Chasing a Short (I think) in Fuse #5

[Jordan]

I absolutely hate electrical problems.  Was out for a drive this past weekend and my fuel gauge suddenly went to empty.  When I got home I found fuse #5 had blown.  Replaced the fuse and it was good for about a minute before it blew again.  Third time it blew as soon as I started the car.  So I am assuming I have a short somewhere, why it developed now I don't know.

Fuse #5 operates the turn signal indicator, fuel gauge, reverse lights, brake/stop lights and low brake fluid light on the dash.  If there is a short will the fuse still blow even if one is not using the function, i.e. I assume only the fuel gauge is using power when one starts the car?  I would like to narrow down my options before I start taking things apart to see where I have the short, unless someone has a simpler and less invasive idea how to locate the problem.   I am very receptive to all ideas.

[Tyler S]

Best way to approach this is to start unplugging components one at a time. Ie tail lights, fuel sender etc.. You can use an ohm meter connected to the fuse terminal with the car off and see if your short "dissappears" when unpluging components. When you get to the end of the list of components on that circuit start looking for a harness issue.

[Jordan]

Thanks Tyler.  The main reason I hate electrical issues is because I am electrically challenged.  I should mention the last time I started the car and the fuse blew, I had the fuel sender unplugged.  Would that eliminate the fuel sender as the culprit? 

When you say hook up an ohm meter do you mean measure across the fuse gap with the fuse in/out?  What reading should I not be getting or should be getting?  How do I unplug the signal indicator?

[Jack Jones]

Hey Jordon,

You can also remove the fuse and with jumper wires install a low wattage bulb such as a dash bulb in place of the fuse. if there is a short the bulb will illuminate and start disconnecting one item at a time that is fed from this fuse. The bulb will go out once you disconnect the circuit with the short than you can focus on that circuit.
This also saves on fuses.   

[Jack Jones]

I also watched a Tech install a "short finder" (bolt)in his fuse box, started the car and watched for the smoke. Not recommended but he didn't care about the old POS.

[ja17]

The entire rear wiring harness has a plug connection at the left front kick panel/firewall. Unplug it to see if the short goes away. This way you will narrow it down to the rear harness or not. Nice tip (light bulb) Jack!

[Jordan]

Thanks for the great ideas.  Hope to get started on it this weekend and let you know how I make out.

[Jordan]

Finally got back to the car.  Used an ohm meter as Tyler suggested and am reading about 130 ohms across the terminals (ignition off).  Unplugged the fuel sender as well as rear tail lights(2 prong and 4/6 prong plugs) individually and still got the same ohm reading.  Does that eliminate all but the turn signal as the source of the short?

Below is a picture of the wiring under the dash.  I want to confirm this by unplugging the plug connection for the rear wiring harness as Joe suggested but not sure which it is.  Is the circled connector the correct plug?  Thanks.

[Tyler S]

Marcus, use the ohm meter with one lead connected to ground and the other lead connected to the load side fuse terminal (the one that has NO power when the key is on.) You are looking for a short to ground so hook it up this way. A short will show up as a few ohms or less. The manual should state everything that is on that circuit.
The issue with using a lightbulb to chase a short is that components are grounded anyways. The lightbulb can still light up even if there is no short. The current path still has a way to ground. IE the windings in the fuel pump. Resistance measurement is crucial. However, The lightbulb trick can be useful if ALL of the components on the circuit have been disconnected and you are chasing a short within the harness.

[Jordan]

Thanks Tyler.  Did as you suggested and I get around 1 ohm (starts at 3 and gradually drops to 1 after about 30 seconds) with everything plugged in (using negative battery terminal for ground).  Get the same reading when disconnecting fuel sender, and each of the tail light plugs (2 on each side).  Still would like to disconnect entire rear harness but want to make sure I am disconnecting the correct plug under the dash.  Only thing north of the rear on fuse #5 would be signal indicator and low brake fluid light if I read it correctly.

[Jordan]

Can someone direct me as to where to find the brake light switch as well as the back-up light switch.  Thanks.

[Benz Dr.]

Most shorts to the rear of the car can be found under the threshold cover. It's easy to pinch the main cable while installing and sometimes it can take years before a wire goes to ground.

You can remove the driver's side inner threshold cover and have a look at the wiring. I've seen where a screw just nicked the cable and caused all sorts of trouble so I wouldn't count this out.

[Tyler S]

Marcus, the brake lamp switch is under the dash high up. Follow the brake pedal arm up and you will find it. The reverse lamp switch is on the pass side of the transmission high up.
When chasing a circuit problem, you can usually ignore things that have some sort of switch or control positioned in the circuit. Such as brake lights. They only activate when the pedal is depressed so you know that part of the harness or lamps themselves are most likely ok. They only tie into the circuit when switched on. Find the components that are always energized by fuse #5 with the key on. The bulkhead connector that Benz Dr mentioned is the only one that its harness disappears into a large grommet on the left side kick panel under the dash. The grommet is barely visible in your pic above. Behind all the wiring you have circled.

[Jordan]

Thanks Dan and Tyler.  Based on the wiring diagram there are 4 wires coming off Fuse 5, the brake light switch and back up light switches, which we can perhaps ignore as they are not energised all the time.  Next is the fuel level indicator, which I disconnected and still had a fuse blow, so that should discount that one. Lastly is the direction signal transmitter, which I guess is also a switch?.  Is the direction signal transmitter the same as the signal indicator or is it located within the signal indicator?

[Benz Dr.]

Disconnecting your fuel sending unit may not do anything if you have a dead short in the wiring harness. If your signal lights work then it's probably the sending unit and more likely a shorted wire. When you do find it you will also find it's something simple and stupid - it usually is........

[Jordan]

Thanks Dan.  So if I had disconnected the sending unit and the fuse still blew immediately upon starting the car would that signify that it is the sending unit, since it is the only item hooked to Fuse #5 that doesn't pass through a switch?

[Pinder]

you mentioned brake fluid light. Did you check if your brake fluid is topped up? could it be that its a bit low and the short is in the brake fluid low level?

[Jordan]

Pinder, brake fluid is full.

Just so I am not misreading this, if the short is in the fuel sender I can still blow the fuse even if I have the sender disconnected?  Is that correct?  If so, how do I isolate the fuel sender, at the fuse box itself?  Would I disconnect the fuel sender wire from the fuse?  I've never removed the fuse box before so I assume this is doable?  I told you I hated electrical issues, they give me headaches.

Thanks for the assistance.

[Benz Dr.]

Yup, you have it right. You can disconnect the fuel sender inside of the trunk but I doubt it will make any difference. You can also remove the harness under the dash which should prevent any further fuses blowing.

 I think your short to ground is likely under the sill trim. Disconnect both ends of the harness from the tail lights, fuel sender, and fuel pump and then test each terminal under the dash with an ohm meter. If you have anything going to ground you will get a reading.

 I'm only slightly better on electrical stuff so don't feel badly. Give me a call if you need any help.   

[Jordan]

Thanks Dan, you are far too modest.  You know oceans more about electrical than I.

On that note, where is the harness connection under the dash?  Can you see it in my picture above?  If not I can take others.

[66andBlue]

Marcus,
take a look at the photo here: http://www.sl113.org/wiki/Electrical/Rheostat
It shows the handshake connectors in my early 280SL. I have not seen a connector to separate the main rear and front harness parts, but perhaps I haven't looked close enough.
What happens when you disconnect the column multifunction switch?

[ja17]

Marcus, the connector to disconnect the rear harness is shown in your picture, but not the one your arrow points to. It is just behind it. You can see about half of it in your picture. It is actually mounted high up on  the side kick-panel, up by the firewall. It is mostly visible in your picture.

[66andBlue]

Joe,
is this the connector circled in red on a 230SL automatic?

[ja17]

Ah yes, that's it Alfred. Notice the cable goes down to the threshold cover and then back to the trunk area.

[Jordan]

Thanks very much.  I found it on mine, mounted to the side wall, not on the firewall as it appears to be on Alfred's earlier 230SL.  Tried pulling on the cover and doesn't seem to want to move.  It does pull apart correct?

I also found this switch tucked under the dash.  Anyone know what it is for?  It pulls out and pushes in.  Seems to be an integral part of the wiring.  Does it get mounted somewhere?  Thanks.  Hope to see what I can find today.