Main.TrailIndexPage | Electrical Systems | Electrical.Troubleshooting


Back to Electrical Systems.

Basic formulas

Ohms law

Ohm's law states that, in an electrical circuit, the current passing through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them. The mathematical equation that describes this relationship is:

 I = V / R

where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a constant, measured in ohms (which is equivalent to volts per ampere), and is called the resistance. The potential difference is also known as the voltage drop, and is sometimes denoted by E or U instead of V.

Alternative representations of this are:

 R = V / I
 V = R x I


The power taken by an electrical circuit (in Watts) is the product of the current in amperes and the potential difference in volts:

 P = I x V

Which can be expressed differently as follows, using Ohms law:

 P = I x I x R
 P = (V x V) / R


A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law:

 R = V / I

The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor. Resistors are used as part of electrical networks and electronic circuits.

Every device in the car, even the wiring itself and lightbulbs, has a resistance.

Series resistance

If two devices are in series, the total resistance is that of the individual resistances added:

 R = R1 + R2

Parallel resistance

If two devices are in parallel, as is frequently the case with light bulbs in cars, the the total resistance is less than the resistance of the smaller of the two. The exact formula is:

     R1 X R2
 R = -------
     R1 + R2

or, for many resistors

 1     1    1    1    1 
 - =  -- + -- + -- + -- .....
 R    R1   R2   R3   R4

Finding a short circuit or battery drain

Finding a short circuit or battery drain is quite difficult.

  1. Put an ammeter (one that measures current) between the -ve battery terminal and the lead connecting it to the chassis/ground. The reason you use the negative battery terminal is that it is much more difficult to create an accidental short this way. If you use the positive terminal, touching it anywhere to the chassis of the car creates an instant short circuit.
  2. Remove all the Fuses in your fuse box. There should now be NO current flowing. If there is, check for any wiring that is non-standard (and therefore does not go via the fuse box). Think of driving lamps, alarm system, after market radio, car-phone etc. It may also be a short in with any of the wires associated with this.
  3. Insert one fuse at a time.
  4. Check the ampmeter. If current flows, there's a drain or short in that circuit. If not, it's ok.
  5. Remove fuse, insert next one, check again. Do so until offending circuit found.

Note: Fuse 1 is always live. Also note, that you should have the key out of the ignition and everything switched off during this process.

Finding shorts using Resistance

When you've found the offending circuit, verify which circuitry is connected to the offending fuse. Take out the Wiring diagram and carefully trace all wires connected to that fuse. Are they intact, no weird, old connections, frayed connections, no opportunity for shorts? This is real tedious and horrible work. Remove the fuse, use the multimeter as an Ohm-meter (resistance) and measure the resistance in the circuit (from fuse to loose end of wire). If there is a short, the resistance will be low. If the resistance is very high (off the scale) you are allright.

If there are lightbulbs in the circuit, try to remove them by removing the bulbs. A lightbulb will have a very low resistance. You can calculate it's resistance if you know the Power (in watts, see the formulas above):

 R = 12/(P/12) = 144 / P.

So a 30W bulb has a resistance of 4.8 Ohm, and a 4W bulb is 36 Ohm. When you have two bulbs (or resistances) in parallel (as you frequently do in cars) the total resistance reduces. A 30W and a 4.8W bulb in parallel (i.e. 4.8 Ohm and 36 Ohm) yield a Resistance of 4.235 Ohm. Hence you will be looking for small differences in resistance when you leave lightbulbs in the loom, making it much harder to find problems.

Finding shorts using voltage

With a healthy battery there is potentially very little voltage drop visible from a short. The best way to check for a short is to verify the current flowing. No current means no short. Current means: something is working (a light, a motor) or you have a short. Once you have determined that you have a short somewhere in a circuit, you will need to start that tedious process of finding it. Verify which circuitry is connected to the offending fuse. Take out the Wiring diagram and carefully trace all wires connected to that fuse. What you should look for are:

  • loose wires, or damaged, or frayed wires with exposed ends able to touch bare or painted metal. Essentially you should not have any. If you find one, use the Voltmeter part of your instrument (attach the -ve black lead to a ground or the chassis, the +ve red lead to the loose, exposed wire). If it registers a voltage when everything is switched off (note: the voltage does not have to be 12 V, it can be anything over 0V) and your key is out of the ignition, that wire can cause a short.
  • burnt-out relays, wires and switches can also cause shorts
  • trace the wire back (physically) to a connector. Try and identify the connector and the post position (you can carefully open up these connectors) and use the connector manual and the wiring diagram (and the colour of the wire if it is original) to determine where/what it should have been connected in the first place. Connect it there using a proper connection.
  • if it is a non-standard, previous owner wire, trace it back to where it is spliced, connected etc in the loom, and remove it or terminate it and wrap it with isolation tape as close to that point as possible.

Old Yahoo content

The following is the content from the old Yahoo documents on the site. It needs to be structured and edited in the correct sections of the entire document. After moving particular content to its correct place in the manual, please delete it here.

Got the car back yesterday from the restoration shop. Today my turn signals don't work, my clock does not work nor the interior light. Checked my fuses and replaced the first fuse. Blew it again when I opened the door. Sounds like a short in my door light switch, but I am curious about the fuse. I can't find anything that tells me the size of fuse that should be used for each of the 12 fuses. Any one out there that can shed some light on this?

I had a similar problem after my restoration. The problem was in the trunk. The body shop had forced one of the tail light plugs in backwards which shorted the 12 V to ground, hence the blown fuse. Check all plugs and make sure they're plugged in properly.

Check your owners manual. they list the proper fuses for each of the 12 fuses. All but #2 and #6 are 8 amp; #2 and #6 are 25 amp.

All of the fuses, with the exception of #2 and #6, are the standard white 8 amp fuses used by many foreign cars. These are readily available in any auto supply shop. Fuses #2 and #6 are 25 amp. It sounds like your problem is with fuse #1 which is the only fuse that is permanently live. Since you said that the fuse blows when you open the door, I would first see if it blows when you open the other door. If it does, then the problem is very likely the interior (entrance) light. If it does not, then the problem is the door switch itself. The entrance light is easily removed by pulling it straight out. You can then examine the connectors and the bulb for possible trouble. When I had my car painted, it took a surprisingly long time to get everything back the way it was supposed to be.

Will: if memory serves, the door open light on the dash is also the low brake fluid light. So check the floats in the master cylinder reservoir. A strange thing happens once in a while. With the floats making contact, the door open, and the car started, the car cannot be turned off. The ignition switch is out of the circuit. You can turn the ignition switch off, remove the key and the car is still running.

The important thing in tracing electrical problems is to stay focused and try to narrow things down. This can sometimes be difficult when faced with a maze of wiring. It's also easy to surmise that several problems are related, when, in fact they may be independent. Regarding your first problem, start with the fact that your #1 fuse is blowing when you open either side door. We know that #1 fuse is permanently live. By opening the doors, electricity is sent to the entrance light. The first step is to examine the connections and wiring to the entrance light. If you do not see anything grounding out, disconnect the light and see if the problem persists. Next, as Will pointed out, I would examine the connections to the brake fluid level warning light control element at the brake fluid reservoir. Since you say that the brake fluid warning light was blinking, see if the reservoir is topped up and check the connections. If the problem still persists, it's possible that when someone was working under the dash, that they pulled something loose. Other than that, the A/C is a "red herring" since it is not affected by opening the door. Regarding the A/C problem, the first step is to determine if the compressor is kicking on. I'm not sure, but I thought the A/C has its own fuse. The blower and the compressor should be on two separate fuses or relays, but I'm not the expert on this. Also, the compressor may not come on while you are sitting at idle.

Let me weigh in on this again. With regard to the fuse blowing when the door is opened, it is likely in the entrance light. If considerable work has been done on the interior and the entrance light and its metal holder has been removed and reinstalled it could be your problem. Here's why. There is very little clearance between the switch contact in the entrance light assembly and the metal mounting ring (which, electrically, is ground). On my 280 SL, I had to "tweek" the metal holder and put some insulating tape on the switch to get it to work. Pull out the entrance light and see if the problem disappears. If it does, you may have the same problem. On the A/C: the compressor clutch and the blower should be on two separate fuses. In my car, both are on the same fuse due to the previous owner's modifications. The compressor clutch will engage ONLY if the blower is set to some position other than OFF, AND the temperature switch is set to any position other than OFF. Both must be on for the clutch to engage. If it's working, the green light on the A/C panel should light and you should hear the engine slow down a bit when the clutch engages. One caveat though, if the interior temperature of the car is cooler than the setting on the A/C temperature switch, the clutch won't engage. Also, the A/C system must be charged with refrigerant and under pressure/ vacuum or the engine will not see any load and won't slow down when the A/C is switched on. But, you can hear the clutch engage (loud click). My fuse was on the driver's side fender wall under the hood. Locate the fuses; if they're good, you'll have to get a voltmeter and troubleshoot it. I connected my A/C to go through the ignition switch so I could not leave the blower on by mistake.

Bob Smith: on my auto 250 SL, I was wondering about the operation of the neutral safety/reversing lights switch. My reversing lights work with a little jiggling of the lever in the "R" position but the ignition lights works in every other position. Was this an option or a standard feature on all models? I looked at the wiring diagram for my 250SL and could not find mention of the `neutral safety switch', only the `back-up light switch'. I have not tried to start the car because I thought I might do damage to something very expensive in the auto transmission. Haynes and the BBB show how to adjust the linkages, cable etc but they don't explain how the switch actually works. Question: with the auto gear lever in any forward drive or reverse gear, should the ignition lights come on?

Dan Caron: I'm sure the wiring diagram you have is for a standard trans car. Your ignition lights should work in all positions but the car should only start in park or neutral. You can adjust the length of the cable and get it so that all the different positions work relative you your gear selector inside of the car.

Joe Alexander: The neutral safety switch is mounted on the fire wall in the engine compartment. A cable from the transmission moves the switch. The cable can be adjusted if needed. The switch controls the power going to a relay which energizes the small solenoid wire on the starter. The neutral safety switch also controls power going to the reverse lights. In addition to this the power to the three position solenoid on the transmission is also routed through this switch. The switch should not control the ignition light unless something is crossed or hooked up incorrectly. Make sure your brake fluid level is up. I have seen strange things happen when a low level light comes on. Power feeds back through the wiring harness when the door open light is tripped.

Bob: Joe, I see the relay you mention as part of the starter solenoid, on the wiring diagram. Obviously it is wired differently for auto transmissions. SO if I try to start the car in any gear I should not do any damage even though my ignition light shows there is power? It works differently from the way I had thought. I thought my neutral safety switch was not working properly, hence my enquiry. I will try starting it in gear, just to test the system.

Tom Sargeant: I had that special experience this evening-my brake fluid warning light came on while driving, and when I stopped the car and opened the door-then turned the key to off-the car kept running. True joy of ownership is not fully experienced until this special quirk of the w113 cars occurs. By the way-I closed the door and then turned the key to off and the engine stopped. This event is documented at: Has anyone experienced the low brake fluid warning light coming on when the brake fluid is full? All doors were closed, the interior light was not on. The battery cables are secured tightly.

Dan Caron: Hi, I think this warning system incorporates a float that goes into the top of each side of the reservoir. If the float leaks it will sink and the system will make contact - even when the bottle is full. You can always unplug the wiring going to the contacts.

Tom again: that was exactly what it was. I pulled the leads and determined the light went out when one of the leads was not attached-indicating that was the likely float. The float was full of fluid. I had an extra float (don't ask why-I don't really know) and installed the float and everthing is back to normal.

I have no brake lights. I have power to the switch, if I jumper the connector I get lights. The switch has continuity in the closed position. I've cleaned the connections one of which get very hot. I'm going to get a new switch but this is just a weird problem that the switch doesn't seam to pass current and has some type of resistance. I opened the switch and there are no signs of heat deterioration. BTW, this is the second switch in three years. Anyone seen this type of issue?

I conjecture that the connection (the one that was hot) is loose, making poor contact. That this is causing high resistance, leading to no (or possibly very weak) brake lights, and heating of that connection. Can you take the female connector apart and squeeze the metal tube a bit to make it snug?

Thanks for the comment. I took the female connector apart and did tighten the connection. Weird, but no improvement. I cleaned the connections first with contact cleaner then with a slight abrasive.... After taking the switch apart I figure replace it.

Does anyone recall discussion about a condition that causes the dash light to illuminate when the brakes are used? I've searched the archives and can't find it, but it sounds familiar... Anyway, here's the situation: 1971 280SL, recently, the dash light started coming on when you hit the brakes... obvious logic of wires crossed with brake lights appears NOT to be the case... for some odd reason I seem to recall a similar discussion in here a long time ago, and it was related to a combination of conditions that could naturally occur, such as brake fluid level low AND wipers on when you hit the brakes, or wearing a striped tie with plaid slacks, or something odd like that... Any ideas will be appreciated...

I think I remember that. You can shut off the car if the brake fluid is low and you have the door open. This is because the used the same dash indicator light for low brake fluid and door open. I always wanted to try that, but I was concerned about running the car with the door open.

I believe your problem is caused by a loosened float in the master brake cylinder reservoir. That is, of course, unless your brake fluid level is low. You can temporarily eliminate the annoyance by unplugging the contact to the reservoir until you correct the problem.

Alternatively, if the brake fluid is low and the door is opened, it actives the ignition system and turns on the fuel pump. Because of this, it has been suggested to remove the fuel pump fuse when working on the brakes.

The problem may simply be low brake fluid sloshing forward under braking, causing the low fluid warning light to come on. Of course, the car may also be telling you not to mix stripes and plaids.

Don't know if anyone else suggested it but I have seen the wrong bulbs used in the tailights/markerlamps on other cars cause this. Usually it is a single filiment bulb where a 2 filiment bulb should be. The one contact on the bulb base bridges the circuit between the brake light and running light circuits which causes what you describe.

I had a similar experience after I had a Satelite tracking system installed into the car, and had this exact problem - warning light comes on with brake lights. Seems this had something to do with the way they wired up the remote shutoff - which included some wiring to the indicators and fuel pump. In the end, they had to remove all extraneous wiring to get the warning light to function normally. Some alarm systems I am told function in much the same way.
< Grounding | Main.TrailIndexPage | Exhaust System >