Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Control Units
Ignition Control Unit (ICU) collects signals from altitude sensor, ignition distributor, knock sensor, temperature sensor, throttle valve switches and fuel injection control unit. Using collected data, ICU continuously updates required degree of ignition advance/retard.
There are 2 separate ignition timing maps programmed into ICU. One is programmed for regular fuel. The other is programmed for premium fuel.
When coolant temperature reaches 149°F (65°C), ICU automatically switches to premium fuel map. Premium fuel map is programmed to have timing with a slightly advanced curve in some of lower RPM ranges.
If ignition knock occurs repeatedly, ICU will automatically switch to regular fuel map. ICU will switch back to premium fuel map after a fixed amount of time or whenever engine is restarted.
Fuel Injection Control Unit (FICU) receives signals from air sensor potentiometer, altitude sensor, oxygen sensor, temperature sensor, throttle valve switches and ignition control unit. Using this data, it varies air fuel mixture as necessary.
Both control units constantly interface with each other. Each control unit contains it's own fault memory. During self-diagnosis ignition control unit will always display fault codes first. Fuel injection control unit will then display fault codes.
On 5000S models, FICU is in a slide-in bracket located behind right kick panel. See Figure . On 90 models, FICU is located behind A/C evaporator in right footwell. On 5000S models, ICU is bolted to a bracket located behind left kick panel. See Figure . On 90 models, ICU is located behind right front kick panel.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.