Saab C900 Heater Control Valve Replacement

and Coolant System Flushing

Compiled by Craig Dewick

December 2004 - May 2005

The heater control valve in a Saab C900 regulates the flow of hot coolant mixture directly from the engine compartment through the heater core that's located in the bottom section of the middle part of the dash. These valves are incredibly annoying to replace due to the lack of direct access to the bolts holding the heater control valve in place on the heater core.

However, if you take it carefully and don't let your frustrations get to you as you pull your hair out trying to work out how to get better access to the heater control valve securing bolts, you will get there in the end!

If you plan on changing the heater control valve, also consider taking the opportunity to flush out and clean the entire coolant clean at the same time. That way you can flush/drain the coolant system (as well as running a radiator flush/clean product through the system first), then replace the heater control valve, then re-fill the coolant system.

Ready to Flow...

Before starting work, decide if you want to do a complete coolant flush in conjunction with changing the heater control valve, or just change the valve. If you decide to include a complete coolant flush, get the supplies for that task ready in parallel with getting a new heater control valve, etc. and take care of flushing and draining the coolant system first.

- To flush the coolant system firstly make sure that the heater control valve is set so that it's half to all the way open otherwise when you put coolant flush additive into the coolant system via the expansion tank it won't flow through the heater core as well and when you drain the coolant the hoses to the heater control valve will probably still have more than a trickle of fluid in them.

- NOTE: If you are going to add a coolant system flushing additive (such as Nulon R40 Radiator Flush and Clean), pour the additive into the radiator expansion tank, run the engine for the specified amount of time to get the coolant up to normal temp, mix the flushing additive and allow it to do it's job. Allow the engine to cool to a safe temperature before slowly opening the expansion tank cap to release the pressure that has built up. Do NOT attempt to open the radiator drain plug until the system has cooled sufficiently AND you've released the pressure via the expansion tank cap!

Disconnect the battery negative before doing anything with the radiator drain plug which is itself located located on the lower-right corner at the back of the radiator when looking from the back to the front of the engine bay. The drain plug will allow the whole coolant system to be drained once the expansion tank cap has been opened. You will need a 17 mm socket and depending on what's getting in the way (cables, etc.), you might need an extension bar on your ratchet spanner. Be careful of the battery terminals even though the negative should have already been disconnected.

Bearing in mind the earlier warning, if the system is full of gunk for whatever reason and the radiator won't drain out by itself after opening the drain plug (as it was with my 900i), you might need to crack the drain plug open slightly and run the engine to pressurise the coolant system and force the gunk plugging the drain plug opening out of the radiator. Be sure to keep the radiator expansion tank cap fully closed and stay at the car when doing this since as soon as the gunk lets go, the radiator will drain quickly while the engine is running. Make sure you turn the engine off straight away and let the rest of the coolant run out normally! It shouldn't take long for the pressure to build up enough to push the gunk out the drain plug. Remember to disconnect the battery negative after the fluid has stopped flowing out of the drain plug and before you go to tighten the drain plug up again.

Once the coolant stops running out (a small amount will stay in the heater core), wash away the coolant left spilled on the inside of the engine bay with some clean water, then pour about 5 to 6 litres of clean water into the expansion tank and let it wash the remaining traces of old coolant, etc. out of the system via the drain plug. That helps to ensure the system is as clean as possible for when new coolant mixture is added after the heater control valve has been replaced.

Re-tighten the radiator drain plug then unclamp the two hoses from the back of the heater control valve and gently remove each one in turn from it's respective pipe port.

- If you decide not to flush the coolant system, you will still need to remove the coolant from the two hoses going to the heater control valve. Carefull unclamp them and pull each one off it's respective pipe port on the back of the valve gently. Let the fluid run out but remember to rinse it away with clean water immediately since it can damage paint, etc. due to the glycol content (brake fluid is harmful to paintwork for the same reason).

Once all this is done, you're ready to get into the fun part which is the actual replacement of the heater control valve! 8-)

Standby for Action!

This is the meaty bit - ie. actually replacing the heater control valve. Before you get to this you must have either fully flushed and drained the coolant system or at least drained the coolant out of the hoses leading to the back of the heater control valve. And of course disconnecting the hoses from the valve.

Removing the old Heater Control Valve
Damaged heater control valve viewed through left speaker opening	Coolant pipe ports through firewall with hoses disconnected	Close-up view of the damaged heater control valve

Accessing the bolts which secure the valve to the heater core pipes is a contender for the most pain-in-the-ass repair job - particularly the two away from the camera which have bugger all access. They will either be 4 mm sunken hex-drive (ie. allen-key) or large Philips head bolts. If they're the allen-key type you can wangle a 4 mm allen key with care to slowly loosen the two bolts with highly restricted access.

You can also get to the bolts from under the dash. I removed the 'air pipe' but didn't remove the knee bolster (but I probably should have). See the later pics for underneath pics!

Comparison of Old and New Heater Control Valves
Bottom of old valve showing accumulations of coolant gunk	Close-up of top section with broken control shaft	Close-up of new valve showing complete control shaft

Once the old valve has been removed (note that yours might not have failed in the same way as the one I removed from my car), you need to use some screwed up pieces of paper towel stuffed into each heater core pipe opening to soak up a bit of the fluid so the level is below the mounting face for the new heater control valve.

Then you need to clean up the mounting face. Tom Townsend from Townsend Imports recommends using a pot scrubber or some other non-metallic scouring pad to clean up the metal. If the old valve had gasket sealer or silicone used to hold the o-rings into the bottom of the valve, make sure all traces of that material are removed from the heater valve mounting face before you get cleaning with the scouring pad.

Installing the New Heater Control Valve
Heater valve mounting face partly cleaned up	New valve in place with pre-vaselined o-rings installed in base	Coolant hoses re-connected to the hose ports

To aid in the cleanup, slightly wet the scouring pad and dry off the heater valve mounting face every once in a while to see if you've cleaned it up enough. You don't have to get all the natural corrosion off the metal - just make sure the section around each pipe opening is clean, smooth and all the roughness is removed so it feels smooth when you run a fingertip around each pipe opening. That will help to ensure the new o-ring seals for the new heater control valve make a good seal when compressed by tightening the securing bolts.

Use something like Vaseline or a similar product to rub all over the two o-rings before pushing them into their seats in the base of the new heater control valve. This will help retain them when you position the new valve and push the hose ports through the rubber 'gasket' into the engine bay, and make it easier to adjust the final position so that you can carefully feed each one of the four new securing bolts up through the mounting face into the captive nuts that slot into clips moulded as part of the new valve's base.

Ode to the Saab C900 Heater Control Valve
Underside view taken from lying awkwardly on the floor under the dash	Lying under the dash half out the door!	Spousal Attack Unit T (ie. my wife Tiffany!) scratching her head

Referring the picture above	Not so squeezy!	Some additional insight
Note the location of the securing bolts in the above picture. The other two bolts are extremely difficult to access from above or from below. The object on the left of the picture is the heater core itself and it makes access from underneath impractical.		Removal of the knee bolster as well as the air ducting (I just removed the air ducting) would probably have made the job a bit easier. 8-) I've also been told that the heater core itself can be removed fairly easily - will need to check into that some more.

There is an excellent document covering C900 heater valve repair/replacement authored by Tom Townsend which is available at the Townsend Imports website. Note that much of the site where this document lives uses a dynamic IP address so it's not always available.

The same document has been reproduced at at saabnet.com but beware of saabnet.com's censorship and access-blocking policies if you find you can't access that link. 8-)

The Final Phase

Once the four securing bolts have been tightened sufficiently (and evenly, to ensure even pressure on the o-ring seals), re-connect the hoses in the engine bay. Make sure the hose from the water pump connects to the top hose port! If you connect them the other way around you will get a small amount of heating with the valve turned off due to the internal design of the valve itself.

Re-connect the control rod that goes to the middle rotary knob on the dash and push the dash fascia back in enough so the valve end can be slid onto the shaft. If you don't plan to do anything else while you have the dash apart, you can push the fascia back into place and re-install the four bolts that hold it securely, otherwise leave them out so you can do more work later on (remembering to be careful with the heater valve control rod).

Turn the knob on the dash to the hottest setting and make sure the outermost vents on the dash have their flaps opened.

- If you only released the small amount of fluid from the hoses going to the heater control valve, reconnect the battery negative, start the engine and top up the expansion tank with some fresh coolant/water mix so the level is correct.
- If you did a full coolant flush and drain, check again that the radiator drain plug has been re-tightened, get your new coolant, and open the thermostat bleeder nipple (the brass fitting which uses two 11 mm spanners - one to hold the base and another to open the nipple itself). This will allow the remaining trapped air pockets to escape and give you an indication when the system is almost full of coolant.
Even though Saab's specs say you need 10 litres of coolant mixture, my C900's seem to take about 8 litres - perhaps a little more - so if you are going to use a concentrated coolant product (such as Nulon Long-Life Concentrated Coolant or Nulon Ultra-Cool), mix the right quantity of concentrated coolant with the appropriate quantity of clean (preferably distilled/demineralised) water in something like a clean household bucket, then slowly pour it into the cooling system of the engine via the coolant expansion tank and watch the bleeder nipple on the thermostat.
Close the bleeder nipple firmly when air bubbles have stopped coming out and all you are getting is a slow dribble of coolant mixture. Wash the spilled coolant away from the top of the transmission with some clean water.
Reconnect the battery negative and start the engine. As the coolant mixture starts to circulate, the small overflow hose into the expansion tank should let the remaining bubbles in the top of the radiator flow up into the expansion tank and you might need to add a little more coolant and water to keep the tank topped up to half-way.

Check that no coolant is leaking from where the hoses attach to the back of the new heater control valve or from where the new valve is mounted onto the heater core inside the dash. If the o-ring seals have been properly compressed and the mounting face on the heater core was properly cleaned there won't be any leaks.

Allow the engine to come up to operating temp so the thermostat operates to modify the path that the flow of coolant takes. Also watch that the cooling fan(s) (one or both, depending on the year model) operating when the thermoswitch mounting at the top-left corner of the back of the radiator reaches it's switch-on temperature.

As you wait for the engine to reach normal operating temperature, make sure the heater valve is letting heated coolant flow through the heater core by checking that the outer vents on the dash are giving a stream of hot air. Once they are, turn off the heater valve and check that the stream of air starts to go cold as the heater core loses it's supply of heated coolant and the airflow draws out the remaining latent heat.

Let the radiator cooling fan(s) cycle a few times, and see that the coolant level in the expansion tank has not dropped below the minimum level. It shouldn't since the coolant mixture will expand as it heats and contract as it's cooled by the fans, but if it has gone too low top it up.

Turn off the engine, clean up and that's the end of it - you have successfully changed the heater control valve (and perhaps also fully flushed and refilled the coolant system) in your Saab C900!

Let the car sit overnight and check that the coolant level is correct when the engine is cold. Adjust if necessary.

If you'd like to contribute any additional or updated information for this page, please let me know.

If you would like to discuss this page and it's contents with other Saab enthusiasts, please visit the Saab C900 Technical Forum at Craig's Classic Saab Workshop.

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