The glow plugs preheat diesel engines allowing faster ignition even when cold. If your engine has any problems starting or you see smoke coming out of the exhaust, then one or more of the glow plugs are not working. Check them out yourself to avoid a trip to the mechanic.
Steps
Method 1 of 2: Check the Glow Plugs in the Engine
Step 1. Get a multimeter
It is a digital tool used to test electrical cables and devices. In the center of the multimeter you will find a large graduated knob with which to set the various types of measurements. To test the current and electrical resistance you will connect a pair of probes to the two unions, generally one on each side, one black (negative) and one red (positive). Such probes generally have metal clamps on one end. While so many numbers on the knob can make it seem like a very complicated tool, you only need to use one particular setting to perform the test illustrated in this article.
- Analog measurement tools may apply extra voltage to the component or device you are going to measure, so they should be used more carefully than their digital counterpart.
- Since the current can fluctuate, the first reading you get from a digital multimeter may not be accurate. This type of instrument struggles to measure a constantly changing current. An analog instrument, on the other hand, allows you to see fluctuations as well, but is still less accurate in general.
- For this test it would be better to use a digital multimeter because the reading directly shows you a number, unlike a needle moving on a graduated scale like in the case of an analog multimeter, which makes measurement more difficult.
- If you are still using an analog multimeter, be sure to choose one that has a sensitivity of at least 20k ohm / V.
Step 2. Set the multimeter to ohms
The symbol for ohms is the Greek omega, a letter that resembles a horseshoe with two horizontal dashes. There are two long vertical lines that delimit the resistance range.
Step 3. Find the intrinsic resistance value of the multimeter
Join the two conductors of the instrument together and note the result that appears on the display. Make sure the conductors are touching, if you are using a digital multimeter a number will appear on the screen.
Subtract this value from the one obtained in the measurement of the glow plug
Step 4. Check the battery voltage
Set the multimeter to volts, connect the negative lead to the negative terminal of the battery and then the positive lead to the positive pole of the battery. The value you read should be close to 12.5 Volts with the engine off and 13 Volts with the engine running.
If not, check the battery or alternator before continuing. The glow plugs will not work properly if they do not receive the proper voltage
Step 5. Locate the glow plugs
Consult the maintenance manual to understand where they are located in your vehicle's engine. The exact location varies from model to model.
Step 6. Remove the plugs or plugs from the glow plugs
These are in fact usually covered by protective caps. Remove them for testing.
Check connectors and plugs for rust or corrosion. In case, take the opportunity to clean them
Step 7. Connect the multimeter's negative probe to an engine ground point
The two main points can be identified by following the cable that leads to the negative pole of the battery or the one that enters the engine from the alternator. Both of these cables are fixed to the motor by means of bolts. Connect the negative connector to one of these grounding nuts.
Always consult the maintenance manual to find the grounding points of your vehicle
Step 8. Connect the positive probe to the tip of the glow plug
If the multimeter's negative probe is still attached to the negative pole of the battery, you can leave it where it is
Step 9. Evaluate the result that appears on the display
Refer to the manual to determine the specific features for your vehicle.
- Subtract the intrinsic resistance of the meter that you noted earlier from the value displayed by the multimeter. For example: if the resistance of the glow plug shown on the display is 0.9 ohms and that of the multimeter is 0.2 ohms, then the real resistance of the glow plug is 0.7 ohms.
- All engine glow plugs should have similar resistance. If one has greater resistance, it will interfere with engine performance, even if the glow plug is in good condition.
Step 10. Replace the glow plugs
If one is malfunctioning (or more than one), replace them all, never change just one. If the area around the glow plugs is dirty, clean before replacing them.
Some manufacturers have special tools to clean the hole where they are mounted in the cylinder head. These tools can clean the carbon that accumulates in the internal area of the combustion chamber. They also serve to clean the thread into which the glow plug is screwed. This tool is called "reamer"
Method 2 of 2: Check the Disassembled Glow Plugs
Step 1. Remove the glow plugs from the engine
Check the maintenance manual to find out what is the best way to proceed. The correct technique varies according to the vehicle model.
Step 2. Set the multimeter to ohms
Select a range between 200 and 1000 ohms. If the value of a glow plug exceeds the range of the multimeter, then it means that it is malfunctioning.
Step 3. Find the intrinsic resistance of the multimeter
Connect the two connectors together and note the number you see on the display.
Subtract this value from what you get from the glow plug reading
Step 4. Put the multimeter's negative probe on the glow plug nut
Make sure it does not touch the latter higher than the die.
Step 5. Put the positive probe on the tip of the glow plug
This is the end that is covered by the cap when the glow plug is mounted in the engine.
Step 6. Read the result
Compare it with your vehicle's specifications on the maintenance manual.
- Subtract the intrinsic resistance of the meter that you noted earlier from the value displayed by the multimeter. For example: if the resistance of the glow plug shown on the display is 0.9 ohms and that of the multimeter is 0.2 ohms, then the real resistance of the glow plug is 0.7 ohms.
- All engine glow plugs should have similar resistance. If one has greater resistance, it will interfere with engine performance, even if the glow plug is in good condition.
Step 7. Replace the glow plugs
If one or more of them are damaged, change them all; never replace just one.
Advice
- Remove the glow plugs when the engine is hot, in fact it is much more difficult when the engine is cold.
- Test new glow plugs before fitting them.
- Always wear safety glasses when working around the car.
