What is LH-Jetronic?
Bosch LH-Jetronic is an electronically controlled petrol port-injection system closely related to L-Jetronic. Its distinguishing feature is a thermal air-mass meter in place of the conventional L-Jetronic spring-loaded airflow flap. The ECU uses the air-mass signal and other operating information to calculate pulses for electrically operated port injectors. [1]
How it works
- An electric pump delivers fuel through a filter to a rail feeding the main injectors. In the Bosch and Volvo examples, a manifold-referenced pressure regulator maintains the pressure difference across the injectors and returns excess fuel.
- Air flows through a thermal air-mass meter between the air filter and throttle. The meter produces an electrical signal related to the mass of air entering the engine.
- The ECU combines this signal with engine speed, coolant temperature, throttle and other inputs applicable to that vehicle. It calculates the pulse duration needed for the electromagnetic port injectors.
- Where lambda feedback is fitted and active, the ECU uses the exhaust oxygen sensor to correct the mixture. Starting, warm-up, idle control and other functions depend on the particular LH version and vehicle. [1, 2]
For an illustrated example, see the system overview and air-mass-meter drawings in Volvo's LH 2.4 service manual for the 240 B230F. Bosch's technical instruction illustrates both vane and thermal meters.
Air-mass meter: hot wire and hot film
In a hot-wire meter, an electrically heated fine wire sits in the incoming airflow. The control circuit keeps it hotter than the incoming air; flowing air removes heat, so the power needed to maintain the temperature provides the basis for the air-mass-flow signal. There is no spring-loaded measuring flap. The temperature of the incoming air is accounted for by the sensor's measurement circuit. [3, 1]
Bosch also describes a hot-film thermal air-mass meter, using a heated film element instead of the fine wire. Do not assume that a particular LH version uses a given sensor without checking its part number and vehicle documentation. Some hot-wire systems, including the documented Volvo B21F and 240 LH 2.4 examples, heat the wire briefly after shutdown to burn off deposits; that sequence is application-specific. [1, 3, 2]
| Feature | Conventional L-Jetronic | LH-Jetronic |
|---|---|---|
| Air measurement | Spring-loaded vane and potentiometer; air-volume-flow-related signal. | Thermal hot-wire or hot-film meter; air-mass-flow signal. |
| Moving measuring flap | Yes, in the conventional vane meter. | No mechanical measuring flap in the thermal meter. |
| Main injectors | ECU-pulsed electromagnetic port valves. | ECU-pulsed electromagnetic port valves. |
| Injector interchange | Depends on the actual injector, fuel pressure, ECU driver and installation; the family name alone is insufficient. | |
Injectors and electrical differences
The LH main injector is a solenoid-operated port valve. The ECU changes its opening time to meter fuel; it is not the continuous pressure-opening mechanical injector used in K- or KE-Jetronic. In Volvo's documented 240 B230F LH 2.4 example, the main injectors spray simultaneously; other implementations must be checked individually. Some applications also have a separate cold-start valve. [2]
The injector technology is broadly shared with L-Jetronic, but electrical and hydraulic specifications are application-specific. For example, the Volvo 240 B230F LH 2.4 manual gives 16 Ω per main injector, while the cited Bosch Volkswagen L-Jetronic workshop example gives 2–3 Ω with series resistors. Neither value defines the entire LH or L family. Match resistance, driver or ballast arrangement, flow at the specified pressure, spray pattern, connector and seals to the actual installation. [2, 4]
Versions and workshop checks
LH has several versions and manufacturer-specific installations. As one example, Volvo describes its 240 B230F LH 2.4 system as having adaptive lambda and idle functions and an on-board diagnostic socket. Those features and their diagnostic procedures should not be assumed for every LH system. [2]
- Identify the vehicle, LH version, ECU and air-mass-meter part numbers before substituting a meter or using a wiring or diagnostic chart.
- Check intake leaks after the air-mass meter, wiring, power and grounds, fuel pressure, ignition condition and relevant temperature or throttle inputs before condemning the meter.
- The thermal sensing element is delicate. Use the vehicle-specific test procedure and avoid touching the wire or film; do not assume every unit has the same cleaning or burn-off procedure.
- Inspect injector supply and control, leaks and spray using appropriate equipment. A stored fault code, where available, is a starting point for diagnosis rather than proof that a component has failed.
Follow the vehicle's fuel-pressure release procedure before opening the fuel system. Compare all readings with the specification for the exact application.
Related Bosch systems
L-Jetronic uses a vane airflow meter in the conventional Bosch system; D-Jetronic uses manifold-pressure sensing. Both use pulsed electrical main injectors. K-Jetronic and KE-Jetronic instead use a mechanical fuel distributor and pressure-opening continuous-spray main injectors.
Sources
- [1] Bosch technical instruction: Gasoline Fuel-Injection System L-Jetronic, 1995/96 edition, independent PDF mirror, particularly the LH and air-mass-meter sections.
- [2] Volvo service manual TP 31361/1: LH-Jetronic 2.4, 240 B230F, 1989-on, independent PDF mirror. Diagrams, simultaneous injection and the injector resistance example apply to this installation.
- [3] Volvo service manual: LH-Jetronic system, B21F, independent PDF mirror. Hot-wire measurement and burn-off example.
- [4] Bosch Volkswagen L-Jetronic workshop manual, independent PDF mirror, test chart p. 151. Accessed 10 November 2026.