| FFBH and Changeover Valve Coolant Flow & General Operati | |
First post from a long-time lurker. Bought an L405 back in April and after researching a few topics it appears there's no where near as much info on the L405 as there is on the older models. So I thought I'd start adding my 2 peso's as and when.
Whilst diagnosing an issue with my heating (link coming soon) I could not find a coolant flow diagram for the FFBH. It's ended up taking me quite some time to understand how the heating in the car operates so I thought I'd write it all up here in case anyone else comes across a similar issue.
The below is how I believe the heating operates, but it is possible I've got something wrong so any corrections are welcome.
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Brief description of the heating in an L405.
The heating in an L405 is, perhaps unsurprisingly, rather complicated. You may be aware that some L405's have 2 heater matrix's, one behind the dash and another in the "luggage compartment" (boot/trunk).
The front heater supplies the windscreen, driver/front passenger face ducts, driver/front foot ducts, rear face ducts, rear foot ducts. This can be seen in this image
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The second heater (called Auxiliary Heater) supplies air to rear face ducts (in the headliner), rear lap ducts (I think these are the vents you see to the side of the seat when you open the rear doors. There is a valve that can isolate the aux heater circuit if it is not being used.
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Some diesel L405's have an electric booster heater. Operation of which is entirely automatic and boosts the heat of the air down-stream of the heater core.
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Some L405's have a FFBH (Fuel-Fired Booster Heater...previous models called it an FBH - fuel-burning heater). This lives back-right of the engine bay and pumps fuel from the fuel tank to aid in heating the cabin and engine, depending on ambient temperature.
If your L405 has timed climate/park heating, then your FFBH is paired with a changeover valve. This valve can isolate the heater coolant circuit from the engine coolant circuit. It's default state is Open. When de-energised, it connects the heater coolant circuit to the engine coolant circuit. When energised, it isolates the heater coolant circuit from the engine coolant.
It is also what I believe is causing my issues at the moment, which is why I ended up putting all this info together.
Ambient temp sensor located in left wing mirror
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Normal Diesel cold-start operation (petrol versions use different temps as status changes):
1. Engine start
2. If ambient temp less than 12C & engine coolant less than 75 then FFBH starts
2a. Changeover valve is energised (isolates from engine coolant)
2b. FFBH combustion fan, glow plug and fuel pump start
2c. FFBH Coolant pump runs continuously whilst FFBH is in idle or active operation.
(When FFBH off, coolant flow is reliant on engine coolant pump)
2d. Recirc door closes for 4 mins or until road-speed greater than 10mph or if windscreen mist is detected
2e. If ambient temp increases to 15 or engine coolant increases to 85 then FFBH turns off
3. If ambient temp less than -20 then the same steps in point 2 take place, but the Changeover valve remains de-energised and cabin blowers are not turned on so that the FFBH can help heat the engine to improve engine starting.
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I managed to find a diagram of the engine coolant circuit, as well as a flow diagram, but neither include the Changeover valve or FFBH. As I'm trying to diagnose an issue without taking the car apart, I have created the below diagrams from reading the service manual / looking at my own car.
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Coolant flow diagram
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These diagrams significantly simplify the "engine" part of the circuit, which also sends coolant off to various coolers for transmission, fuel, oil etc.
• When the FFBH is on, the Changeover valve is energised, blocking coolant from the engine. Cold coolant from the heater enters through the top of the changeover valve, into the FFBH waterpump that is energised and mounted on the side of the FFBH, through the pipe with a bleed screw (orange in diagram) and into the FFBH
• Heated water then comes out the top of the FFBH (right next to the cold-in pipe with bleed screw) and towards the bulk-head where it T's off to the aux heater (if looking yourself, this is where the rubber hoses meet the metal pipes at the bulkhead.)
• Returning from the heater's the cold water comes back through the top of the changeover valve and the cycle restarts.
• Once the engine coolant temp is above 85, the FFBH is told to turn off, at which point, the changeover valve and FFBH water pump become de-energised.
• Hot water is now able to flow from the engine, into and back out of the FFBH, which is no longer operating, and into the heaters before returning to the engine.
Which, on the engine coolant diagrams, would be something like this: (apologies for crudeness)
Hopefully this makes someone elses life a little easier! Previously owned: Range Rover L322 2007 TDv8 Vogue SE in Santorini Black
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17th Nov 2024 10:52pm
