. At this point, it may be hard to fix the problem without replacing the fuse block. It may be possible to carefully clean the fuse block contacts and connections and make it work again with a new fuse. I would recommend you spray the fuse block with something like WD40 and replace all the fuses. If you can't get the bad fuse holder to work, you can rewire it to an external fuse holder available from the dealer.

Paul

pwogaman · #6 10-08-2012, 01:17 AM

Your "D" fuse slot (Green Circle) melted because the auxiliary fan was running non-stop. Could be the temperature sensor (Red Circle, B10/8) on the engine was not sending a proper signal and the ACC Push Button Controller (N22) defaulted to high speed fan-on, which is not via the lesser voltage resister part of the circuit.

I doubt the problem is with the refrigerant high pressure switch (Blue Circle, S32), as that circuit runs via resistor and would not as likely generate the heat necessary to melt the fuse slot as yours has.

Test the B10/8 sender and try reseating its connection. I have gotten an erroneous signal after washing the engine.

Yet another possibility is a sticking K9 (or K10) relay. With the relay removed you should have no connection between pins 1 & 3. If so, then it is bad. When you put 12 volts to pins 4 & 5 you should have connection between pins 1 & 3. If not, then it is bad.

Use a generic blade fuse holder from any auto parts store as a substitute for the damaged D slot.

JOE49 · #7 10-08-2012, 11:51 PM

Fuse "D" is a 16 amp fuse & was melted to bottom of fusebox & did not blow. All wiring beneath the fusebox is not burnt or brittle. Used a dremel & pick to remove melted area & get fuse un-stuck from fusebox. Cleaned the bottom seat for fuse & put a new 16amp in. Now I see that the top part that holds fuse is "sloppy". Removed fuse & pulled/bent top of fuse connection down. All fuses are tight now. I probably didn't realise the fuse wasn't seated properly when i replaced all fuses to copper.

JOE49 · #8 10-08-2012, 11:58 PM

O.K. tomorrow will begin test for aux. fans (hi & lo speeds), B10/8 sensor, K9,& K10 relays.

pmckechnie · #9 10-09-2012, 07:30 AM

Good job on the fuse holder. You can check but I believe you have taken care of the problem. If the aux fan runs all the time, then that needs to be fixed, but it is not the problem you had with the fuse.

Paul

97 SL320 · #10 10-09-2012, 06:18 PM

Something else to consider, these fuses only make contact through a very small / thin line where the cone meets the hole in the contact tab.

Having battled poor contact on my 1980 SAAB 900 ( owned since 86 but now too rusty to continue ) take a 1/4" dia drill bit and , by hand, turn the bit to clean the hole in the tab. This also makes the contact area slightly larger by making a slight taper. To get really tricky, sharpen the bit to match the angle.

JOE49 · #11 10-10-2012, 02:34 AM

Seems that contact with the top part of the "D" fuse was "sloppy". Top retainer contact with the hole in it was up too far, so I reamed the hole in it & bent it down & put a new 16amp fuse in. Cleaned,checked,& compared all fuse seatings for tightness,connected battery. Started car & let idle to operating temp. All seems well. Thank you all for direction,diagrams,& inputs. Investigation/testing of aux.fan & relays for AC & heat continuing.