1983 300D - Electrical (or what I did to my W123 today) - Page 4 - PeachParts Mercedes-Benz Forum

konstan · #46 11-29-2016, 05:57 PM

Quote:

Originally Posted by 97 SL320

Using a digital meter can lead you to miss diagnosis. Most digital meters don't load the circuit down and leakage will make it look like you have 12 but when any real load is applied, voltage goes away. An incandescent test light works better here.

True but pin 6 has only 6V with the cluster disconnected, so we know there is at least that problem outside of the cluster... I thought about applying 12v to the cluster side of pin 6 to test but decided against it. May do so later, after I figure out why pin 6 does not have 12V

konstan · #47 11-29-2016, 07:19 PM

Quote:

Originally Posted by funola

Did putting incandescent bulbs in the brake wear and brake warning sockets fix the problem of the LED's being always faintly lit? Did you probe with your Fluke meter further up the chain starting from pin 6?

Yes the incandescent sin the brake wear and warning lights fixed that problem. They go off after I start it up and rev it a bit

Having trouble getting behind pin 6. Those cables are very rigid. Do I start disconnecting all the square connectors and removing the ziplock ties that hold the cables in place?

funola · #48 11-29-2016, 07:44 PM

No need to get behind the wire on pin 6 yet. Disconnect battery + cable (remove power when checking resistance), remove fuse 12, key off, use the Fluke to measure resistance of the wire from pin 6 to the junction of fuse 12.

If the wire has good continuity, put fuse 12 back in, turn key on, measure resistance from pin 6 to terminal 30 of light switch, then to C105. C105 is the terminal block on passenger wheel well. I'd tighten those 3 screws which may be lose and thus your problem. Also check tightness of the screw on terminal 30 of key switch.

konstan · #49 11-29-2016, 08:06 PM

Weird. NO continuity i.e.open circuit between pin 6 and the copper fuse bus in the fuse box. That's with the key on too. Those 6 volts on pin 6 must be coming from somewhere else

konstan · #50 11-29-2016, 08:21 PM

Also I got cold so I attached the oil pressure feed and moved the car to the garage. While it was running I measured pin 6. Same 6 volts. FYI, the voltage on pin 6 fluctuates if I try to stick my hand in there, close to the fuse box, and try to dislodge em. I've seen it get up to 9 volts.

Ground pin 15 is good, I checked that too

funola · #51 11-29-2016, 08:39 PM

Time to find and check the BU/RD wire connection at the fuse box fuse 12 terminal 15 per diagram

Diseasel300 · #52 11-29-2016, 08:39 PM

Looking at the schematic posted above, if you have a solid 12V on Fuse 12, but a different or no voltage on Pin 6, you have an open connection between the Fuse 12 terminal and Pin 6 in that Blue/Red wire. Either the wire doesn't have a solid connection to the terminal on either end, or the wire itself has a fracture in it. There isn't much going on in that circuit.

Phantom voltages measured on Pin 6 can simply be "ghost" voltages that the digital meter thinks are real due to a capacitive effect of a poor connection. Use a 12V light from Pin 6 to 15. If it glows, you really do have 6V present, likely from a highly resistive connection. I'd lean more towards the DMM reading a phantom voltage.

Diseasel300 · #53 11-29-2016, 08:44 PM

Funola beat me to it! Out of curiosity, what do you read from 5 to 15? According to the schematic, if Fuse 12 is powered and continuity is good, it feeds 12V back through Pin 5 to the brake wear and warning indicators.

If you have a good 12V there, you know the problem lies in the blue/red wire to 6

funola · #54 11-29-2016, 08:45 PM

I'd lean towards a loose BU/RD wire connection at the fuse box since he was able to make the 6V change to 9V by shoving his hand on the fuse box. No disrespect but phantom voltages not in play here. A wire can't pick up 6V capacitively next to a wire with 12 V.

konstan · #55 11-29-2016, 08:52 PM

How do I go about getting all those wires out of the way? Really start disconnecting everything there?

Diseasel300 · #56 11-29-2016, 08:58 PM

A DMM can and does read voltages not really present. They place virtually no load on the circuit they're testing, and if a wire is fractured internally, there's enough of a capacitive effect between the broken pieces of conductor to fool it into thinking there's a voltage there. An analog meter or test light will not report such a thing since they load the circuit they're testing.

To Konstan: Check for 12V on Pin 5 on your plug. If you have 12V, find Terminal 15 on the bottom side of your fusebox feeding Fuse 12. Verify that you have a Blue/Red wire connected to it, you may find that the wire itself is broken off or clinging on for dear life.

As an aside/PSA: keep your eyes open when you're inside the fusebox for melted or chafed wires. My SDL had a hot relay that created quite a mess of melted insulation leading to a horror-show of shorted wiring.

funola · #57 11-29-2016, 08:59 PM

Can't help on how to get to the back of the fuse box. Do a search. Don't start disconnecting every wire, you'll lose track and won't be able to put it back together.

Diseasel300 · #58 11-29-2016, 09:03 PM

Another thought comes to mind: Disconnect the battery and set your multimeter on Ohms. With the ignition switched on, measure from Pin 5 to 6 on your cluster plug. You should read a short, or near short circuit (extremely low resistance).

Regarding the fusebox: If you're careful, you can lift it enough to poke around in there without disconnecting anything. Make sure the battery is disconnected before lifting anything!

konstan · #59 11-29-2016, 11:04 PM

Quote:

Originally Posted by Diseasel300

Another thought comes to mind: Disconnect the battery and set your multimeter on Ohms. With the ignition switched on, measure from Pin 5 to 6 on your cluster plug. You should read a short, or near short circuit (extremely low resistance).

Regarding the fusebox: If you're careful, you can lift it enough to poke around in there without disconnecting anything. Make sure the battery is disconnected before lifting anything!

5 to 6, open circuit.

I undid the nut that held the fuse panel from the engine compartment, removed a big aluminum thing under the dash and managed to access the fuse panel that way. Working on tracing wires now.

funola · #60 11-29-2016, 11:14 PM

Quote:

Originally Posted by konstan

5 to 6, open circuit.

I undid the nut that held the fuse panel from the engine compartment, removed a big aluminum thing under the dash and managed to access the fuse panel that way. Working on tracing wires now.

From your earlier open circuit resistance measurement from pin 6 to fuse 12's terminal 15, you would expect from pin 5 to 6 to also be open circuit.