What could be causing the power seat fuse to blow? - PeachParts Mercedes-Benz Forum

HoustonM3 · #1 03-01-2010, 02:55 PM

Background : 98 S500 80,000 miles , owned by my 85 yo grandmother and is rarely driven. I went out to start it and of course the battery is dead. I charge the battery / take it for a wash and find that the power seats/mirrors/ steering wheel are not working. Check fuse , its blown. Try putting the spare fuse in and it blows , buy a few new fuses and the same thing happens what could be causing this?

Also I had asked about a year ago about the truck , it is no wanting to shut. The power assist doors work fine and the truck will work sometimes but most of the time I have to slam it closed and the handle never retracts into the deck lid. I have tried pulling the fuse for the assist pump but this does nothing. Also sometimes if i slam the trunk closed and then start the car it pops open again? Any ideas? Go to love the W140s. She spent $4,000 on servicing the car a year ago but it has not moved 50 miles since then. Also the blower motor for the A/C and heat is not working, anyone have a blower motor?

benhogan · #2 03-01-2010, 03:33 PM

Quote:

Originally Posted by HoustonM3

Background : 98 S500 80,000 miles , owned by my 85 yo grandmother and is rarely driven. I went out to start it and of course the battery is dead. I charge the battery / take it for a wash and find that the power seats/mirrors/ steering wheel are not working. Check fuse , its blown. Try putting the spare fuse in and it blows , buy a few new fuses and the same thing happens what could be causing this?

Also I had asked about a year ago about the truck , it is no wanting to shut. The power assist doors work fine and the truck will work sometimes but most of the time I have to slam it closed and the handle never retracts into the deck lid. I have tried pulling the fuse for the assist pump but this does nothing. Also sometimes if i slam the trunk closed and then start the car it pops open again? Any ideas? Go to love the W140s. She spent $4,000 on servicing the car a year ago but it has not moved 50 miles since then. Also the blower motor for the A/C and heat is not working, anyone have a blower motor?

The fuse keeps blowing because of a bad ground. This is easy to isolate with a cheapo multimeter. In the absence of one, you can look around the wires leading to the seat and look for frayed wires or wires where the copper is exposed. That might be because something has rubbed against it enough to expose the wires beneath. The fuse will blow whenever the copper wire touches any ground (basically any metal on the car).

If you have a multimeter, respond back and we'll give you the play by play

HoustonM3 · #3 03-01-2010, 03:55 PM

No I don't have one , but how could this all of a sudden happen , they were working fine a few weeks ago and the car has not moved.

benhogan · #4 03-02-2010, 08:34 AM

It's definitely a short.

The fuse blows to 'save' the electric motors from blowing. Like a sacrificial lamb so to speak.

If you can find the short (a loose wire, a stripped wire that is touching metal), you will solve your problem.

This is actually a good thing that the fuse keeps blowing because fuses are cheap compared to cost of replacing the motors.

Time to go short circuit hunting.

goldstone · #5 03-28-2010, 02:04 PM

Perfect timing! I've just discovered the very same problem with the very same circuit in my '89 420SEL. 8 Amp Fuse #15 (accessory lamps, door locks, antenna, seat adjustment, et al) keeps blowing.

BenHogan generously offered to supply a play by play for tracking down shorts with a multimeter. For those of us with the right tools, but without the know-how, this type of basic tutorial would be VERY much appreciated.

Where does one start, and how can one succeed without blowing 100 fuses in the process? Please, do tell! (Thanks in advance.)

RadioTek · #6 03-28-2010, 04:13 PM

An inexpensive multimeter can be easily gotten at Radio Shack or like stores. Many auto stores such as Checker, Autozone, or Pep Boys offer meters that are set up for automotive work. I would strongly recommend that you not get the cheapest $2.95 Harbor Freight type meter. You get what you pay for.

Read the instructions for the meter and get familiar with it. There are some things you can do to damage or ruin it by putting the leads in the wrong place with the switches in the wrong position. e.g.; trying to measure voltage with the meter set up for Ohms. (can destroy meter, or at least blow it's fuse)

To look for a short without going through a fistful of fuses, you can disconnect the positive terminal of the battery and put the red meter lead on the positive cable and the black lead on ground. Set the meter to measure Ohms and look for a low reading. Then you disconnect fuses one at a time until you lose the low reading; you have now isolated the circuit that has the trouble. (You are pretty much at this point, since you know which fuse is blowing.)

The next part is where you need patience. You need to leave the fuse out and hook the battery back up. You shouldn't have any other fuses blowing, but if you do, there is another short somewhere. You can repeat the procedure until you find the circuit with the problem.

If you've hooked up the battery, left the one fuse out, and everything except what is on that fuse now works, you have taken a large step forward. You now need to determine what components are powered by the now missing fuse.

There is usually a label on the fuse cover or in the manual that tells you this, and the next step is to put the meter on the side of the fuse terminal away from the power supply and check to see if you still have the low Ohms reading. Theoretically, it should still be there.

You should now start looking for wear spots on wires, places where wires are bent over metal corners, bare wires, loose connectors or wires, and components or connectors that are out of place.

If you move things around and gently tug on wire bundles, connectors, or components, you need to check your meter often to see if you've lost your short reading. You may just have 'accidentally' fixed something. Try to find out what, if possible.

If you still have the short reading, you need to disconnect each component on the circuit one by one until the reading goes away. Disconnect one component, check meter, leave disconnected, then disconnect another, and so on. There are usually connectors that let you do this, and you may have to get behind trim panels, under the dash, in wiring channels, or under carpets to get to things. You may have to remove things such as seats or consoles. Check your meter often, you may pull a wire bundle away from a sharp edge, or remove a screw that was run through or pinching a wire.

If you find a broken wire or worn away insulation, I would not recommend simply twisting it back together and wrapping it in electrical tape. This would be a temporary fix, but you will have the problem recur in the future. There are connectors available at auto supply stores that are crimped on the wires and come with heat shrink tubing filled with an adhesive, like hot glue, that will prevent moisture from getting into the repair in the future.

If you find a component that is causing your short, you will probably have to replace it, but first look for obvious failures, like exposed or crushed wires, or burn marks. Older Becker radios are known for failing and causing odd problems with seemingly unrelated systems.

Good hunting, and keep us up on your journey.

Scott

goldstone · #7 03-29-2010, 01:28 AM

Hey Scott,

That was very generous of you to post so detailed a response regarding short detection!

This all makes sense to me. In effect, your described technique will re-establish the #15 circuit using the multimeter/ohm meter without the battery inline. It's all so simple once someone spells it out as you have.

Looking at the electrical schematics, I see that most of the paths fed from the #15 fuse branch from connector "X18", located in the driver's footwell near the parking brake. That seems like a logical starting point for my investigation.

Thanks again for the primer!

JohnM. · #8 03-29-2010, 02:29 AM

Oh, I love nothing more than hunting some electrical gremlins. So satisfying when you find the culprit!

Pooka · #9 03-29-2010, 04:56 PM

Many years ago I was helping a fellow dismantal an early 1950's Plymouth. I noticed most of the fuses were wrapped in alum. foil.

He told me that was because they kept blowing!

I cannot recommend this solution for a Mercedes. Something about FIRE!

The Clk Man · #10 03-30-2010, 05:40 PM

I have a similar problem with my 2002 clk 320. I will use your advice.

goldstone · #11 04-04-2010, 01:46 AM

Over the weekend, I set out to trace the short in the circuit associated with fuse #15 in my '89 420SEL, and I ran into problems using the instructions Scott supplied above. Here's the thing:

1) When I connected one lead of my ohmmeter to the side of the #15 fuse terminal away from the power supply and the other lead to the frame of the car (as instructed), I got not just a low resistance reading, but a NO resistance reading--just like touching the leads to each other. This wasn't what I was expecting.

2) To convince myself that the technique worked (as I understood it), I removed another few fuses from their respective sockets and tested them in the same way. To my surprise (and frustration), a lot of them (particularly the ones with low amperage fuses) gave the same NO resistance reading. Some (including #15) did so from BOTH fuse terminals--which I REALLY don't get, since the battery terminals were both disconnected. Am I dealing with undetected shorts in all these circuits??? I don't think so, but how to explain my test results otherwise?

I don't get it. What am I missing here. I expected #15 to behave differently than all the others, but it isn't.

HELP! (Please...)