In the past couple of articles we’ve been talking about things that go wrong with electrical systems and how to diagnose them. To review those articles, click HERE. In this installment I’ll briefly discuss two more electrical challenges:
1. Parasitic Draw
2. Excessive resistance (i.e. the Green Monster)
Skilful use of a quality DVOM to measure voltage drop, and a good understanding of wiring diagram use will lead to quick, profitable resolution of these complaints.
A common challenge in repair shops is the diagnosis of “my battery keeps going dead overnight” complaints. This is usually caused by parasitic draw. Some component in the vehicle is using electricity all of the time, rather than shutting down when it should.
Parasitic draw could be caused by a light staying on because of a faulty door switch, a component with an internal short circuit causing current to continually flow, or a faulty ECU (electronic control unit).
To diagnose parasitic draw you must first determine how much current is being drawn in the overall electrical system with the ignition key ‘off’.
Measure the current at the battery positive or negative cable by either inserting an ammeter in the circuit between the cable end and the battery (not recommended because it requires disconnecting the battery cable which leads to other problems and a potential faulty diagnosis), or by using an “amp clamp” on either battery cable. Newer battery testers can sometimes measure current draw as well.
Note: Make sure to measure current draw at least 30 minutes after vehicle shutdown and don’t open any doors while performing your current draw test!
Some parasitic draw in an electrical circuit is normal and is created by the control units in the vehicle. All computers must draw current for their “keep alive” memory. Each vehicle and manufacturer will have a specification of how much parasitic draw is acceptable.
Sometimes it is difficult to find a specification for what is acceptable parasitic draw. In this situation you can either compare the problem vehicle to a known good vehicle of the same make, model, and year, or you can use a general guideline of less than 100mA.
Note: The more computers in a vehicle, the higher the parasitic draw will be. I have seen NORMAL parasitic draw in excess of 250mA!
Once you have identified that you have excessive parasitic draw, you now need to find the component causing it.
With an ammeter displaying the current value, begin to remove fuses until you see the draw drop to an acceptable level. Reinstall the fuse and use the wiring diagram to identify what circuits are on that particular fuse.
Disconnect each component, one at a time, until you again see the excessive draw disappear. The component, or circuit, that you last disconnected is the culprit! Now it’s time to use your skills to determine if you have a wiring problem or a component problem.
Depending on where in the country you live, excessive resistance is most likely the biggest cause of electrical problems. Wires chafe and are exposed to elements, poor diagnostic practices create open in insulation that invite corrosion, water gets into connectors and causes corrosion, or dirt and grime cause poor connections in switches.
Finding excessive resistance is a matter of utilizing a voltage drop test and recognizing what the DVOM is telling you.
Excessive resistance causes voltage to be “dropped” at an area of the circuit that generally doesn’t use up any voltage. Often this occurs at switches and relays, although it may happen in a wire if the wire is somehow exposed to the elements.
If enough voltage is dropped, there will not be enough voltage left to operate the components in the circuit.
To find an area of excessive resistance, use the wiring diagram to identify areas of the circuit that are easy to access. You do not have to worry about testing across a load – you already know that those are supposed to drop voltage. Pick areas where you’ll be testing a length of the harness or across switches and connectors. If there is any component (including “switches” or “relays” that are solid state control devices) you will have higher than expected voltage drop.
Always double check when you get an unexpected result to be sure that there is nothing but a wire in the circuit between your meter leads!
Remember, voltage drops of greater than 0.2 volts are what you are looking for! Anything more than 0.2 volts and you have an excessive resistance problem.
Continually practice using wiring diagrams and your DVOM on the vehicles you work on. Practicing on known good vehicles when things are slow at the shop is a great way to prepare to diagnose the challenging ones when time is at a premium!
As always feel free to contact me with article ideas, questions, comments, or complaints! Don’t forget to subscribe to this column to be notified every time a new article gets posted! I can be reached through Examiner or at www.intelligentmechanic.com.