Some Useful Tips For Simple Fault Finding On Cars.

1) Car Starter Motor Drags.

Although this can be caused by a faulty starter motor or battery, some times it can be a simple thing such as a corroded battery terminal or poor connections on the earth wire or positive wires to the solenoid and starter motor.

A useful tip to use if the vehicle is at standstill and cold, and the starter motor drags is to feel the connections with your finger at the battery terminals, earth cable, and lugs at starter motor. If it is a loose connection it will cause a tremendous amount of heat and it will be fairly warm to the touch.

Switch the head lights on. If the lights are very dull suspect your battery.

If the lights are on and bright crank the starter motor. If the lights dim completely suspect a faulty stater motor or tight engine.

2)  Starter Motor Spins Freely Without Engaging.

It might possibly be that your key switch has a high resistance. For a test you can take 12 Volts from the battery straight to the terminal on the solenoid that is fed from the key switch. If the starter motor engages correctly your key switch is faulty.

 You can fit a new key switch. A cheaper option is to fit a relay as shown below. Any hooter or  relay will work. It is a lot cheaper and easier than replacing the key switch.

Ensure that you use no less than 2.5mm square wire on the contact circuit. The thicker the better. Insert a 20 amp fuse into the inline fuse holder.

Tail Lights are dim, and flash with indicators.

This is normally a sign of a corroded earth connection. Check earth connection and clean if necessary. If you cant find out where they earthed it from, trace the earth wire from the tail light and fit a new wire and lug to form an earthing point.

Simple Headlights On Alarm Circuit.

Although this is not problem solving I thought I would just add this in. All you need is a 12v Piezo Sounder which you can obtain from any electronic shop as shown below. It must have its own internal oscillator circuit.

You will also need an inline fuse holder with a 1 amp fuse and reed switch as used in burglar alarms and wire.

Reed Switch which is operated by magnetic field

Inline Fuse

Some Heat Shrink Sleeving

 Instructions

1. Take a wire from the negative terminal of the sounder and earth it to the body.
2. Take a wire from the positive wire and route it carefully to the headlight. Leave a bit extra. 
3. Slip the correct size heat shrink over the wire in the head lamp side. It must be 30mm longer than the reed switch and be able to slip over the reed switch.
4. solder the wire that you routed onto any side of the reed switch.
5. Take another piece of wire and solder it on the opposite side of the reed switch. It must be long enough to reach the positive terminal of the battery.
6. Slip the heat shrink sleeving over the read switch. Heat up with a hot air gun or lighter to shrink sleeving.
7. Carefully remove some of the tape around the loom at the headlight. Find the earth wire that comes from the head lamp. Wrap this around the reed switch about 6 times. Do not cut the insulation of the earthwire.
8. Tape the loom closed with the reed switch inside.
9. Slip a piece of heat shrink wire over the opposite end of the wire. Strip wire and solder to inline fuse holder. Slip heat shrink sleeve over join and shrink with heat.
10. Fit lug on opposite side of fuse holder to match bolt on positive terminal of battery.
11. Disconnect the negative terminal of the battery.
12. Remove the nut from the positive terminal and slip the lug onto the bolt and tighten.
13. Refit the negative terminal.

How it works!

The reed switch is operated the moment a magnetic field is nearby. By wrapping the wire around the reed switch you are forming an electric magnet. The negative wire (or common) is used to make sure that the alarm is triggered irrespective of if the bright or dim lamps are left on.


 

Wiring Of 7 Pin Trailer Plugs

I often get asked how to wire up a 7 pin trailer / caravan plug. The drawing below indicates the standard used by most trailer or caravan manufacturers.

7 Pin Trailer Plug

Note that some vehicles do use alternative codes. In some cases Pin 2 is used for the fog lights. In other cases trailer manufacturers link pin 2 to pin 7. On some caravans pin 2 is used for the fridge.

Must dealers do stock the standard 7 core flexible electrical cable with the correct wiring colors as indicated in the drawing.

There are many ways to connect your wiring harness in your vehicle if fitting a new trailer socket. You can bare a piece of insulation on the wiring harness of the vehicle, bare the ends of the wire from the trailer plug and twist them around the bared wires of the vehicle. Note that these connections must be soldered to ensure excellent conductivity and prevent corrosion. Make sure that your solder is resin cored and not acid cored. Resin core is for electrical wires and acid core is for sheet metal work.

Poor conductivity is the cause of most electrical fires. The reason is that the current can be enough to cause heat build up on a area of high resistance, but not enough to blow the fuse. As the heat builds up it causes carbonizing which increases the resistance even more. This can continue until the heat build up is enough to ignite flammable materials in the area. So ensure that your joints have the best conductivity possible

I prefer using HT self vulcanizing insulating tape to tape up the joints.

 

This can be obtained from any electrical supplier. I normally use the 3M or Pratley type.  Although it is expensive it will not unravel like normal insulation tape. You will not be able to unwrap it once it has vulcanized. You will have to cut it off to remove it.

Most modern cars have fancy tail light holders as shown below.
 

One can solder your wires directly to the galvanized strips instead of stripping the insulation on the wiring. However you will have to remove some of the galvanizing to ensure an excellent joint. It is advisable to coat the soldered areas with insulating varnish to prevent corrosion when all the joints are completed. 

Another method is to use splicing connectors. This involves no striping of insulation or soldering. However you must ensure that you size the connectors to mach the size of the conductors used. Here is an image of splicing connectors.


If you look at the connector, you will find that the groove in the front of the connector will simply clip over the wire in the cars loom.  The groove at the back is only accessible from  1 side only. Simply insert the wire from the trailer socket into this groove, clip the connector over the wire in the car loom, and gently press the splicing clip down with a pair of pliers. Ensure that the splicing clip is depressed evenly and squarely, otherwise you will have to extract the splicing clip and start again. Once the splicing clip is fully home, simply clip the top cover over, and your connection is complete. Once again I prefer to use the 3M type.

It is easy enough to trace the wires if you are not sure. I prefer to use a 12V test lamp to a multimeter. The reason for this is that most modern multimeter s have a very high internal reading. This can give you incorrect readings when looking for the correct wire. However a multimeter can be very useful in checking the joint resistance after completion. Do not forget to subtract the reading of the lead resistance when checking the resistance. This can be found by connecting the two leads together of the multimeter and taking a reading. I would be very concerned if the resistance from the trailer plug to the connection is more than 0.2 ohms.

It is imperative to ensure that the wires from the trailer plug is equal to or thicker than the size of the wiring in the vehicle. Using thinner wire might prevent the fuse from blowing in the case of an electrical short circuit.

Last of all always check the wiring on the trailer or caravan as well. Ensure that this also matches the standard wiring code before plugging in.












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