Electrical Puzzler - PeachParts Mercedes-Benz Forum

mtnaggie · #1 01-17-2012, 08:37 PM

Greetings All ~

I have a quandry I need help with. Here's the story:

Dad & I were switching glow plug relays out on his 1985 300 CD California. Dad accidentally touched the fender side with the nut driver when he was putting on the nut to the battery power. Sparks fly and I just know that we have fried some fuses.

Today he gets out and starts to test the fuses. He uses a tester with the key in the ON position and finds that the top row of fuses 2-14 are all dead, though none are burned through.

I have verified that his tester is working properly and does detect current on the bottom row of fuses. Then I verified that when he touches the top and bottom terminals on these fuses, he has no electricity. Fuses A and B on the outside of the first row all work.

Here's where it really gets very weird. The wipers, powered through fuse 6 work. He has triple checked and no current is running through the fuse.

Any ideas what we have blown?

Yak · #2 01-17-2012, 09:26 PM

First, are you measuring current or voltage? It'd be difficult to measure the current through a fuse when it's in operation unless you interrupt the circuit or have loop-style meter. If he's got the meter set to volts and he's measuring either side of the fuse, he should read zero. There should be little to no voltage drop through the fuse. If he's trying to measure amps by probing either side of the fuse with it plugged in, the electricity will bypass the meter and pass through the fuse. He might measure something, but it'd be small.

The top row of fuses is mostly powered by 15 and 15R. 15 gets power in position 2 and start on the ignition, not position 1. That should mean no power on 10, 12, 14 in 1.

Fuse 8 is 15X, that's also only in 2, not 1 or start.

Fuses 4 and 6 are 15R, that gets power in 1, 2 and start. So it's not a surprise it works in 1. He could check the glove box light, too, that's off 4. And maybe the radio.

But fuse 2 is off 30, so that should have power. And many of the odd numbers are for the lights. They'll work regardless of ignition position. Are they on, by chance? If not, then maybe you cooked your light switch.

vstech · #3 01-17-2012, 10:24 PM

how are you measuring the blown fuses? if they are not burned through, then they should be good fuses... unless you melted the holders, or the end caps are burned. you should ground one side of the meter, and probe the top and bottom clip of each fuse to determine if you have voltage to the fuse, and through it.

mtnaggie · #4 01-17-2012, 10:38 PM

O.K., it appears he is using a simple continuity type tester where one end of the tester is grounded, and the other end has a pointed probe. When the probe connects to current, the light comes on.

I'm real lost when you describe the different sources of power for 15, 15R and 15X.

mtnaggie · #5 01-17-2012, 10:41 PM

Oh, and he is touching each of the points in the fuse box where either end of the fuse rests. In other words, if the fuse was not blown, the current on one end of the terminal will flow through to the other terminal.

I don't know that I can describe it any better without drawing a picture with my crayons! I beginning to think I need to go back to riding a bicycle. I can understand the chain thingy pretty well.

vstech · #6 01-17-2012, 10:43 PM

OK...
with the meter set to read DC volts, and in the max 20volt range, the black probe should be on the negative battery terminal, and the red probe should be touched to each fuse top then bottom to see if volts are present in both places.
shorting the supply wire to the glow plug controller should damage nothing more than the supply wire, or the nut driver... or your hand of course...

Yak · #7 01-17-2012, 11:50 PM

Quote:

Originally Posted by mtnaggie

O.K., it appears he is using a simple continuity type tester where one end of the tester is grounded, and the other end has a pointed probe. When the probe connects to current, the light comes on.

I'm real lost when you describe the different sources of power for 15, 15R and 15X.

15R, 15X, 15 and 30 are designations for switched or steady power. They are controlled by the position of the ignition switch.

Ignition off = power to terminals marked 30 (or variations of 30, e.g. K30).

15 and its variations also get power in some fashion depending on the position of the ignition switch, marked by a I or II on the switch, or a 1 or 2 in the manual. Or, in "ON" or "RUN" or "START"

I didn't notice where you said things didn't actually run or start, so is there really a problem to be solved, or looking for info just in case.

If the fuses all look good, then they might be okay. If he connected the wrench from the fender directly to the hot post on the battery, there's no reason to suspect fuses. The power went from the battery through the wrench to the fender. Look for a small burn mark. The fuses are probably fine since they weren't involved.

But to be thorough:

Cycle the headlight switch through all positions. Do the lights work? If so, most of the odd number fuses are okay.

Turn to position I (or ON) try the wipers. If OK, fuse 6 OK. Open the glove box. If light, fuse 4; or turn the radio on.

Turn to position II: try the horn (12 or 14).

Then try to start and see what happens. He may need to check the GP strip fuse if no GP light.

There should be a fuse diagram in the lid of the fuse box, or look for the fuse diagram sticky.

The fuses in the four corners are letters a-d. Two are for the window circuits (diagonal, not F/R or L/R). I forget what the other two are for.