- Automotive Uses for a Multimeter
- Checking the car battery
- Checking the car battery charging system
- Checking for excessive current drain on the battery when the car is off
- Testing whether the electrical system is getting power
- Checking wire integrity
- Testing spark plugs & glow plugs
- Testing sensors & solenoids
- Checking a car’s grounding
- Testing a rear window defroster
- Testing temperature in different parts of the engine
- What to look for when buying a Digital Multimeter for Automotive Work
- Popular Digital Multimeters for Automotive Work
One of the primary uses for a multimeter in the hands of the DIY-er is to help diagnose and fix a car’s electrical problems, and the right multimeter can be an inexpensive yet invaluable piece of kit in the automotive tool box. So what is the best multimeter for automotive work? Well, before we can figure that out, we have to know what a multimeter can be used for when it comes to the family motor.
Automotive Uses for a Multimeter
Today’s cars are packed with all sorts of electrical systems and electromotive components, any of which could go wrong or wear out, and future vehicles are only going to get more complicated as we switch away from the combustion engine towards a fully-electric future. So what car problems currently can be diagnosed with a digital multimeter? Let’s take a look at some common ones:
Checking the car battery
The first obvious electrical problem that can arise in any car involves the root source of stored electrical power in every vehicle – the car battery. Most people have experienced or at least know of someone whose car did not start due to a low battery charge. So one of the primary diagnostic uses of a multimeter is to check the battery voltage in a car that is having problems starting. Car batteries are 12V affairs, but a fully-charged battery should be expected to read above 12V. So, a car battery that is below this value, even if it is at 12.0V exactly, is likely to be having engine-starting difficulties especially in cold weather. The solution to this is to remove the battery from the car and recharge it externally.
However, even if the battery voltage in car that is having trouble starting is above 12V, this does not necessarily mean that all is well with the battery. It is also important to check what the battery voltage falls to when the engine is actually starting up as this can flag a defective battery that might need replacing. If you observe a multimeter hooked up to the battery terminals as the engine is turned on, you will notice that the voltage level will fall as the starter draws large amounts of power. The fall in voltage is normally only a couple of volts, with the battery voltage falling briefly to approximately 10V. If however, this voltage falls significantly below 10V, say to 4V, this usually means that the battery is defective and needs replacing. Even attempting to recharge such a battery is unlikely to solve the engine starting problem permanently as a large fall in battery voltage at start up most often means that the battery can not retain its charge for any normal length of time even in its quiescent state.
Checking the car battery charging system
Another vehicle system test that is often done with a digital multimeter is to verify that the car’s internal battery charging system is working correctly. When the car engine is on, a component called the alternator converts some of the engine’s kinetic energy into electrical power that is directed at the car’s battery in order to replace energy lost during engine start or used by other electrical systems. There are several checks that can be done on a car’s battery charging system, of which the most obvious is to measure the voltage at the battery terminals while the engine is running. The voltage at this point should be about 1.5 – 2V higher than the base voltage of the battery, allowing charge to flow into rather than out of it. If the voltage from the alternator is too low, this may indicate the presence of a fault within the car’s battery charging system, but also by the same token, if the alternator voltage is too high, this may be beyond the battery’s specifications for recharging it and the battery could become damaged over time. As a consequence, checking to see that the recharging voltage is within the specified limits of the battery in use is an important check to prolong the car battery’s life.
Another set of checks that can be done with a multimeter to ensure the charging system is working correctly, is to look at the integrity of the connections. By looking at the normally small (mV) voltage drops across different connections of the charging system (when the engine is running), any values that are excessively large will reveal a deficiency in the connections, and potentially compromising the charging of the battery. The typical connections on the charging system that are normally checked are:
- Negative battery terminal <—-> Engine grounding point (normal drop ~ 400mV)
- Negative battery terminal <—-> Body (Chassis) grounding point (normal drop ~ 40mV)
- Positive battery terminal <—-> Alternator output (normal drop ~ 230mV)
Another charging system test that can be carried out with a multimeter which is usually the preserve of professional car mechanics is the AC ripple test. This looks for excessive AC voltage generated by the alternator that works against the DC voltage charging of the battery. To perform this test, the digital multimeter probes have to be connected up as follows:
- Negative battery terminal <—-> Alternator output
An AC voltage of greater than 500mV when the engine is at 2500rpm would indicate a problem with the car’s charging system.
Checking for excessive current drain on the battery when the car is off
When a car is turned off and left to stand, the last thing one wants is for one of the electrical systems to draw too much energy from the car battery over an extended length of time – this will drain the battery excessively and make future starting of the vehicle a problem. So checking the current draw on a car that is turned off is an important check especially if there are repeated problems with a car battery even after it has been recharged. To carry out this test, the digital multimeter is placed in series between the battery and the car’s electrical systems allowing it to measure the current passing through it. Here the magic number is 50mA, anything much above that would indicate a problem (NB: it is important to let a car stand for a little while after switching it off as there are some electrical systems that continue to run for a short time after engine shut down).
Testing whether the electrical system is getting power
Whenever a fault develops in one of the car’s electrical systems, one of the first checks that can be made is to check to see whether it is getting any power in the first place. This can easily be done by hooking up a digital multimeter to the power pin or plug of the electrical system in question and checking that it is receiving the correct voltage when the system is switched on.
Checking wire integrity
Sometimes wires in a car get old or are twisted back and forth so much that they break. So having a multimeter at hand to check the integrity of the wires connecting electrical components can be very useful. Digital multimeters have two primary ways to do this, either using a bespoke feature, known as the continuity setting, which beeps when the multimeter’s probes simultaneously touch both ends of a intact wire, or via resistance testing of the wire – a normal wire will have a small amount of resistance to it whereas a broken wire will show infinite resistance usually represented by ‘OL’ or ‘1.’ on the multimeter display.
Testing spark plugs & glow plugs
As with wires, you can test the integrity and proper functioning of spark plugs and glow plugs from your vehicle. The spark plugs or glow plugs will have to be removed, and then using the continuity setting or resistance reading, you can check that the elements of plugs are intact and are free from any obstructive corrosion that might be interfering with their ignition function.
Testing sensors & solenoids
Many of a car’s electrical components function as a result of how electricity flows through them. For example, car temperature sensors often work on the basis of resistance values varying with temperature, or door locks and fuel injectors which are solenoids consist of wire coils wound around elements that become magnetised when electricity flows through the coils. Digital multimeters can be useful here as many of these components can be checked for faults by looking at their impedance (resistance) values and then comparing them to manufacturer specifications. Any differences in impedance outside of the component’s normal range would indicate a problem and probably warrant replacing.
Checking a car’s grounding
The whole metal chassis of a car is normally connected to the negative terminal of the car battery making it the car’s ‘Ground’. However, the chassis is made of steel which is not always such a good electrical conductor, and as a result, some parts of the chassis will be better grounded than others. This is particularly important when fitting after-market kit such as stereo amplifiers where a good ground connection is needed for the equipment to work properly. A digital multimeter can be useful here to compare different points on the car’s chassis to find the best grounding points for new equipment.
Testing a rear window defroster
An electrical system found on most cars that sometimes goes awry is the rear window defroster. Using the DC voltage setting of the digital multimeter, you can first check that the connection to the defrosting element is sound by ensuring that it is receiving the correct voltage when switched on. In addition, using correct multimeter probes that can penetrate to the conducting surface of the defrosting element, the whole defroster array can be checked for integrity by looking at the resistance values for each of its sections. Any resistance that was too high would indicate a compromised region of the defrosting array.
Testing temperature in different parts of the engine
Another less obvious use for the automotive digital multimeter is to measure temperature. There are a number of car automotive systems where temperature is an important factor and where abnormal values may flag a developing problem. Car systems like the air conditioning system or the engine cooling system are typical places where you might want to keep an eye out for excessive temperatures. Not all multimeters have a temperature sensing function but the ones that do can usually measure temperature at the higher levels that you might find in a car’s engine.
What to look for when buying a Digital Multimeter for Automotive Work
Unsurprisingly given the myriad of automotive uses for the humble digital multimeter, there are a whole array of features that can be on this handy little device. Some features are standard and common to all multimeters while others are optional, often only present on the more expensive ones. But even the standard features can have differences in their specifications, so one needs to keep an eye out to make sure that they too will meet their needs. For basic automotive work, we recommend the following features and specifications (if appropriate) at a minimum:
- Auto-Ranging (preferred)
- Voltage (DC) – measuring from milliVolts (mV) up to at least 20V (since cars are based on a 12V system)
- Current (DC) – measuring from milliAmps (mA) up to 10A or 20A
- Backlit Display (preferred)
- Voltage (AC) – measuring milliVolts (mV)
Although one can diagnose many electrical problems with the essential features above, there are some additional features that advanced users may require or make the process of measurement more efficient. These include:
- Auto-Hold or Minimum / Maximum
- Duty Cycle
- Detachable Readout Screen
Fortunately, even the most basic digital multimeter has most of the standard measuring functions that one wants for essential automotive work, so having a multimeter at hand for automotive maintenance should not be out of anyone’s reach. Let’s take a look at what is on offer in the UK when it comes to popular digital multimeters that can be used for automotive use.