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If your R107 blows hot air in the middle of summer or no heat in winter, the heater valve is a prime suspect. It is an electrically controlled valve in the coolant line to the heater core.
Which valve each model has
Later R107s with automatic climate control use an electrically controlled valve on the inner fender, often called a monovalve (one circuit) or duovalve (two circuits, on some cars with separate left and right control). Earlier cars with manual heater controls use a mechanical or vacuum-controlled valve. Check your model and year in the parts catalog.
How it works
The monovalve is normally open: without power, coolant flows to the heater core. The climate control system pulses it closed to reduce heat. More closed time means less heat. That is why a failed valve often causes full heat.
Symptoms
- Heat stuck on: valve stuck open, or no control signal (it stays open without power).
- No heat: valve stuck closed (debris or a jammed plunger), or a blockage elsewhere.
- Uneven heat side to side: one circuit of a duovalve failing.
- Clicking from the valve area: it is being pulsed, which proves the control signal.
Unplug test
- With the engine warm and the climate control set to full cold, listen for the valve clicking.
- Unplug the valve. Heat should come on fully.
- Set the control to full hot. If there is no heat with the valve unplugged, the valve is stuck closed or there is a blockage.
- If heat stays on at full cold with the valve plugged in, either the valve is stuck open or the control signal is missing; check the signal with a test light.
Rebuild vs replace
Rebuild kits with a new plunger and seals are available and often work well. Some owners report that the original plunger is stronger than some rebuild-kit plungers, so keep it if it is undamaged. A new valve is the quicker fix. Flush the cooling system if you find debris in the valve.
Bleeding after
Once the valve is back in, top up the coolant and run the engine with the heater on full hot to purge air from the heater core.
Quick answers
- Can I bypass the valve? You would lose heat control. A temporary manual valve can get you through a season but is not a real fix.
- Is the valve the cause of a coolant leak at the fender? Often yes; check its hoses and seals.
