Webasto Heater HL125 Service Manual

Webasto Heater HL125 Service Manual
Webasto Heater HL125

The HL series of Webasto Air Heaters were manufactured from the 1960’s and as a result formed a large proportion of Webasto Heating Installations within the UK at that time. The heaters were solidly designed using well proven technology, and as such have a reputation for rugged reliability and service ability. Many of them have lasted in excess of 20 years or more.

Since the mid 1980’s, a new generation of heaters has appeared from Webasto, using new burner technology and much simpler design. These heaters made the HL30 obsolete and offer many advantages over older designs. This manual is intended as a guide for those who have had no experience of the older series heaters,
and should be read as a whole before any service work is undertaken.

Operating Principles
The principle of operation for all heaters is similar. Figures 1, 2 and 3 refer to model HL30, minor differences with other models are discussed separately. When the heater is turned on, power is switched to the glowplug (2). This pre-heats the combustion chamber in it’s vicinity.

After 2-3 minutes pre-heat, power is switched to the main heater motor (78) and simultaneously the fuel solenoid (10). Since the fuel pump is driven directly by the motor, fuel is now pumped through the open solenoid, into the fuel pump and injected into the fuel delivery pipe (E, fig 3). From here it is passed into the rotating atomiser (66) and sprayed past the glowplug into the combustion chamber (40). Simultaneously, the combustion air fan (64) draws air through the combustion downpipe (20) and blows it into the combustion chamber to be mixed with the fuel.

This mixture is ignited by the glowplug, and hot exhaust gases pass through the heat exchanger (40) to be exhausted through the downpipe (22). Simultaneously, the fresh air fan (30) blows air over the hot heat exchanger and this is discharged as hot heating air into the ductwork. It is to be noted that combustion (primary) air and fresh (secondary) air cannot mix, and that heating air is therefore free from fumes.

As combustion becomes established, the temperature inside the heat exchanger rises until the heat detection thermostat (HD thermostat) (4) contacts close. The glowplug now de-energises and combustion is self-sustaining and stable. When the heater is turned off, the solenoid is de-energised and closed, interrupting the fuel supply. Combustion ceases and the heater cools to the point where the HD thermostat contacts open, shutting the heater down. The blower motor is energised until this happens.

Semi-Automatic Control (SA Control)
Before approximately 1980, most heaters were supplied with SA Control. Under this system, all timing sequences are carried out using a clockwork timer switch (12, figs 4 & 5). Closing of the HD thermostat contacts energises a relay (14, figs 4 & 5) and the green light glows. This circuit also bypasses switch circuits so that the heater remains running when the switch knob reaches the fuel heat position. If the HD contacts do not close the heater cuts off at this point. With the heater running, reduced heat can be selected. In this case, current is switched through different motor windings

Overheat protection is provided by a mechanical cut out (7, fig 1 and 5 , figs 4 & 5) which interrupts the feed to the relay coil so that the heater stops dead. After some minutes, the cut-out should reset itself. Webasto HL65, HL95 and HL125 SA heaters all use 5 and 7 way Hella caravan type plugs and sockets to connect heaters to wiring components. Early HL30 SA heaters use the same system, although from about 1977 onwards a 7 way plastic junction box was used (fig 5). With the exception of this box, SA wiring components are interchangeable for the entire range.

Fully Automatic Control (FA Control)
FA Heaters first appeared in about 1980 in the UK and for some years there was an FA/SA option.After two or three years, however, fitting of FA heaters become almost universal. Under this system, timing and switching is carried out by an electronic control box (11, fig 6). This box and wiring system is common to all HL FA heaters. Closing the HD contacts results in the instantaneous de-energising of the glowplug, unlike SA systems where it continues until the time cycle is completed. Failure to ignite first time results in the start cycle resetting itself after about 3.5 minutes. Thus FA systems will attempt two automatic starts, after which the switch must be reset if combustion is not established.

Overheat protection is by means of an overheat fuse (9a fig 1, 5 fig 6). Continuity through this fails when air temperature around it reaches 100°C and this interrupts power to the fuel solenoid, which closes. The heater therefore shuts down in the normal way. If the heater started first time and subsequently overheats, the control box will automatically repeat start when the heater stops. It will obviously not re-ignite however. Overheat fuses are not repairable.

Most of the later HL125 FA heaters still used the red mechanical overheat thermostat, but this is wired using the third contact so that if it overheats, an electrical in-line fuse (2 amps) blows, cutting power to the solenoid. This fuse is usually to be found taped to the overheat thermostat leads.

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