Today we’re getting technical: It’s all about heating in laminated and tempered safety glass. We answer the questions “How does a heated automotive glass work and how can it be tested professionally?”
Nowadays, heated backlights are part of the basic design of every vehicle, but heated windscreens are becoming more and more popular. If the heating doesn’t work after glass replacement, the automotive glass is often the first thing that is suspected. But that’s not always the case: as an electrical consumer in the vehicle, the heating itself can also be affected by contact resistances and poor plug connections.
Let’s start at the beginning
Which material is used for the heating element in automotive glass?
This question cannot be answered easily – it depends! 😉
Heated automotive glass with visible heating coils is made of tungsten. Depending on additional materials, the appearance of the heating coils may vary.
The use of tungsten has two advantages:
1. Hot glass, cold supply
The specific resistance of tungsten is almost three times higher than that of copper. For copper, p = 0.017, for pure tungsten p = 0.053. Tungsten has a lower electrical conductivity, and the conductor heats up more than a copper conductor with the same supply.
What does this mean for heated automotive glass? Remember: The heating coils are largely made of tungsten, while the supply line of the heated automotive glass is made of copper. This means that the heating coils get hot, while the supply line remains cold. Clever solution 😉
2. Temperature-dependent resistance
Due to its design, tungsten also has PTC properties. PTC stands for "Positive Temperature Coefficient". This means that the electrical resistance also increases with rising temperatures and the current flow decreases.
This is really practical, as it means that the glass temperature is influenced independently of the control element, so that the maximum glass temperature of 70°C isn’t exceeded.
Other variants for heated automotive glass
The heating conductor for a backlight can also be made of silver lacquer. Silver is more conductive than other materials and has a high thermal conductivity, meaning that fewer heating conductors are needed.
Windscreens can also be coated with a silver layer. The Eurocode "ACKK" or "ACKCK" helps to identify those parts. In sunlight, they can shimmer in different shades from green to purple. To achieve an acceptable heating performance, a 48-volt supply voltage must be used, as the coating is very thin. In vehicles with a 12-volt electrical system, a 48V/12V DCDC converter is used for this purpose. In hybrid vehicles, which already have a 48-volt on-board power supply, the converter is not needed.
How can a heated automotive glass be tested professionally? What are the most common sources of error?
Let's look at the measurements of heating first:
• Resistance measurement
• Voltage measurement
• Temperature measurement
• Current measurement
If a heated glass doesn’t work, there can be four reasons for this:
1. No voltage
2. Insufficient voltage
3. Too much resistance
4. Short circuit
How to summarize best? Let’s start with the basics.
Resistance, voltage, and temperature measurement
1. Every heated glass has PTC properties. This means, when the resistance is measured with a multimeter, the glass temperature needs to be considered as well. If the vehicle is parked in the blazing sun, a higher resistance is measured than if it is parked in the cool shade.
2. Which electrical resistance can be expected from a heated windscreen with a glass temperature of about 20°C if the heating fuse is protected with 20 amps?
R = U / I
R = 14.4 V / 20 A
R = 0.72 Ohm
This means that an electrical resistance of R = 0.72 Ohm can be expected when the motor is running, and the windscreen is at normal temperature.
3.
Good to know: Direct resistance measurements are always carried out in a voltage free condition. This means that the connections are disconnected, the plugs are open and there is no current flowing.
4. Whether automotive glass works under load and has the appropriate resistance can be determined using a four-wire measurement. The resistance is determined using the voltage drop and current. However, at least 3 measuring devices (ammeter clamp, multimeter and infrared thermometer) are required for this. Based on the measurement results, the corresponding resistance is calculated according to Ohm's law (U = R x I).
5. This procedure is very precise, but it is very time consuming.
6.
My tip: If the glass is at normal temperature (glass temperature between 10-20°C), a quick resistance measurement can be carried out with a multimeter. When a resistance between the plugs for a not-connected glass heating of less than 1 Ohm is measured, there is no disruption in the automotive glass itself and the heating should work. However, the Ohmmeter must not read 0.0000 Ohms. In this case, we would have a short circuit. This procedure can be carried out with both an uninstalled and installed automotive glass. The four-wire measurement can only be used with an installed glass.
Voltage measurement
The next step is to check whether the voltage is stable when the heating is switched on. When the ignition is switched on, the full battery voltage should be present between the positive and negative connection of the heating. This is usually 12 to 12.4 V. If this is correct and the automotive glass still doesn’t heat, the issue is in the plug connection.
A badly soldered connection or corroded/bent plugs can cause contact resistance. As a result, part of the battery voltage drops in the plug connection itself and forms a voltage divider together with the downstream heater. In other words, the heater isn't getting enough power.
To summarize
1. No voltage
Check the fuse and power supply directly in the vehicle's wiring harness. Minimum 12 volts.
2. Not enough voltage
Voltage drop between connection and automotive glass should be <0.1 volt.
For backlights: Check contact resistance between solder connection and busbar.
For windscreens: Check plug connection between vehicle wiring harness and heating plug.
3. Resistance too high
Check resistance of heating at normal temperature. Standard value: <1 Ohm.
Four-wire measurement can be carried out for precise results.
4. Short circuit
If the multimeter shows exactly 0 Ohm, there is a short circuit.
If no resistance can be measured, there is an interruption.
In both cases, the automotive glass must be replaced.
If you are not familiar with the basics of electrical engineering yet, understanding the processes can be confusing first. But it sounds more complicated than it is. As always, practice makes perfect 😉 And if you have any other questions, please feel free to call our technical expert hotline.