The Webasto Thermo 90 ST is an auxiliary heater fitted to cranes and heavy vehicles. It warms the cab or the coolant circuit so the machine starts and works in the cold. When one fails out of warranty, the shop or fleet mechanic is the one fixing it, and they need workshop-level procedures, test steps and fault diagnosis in front of them.
What this manual helps you do
- Diagnose a heater that won't start or shuts down, using the troubleshooting section.
- Run the functional tests to confirm what's working and what isn't.
- Trace wiring faults with the circuit diagrams and the electrical connections.
- Replace worn components by following the repair and servicing work procedures.
- Check the technical data before you close the job up.
Who this is for
A crane service technician, a rental fleet mechanic or an independent repair shop that's taking on heater work in-house. It also suits plant maintenance staff who inherited a machine with no paperwork. If you're the one holding the tools, this is written for you.
FAQ
Q: What do I receive and how is it organised?
A: You get the set: 3 PDF files, 164 pages in total, in English and German. The main manual is publication 1301496B02_DE (55 pages), with publication LIT1301498A (54 pages) and Manual 3 (55 pages). The text is searchable and copyable, so you can jump straight to a fault or a procedure.
Q: Which heater does it fit?
A: The Thermo 90 ST, and the same manual also covers the Thermo 90 S. The title reads Thermo 90 S and ST Workshop Manual.
Q: How does it fit the job I'm doing?
A: It's the complete workshop manual for this heater. It runs from introduction, general description and functional description through technical data, troubleshooting, functional tests, circuit diagrams, servicing work and repair, so you can follow one fault from symptom to fix.
Print it, search it, or keep it on the shop laptop, and you'll have the procedures at the bench when the heater comes in.
Click here to view the preview (PDF)
Models covered by this manual
This service manual is also used for one further Heater model: Thermo 90 S. Check the model and serial number on your data plate before ordering.
📘 Show Index
Table of Contents:
- Publication 1301496B02_DE (55 pages)
- Publication LIT1301498A (54 pages)
- Introduction
- Contents and Purpose
- Meaning of Signal Words
- Additional Documentation
- General Description
- Combustion Air Fan
- Heat Exchanger
- Temperature Sensor
- Functional Description
- Switching On
- Heating Mode
- Control Mode
- Technical Data
- Troubleshooting
- General
- General Fault Indications
- Fault Indications on Fault Lock-out
- Functional Tests
- General
- Settings
- Testing Individual Components
- Circuit Diagrams
- General
- Thermo 90 ST Circuit Diagram Legend
- Servicing Work
- General
- Work on the Heater
- Work on the Vehicle
- Repair
- General
- Dismantling and Assembling
- Product Information
- Manual 3 (55 pages)
- Introduction
- Contents and purpose
- Meaning of signal words
- Additional documentation to be used
- General description
- Combustion air fan
- Heat exchanger
- Temperature sensor
- Function description
- Switching on
- Heating mode
- Control mode
- Technical data
- Troubleshooting
- General
- General fault indications
- Fault indications on fault lock-out
- Function tests
- General
- Settings
- Testing individual components
- Circuit diagrams
- Service work
- General
- Work on the heater
- Work on the vehicle
- Repair
- General
- Dismantling and assembling
- Packing, storage and shipment
- Sequence of functions
- General fault indications
- Fault indications on fault lock-out
- Visual check, burner back wall
- Visual check, pilot flame outlet opening
- Visual check, complete burner
- System circuit for Thermo 90 S, 12 and 24 V, with standard clock
- circulating pump control
- System circuit for Thermo 90 S-ADR, 24 V, with switch
- System circuit for Thermo 90 S-ADR, 24 V, with switch without power take-off
- System circuit for Thermo 90 ST, 12 and 24 V, with standard digital timer
- System circuit for Thermo 90 ST-ADR, 24 V, with switch
- System circuit for Thermo 90 ST-ADR, 24 V, with switch without power take-off
- Specimen installations for heater in a truck
- Fuel supply
- Webasto fuel extractor
- Fuel extraction from the plastic tank (extraction through tank drain screw)
- Fuel extraction from the plastic tank (extraction through tank fitting)
- Pipe/hose connection
- Metering pump, installation position and attachment
- Exhaust pipe opening, installation position
- Electrical connections (Thermo 90 S)
- Changing the circulating pump
- Changing the temperature limiter and temperature sensor
- Changing the combustion air fan
- Changing the burner, flame monitor and glow plug
- Changing the burner head
- Contents and purpose
- Meaning of signal words
- Additional documentation to be used
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