Wednesday, January 18, 2017 Detailed Auto Topics
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Detailed Topics

In part one, we introduce the fundamental of voltage-drop testing. This segment, explains more examples of this powerful tool.

Low power and check engine light

An example of voltage-drop testing is finding the cause for a lack of power and a check-engine lightDiagnostic trouble codes P0171 and P0174 indicate a lean fuel to air mixture. These suggest not enough fuel available to the injectors. A fuel-pressure test confirms insufficient pressure. A logical first step may be replacing the fuel filter. Here it does not help and the low pressure persists. Many enthusiasts might next condemn the fuel pump. This is expensive, and the decision is made with incomplete information.

We use voltage-drop testing to gain more insight. Checking system voltage reveals 12.5 volts, full current available at the battery. With the fuel pump running, we check voltage, from the negative battery terminal to the fuel pump ground. This shows one-tenth volt, almost no current flow, so the ground is good.

The next check is voltage-drop from the positive battery terminal, to the fuel pump.  We find three volts and this shows a problem. A restriction to flow exists in the fuel-pump power circuit. Three volts are indicated, because current flows through our voltmeter rather than through the restricted circuit.

Running at less than twelve volts, the fuel pump does not produce proper pressure.  The engine runs lean, because fuel pressure is low, due to low fuel pump voltage.

Voltage-drop testing a relay

To find the problem, we check voltage-drop from battery positive to the fuel pump relay and find it is 0.1 volt.  Full voltage is reaching the relay. Next, we check across the relay terminals. Three-volts are flowing around the relay, showing a problem.

We replace the relay, voltage-drop is one-tenth of a volt and fuel pressure returns to normal.  Testing the circuit from battery positive to the fuel pump indicates one-tenth of a volt, normal for a good circuit.  Fuel pressure is now normal. We clear the codes and test-drive the vehicle.  No check engine light and restored engine performance confirm the problem is resolved.  These simple tests point to the problem and save needlessly replacing a good fuel pump.

Repeat electrical failures

Voltage-drop testing adds a new dimension to our diagnostic arsenal. Another example is a bit different but shows the same principles. This vehicle has repeated electrical problems. The condenser cooling fan fails twice. The air conditioner blower, two alternators and several lights also fail, in less than two years. No single component is involved, so we assume the issue involves the entire vehicle.

Testing starts as before, by checking voltage at the battery. A reading of 12.5 volts, between the battery posts, shows full power available. To check body grounds, we connect from the battery negative post, to a good chassis ground. We switch as many circuits as possible, to the on position. This includes the blower, radio, headlights and air conditioning. Voltage-drop measures 2.0 volts, meaning a problem exists.

Poor terminal connection with voltage drop

We measure the voltage drop from the negative battery terminal to the temporary cable end. This shows a drop of one-half volt, high, but not the entire problem. The cable end is cleaned and reinstalled. Testing now shows, 0.1 volts drop from the battery post to the cable end. Checking from the battery post to the chassis ground reads a voltage of one and a half volt, still too high.

Voltage drop, bad cable connection

We take a reading from the battery post to the battery cable and it shows one and a half volts. A poor connection causes too much resistance in the circuit. To correct this, we replace the temporary cable end with a crimp-on style cable end. Testing now shows two-tenths of a volt, from the terminal to chassis ground, meaning the circuit is good.

Tests from battery positive to each component read within specifications. The owner experiences no further problems with the electrical system. Experience with voltage-drop testing makes it easier to use. The methods are simple to learn and help prevent many wrong decisions. Voltage-drop testing is a test method of which every repair enthusiast should be aware.





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