944 No Heat

[zooklm1]

@CarBot I have an 86 944 Turbo that the heat won’t come on. What should I check/test to determine the cause?

[CarBot]

I have an 86 944 Turbo that the heat won’t come on. What should I check/test to determine the cause?

On an 86 951, “no heat” is usually either coolant not flowing through the heater core, the heater valve not opening, or the HVAC box not directing air through the core.

First confirm the basics: engine fully warmed up, coolant level correct, and the temp gauge behaving normally. If the engine itself isn’t getting to temp (stuck-open thermostat), you’ll get weak heat.

Next, feel the two heater hoses at the firewall after the car is warm and the heat is commanded on. If both hoses are hot, coolant is flowing through the heater core and the problem is likely inside the dash (blend/air direction foam, control cables, or the heater valve not actually changing airflow). If one hose is hot and the other is much cooler, flow through the core is restricted (partially plugged core, air trapped, kinked hose). If both are cool while the engine is hot, the heater control valve may be stuck closed or not being actuated, or you’ve got a bigger coolant circulation issue.

On these cars the heater valve is a common culprit. Check that the valve moves when you move the temperature lever/knob, and that the linkage/cable isn’t disconnected. If it’s vacuum-operated on your setup, verify it’s getting vacuum and the diaphragm holds vacuum; if it’s cable-operated, verify the cable sheath is anchored and actually pulls the lever through its full travel.

Also verify the blower is actually moving air. If you have airflow but it never gets warm, that points back to coolant flow/valve/blend door. If you have little/no airflow, that’s a blower motor/resistor/fuse issue rather than “heat” specifically.

If you tell me what you observe at the two heater hoses (both hot, one hot/one cool, both cool) and whether the blower airflow is strong, I can narrow it down quickly.

[Tom]

That's a rare issue because nearly every HVAC fault on these cars defaults to heater on. I'd start by just pulling the vacuum hose off the heater valve under the engine and see if the valve opens. If the blower is blowing and the heater valve is open, heat will get into the cabin somewhere, even if not from the right vents. If the heater valve remains closed with no vacuum line, it's stuck and needs to be replaced (or fixed, but good luck with that). If the heater valve does open and let hot coolant into the cabin, then my best guess would be either a defective (leaking, stuck open) solenoid on the side of the heater core console (see video), a temp sensor, or the heat selector on the climate control unit.



You can diagnose the climate control unit faults using our diagnostic charts, which I'll attach here so that @CarBot can see them too (I think).

[CarBot]

That's a rare issue because nearly every HVAC fault on these cars defaults to heater on. I'd start by just pulling the vacuum hose off the heater valve under the engine and see if the valve opens. If the blower is blowing and the heater valve is open, heat will get into the cabin somewhere, even if not from the right vents. If the heater valve remains closed with no vacuum line, it's stuck and needs to be replaced (or fixed, but good luck with that). If the heater valve does open and let hot coolant into the cabin, then my best guess would be either a defective (leaking, stuck open) solenoid on the side of the heater core console (see video), a temp sensor, or the heat selector on the climate control unit.



You can diagnose the climate control unit faults using our diagnostic charts, which I'll attach here so that can see them too (I think).

You’re on the right track. On the 951 with the automatic climate control, the heater valve being vacuum-closed means a vacuum/control fault can absolutely leave you with no heat, even though a lot of other HVAC failures tend to default warm.

Quick sanity checks that usually isolate it fast.

With the engine fully warm, confirm the blower is actually moving air. If airflow is weak or dead, chase blower power/resistor/fuse first because you can have perfect heat available but nothing pushing it into the cabin.

Then go straight to heater core flow. Feel the two heater hoses at the firewall. If both are hot, coolant is flowing through the core and you’re looking at an air-mix/controls issue. If one is hot and the other is much cooler, the core/valve is restricting flow. If both are cool while the engine is hot, the valve is staying shut or you’ve got a circulation/air pocket issue.

On the heater valve test, pulling the vacuum line off the valve is a good quick check. With no vacuum applied, the valve should go to the open position. If it stays closed with the vacuum line removed, the valve is stuck and replacement is usually the realistic fix. If it opens and you suddenly get hot heater hoses, then the valve itself is OK and the problem is upstream in the vacuum control side.

If the valve opens but the cabin still won’t heat, I’d focus on the climate control’s vacuum solenoid(s) and the in-car temp sensing. A leaking or stuck solenoid can keep vacuum applied when it shouldn’t, and a bad interior temp sensor or control head can command “cold” all the time. Also worth checking the small vacuum lines for cracks/disconnects at the control unit and in the engine bay, since one split line can make the system behave backwards.

If you post the diagnostic charts you mentioned (or a photo of them), and tell me what you find on the heater hoses and whether the valve opens with vacuum removed, I can help you interpret the chart results and point to the most likely component. Also, if you can confirm whether your car has the automatic climate control panel (with the temp wheel and AUTO) versus the manual sliders, that changes which exact tests apply.

If you want a related Carpokes reference, search the 944/951 section for “951 heater valve vacuum” and “auto heat no heat solenoid” threads; there are a couple good walkthroughs with photos of the valve and the vacuum routing.