Michael Benno

1988 928 S4 5spd.

I always have heat in my car. The temp slider on the HVAC gives me warm to very hot. I have done some initial trouble shooting an have determined my combination valve and footwell valves are leaking. I plan to replace them this winter with a dash project. However, in the meantime I'd like to be able to drive without being heated on 70-degree days.

It seems to me that the heater core is always getting hot coolant through it, which leads me to believe my heater valve is not closing all the way despite having tested with good vacuum. So it would seem that valve is not closing all the way because 1. the valve is defective and wont mechanically close, or 2. the control unit is not sending enough vacuum to close the valve.

Here is the troubleshooting I have done so far: (using these instructions https://www.928gt.com/default5.htm)

I have tested the temperature control unit. At the coldest setting the flap fully opens and fully closes.

I have tested the vacuum system source and all the lines from the vacuum control manifold. While I do have leaks to the footwell flap (yellow) and combination vent (blue line), I have strong vacuum to the heater control valve (white line) and can see it functioning.

I'd like to see the valve in action using the temp control while the car is in action but not sure how that can happen with the air cleaner off.

I'd appreciate your thoughts on how to figure this out.

StratfordShark

When you say you have "strong vacuum" to the heater valve, have you actually measured on a gauge? With leaks at two of the actuators, you will be pulling vacuum down through the entire system.

Do Wally's tips on the 928GT site also discuss the internal and external sensors? I don't remember but I do recall they are the best instructions on how the whole system operates. The fact that you get warm to very hot indicates your sensors may be OK, but the system overrides to full cold at the coldest setting irrespective of the sensors. Is your variation across the range of the controller, or does it go full hot from just off the left hand cold end which suggests sensor fault?

If you want to observe the heater valve arm operating it's fine to run the engine with air filter off and idling.

You haven't done any tests yet on the heater mixing flap motor. You can check this just with ignition on, seeing if it travels between the cold and hot positions between the two ends of controller scale. If you have a fault here then the mixing flap may not be closing off hot air from the matrix. If the mixing flap is closing correctly and the seal is good, then even with faulty heater valve allowing hot water to pass you would still get pretty good control over the temperature in cabin.

dr bob

Short-term fix is to use a cable tie to hold the heater valve closed until you are able to restore the integrity of the rest of the actuators.

The HVAC vacuum system is tied to the vacuum reservoir in the rear of the driver's-side front fenderwell. Vacuum from the engine comes via a tap on the brake-booster check valve fitting, and another check valve.

In anticipation of having leaks in the HVAC system possibly disabling more critical vacuum function like the intake resonance flap in S4+ cars, vacuum doorlocks in early cars, cruise control servos in all cars and more, there's functionally a restriction in the HVAC vacuum supply line. Leaks in a couple actuators allow air to bleed into the whole HVAC system leaving it without adequate vacuum to hold the heater control valve fully closed. You will be fixing those leaks this winter you suggested, and at that time you'll remove the cable tie from the heater valve so it can again allow flow to the heater core.

The 'failure' positions on the various actuators and valve consider the need for heat and defrost functions critical. So as you lose vacuum due to leaks, you'll get hot air to the windscreen, reduced flow through the dash center vent, and some flow from the floor vents. The recirc air flap won't fully close either, so you'll have outside air coming in. And the heater valve won't close.

GlenL

Or run a vacuum line from manifold vacuum to the heater valve. Get a "Y" and add the line from the connection to the HVAC at the brake booster. You can experiment with putting it on the engine side and HVAC side of the one-way valve (if you've got one.) This will also tell you if the heater valve will close all the way with full vacuum.

Michael Benno

First of all my apologies for not including the model year: 1988 S4,

Thanks everyone for the suggestions. I will try and answer all your questions below.

I have been using a mighty vac to do the vacuum to do the testing with the ignition off and followed the progression in the Wally's word.

I have not tested the sensors. But temp seems to get gradually hotter as I mover the slider.

I have tested (witnessed) the heater mixing flap motor flap moving full range as I move the slider. I also checked the adjustment arm to ensure the flap fully opens and fully closes.

The suggestion that the two known defective diaphragms are bleeding vacuum from the temp valve seems logical. However shouldn't that only be the case when the manifold as those lines open? Shouldn't the manifold act like check valve and only open those lines up on the appropriate setting.

It does sound like there could be many factors causing heat when no heat is selected.

What do you all think of this, in an effort to identify or eliminate the heater valve function as the problem. I will disconnect and plug the vacuum line from the control manifold. Then I will run a vacuum line from the valve to a might vac in the car and test heat / cool by actuating the valve manually.

Seem reasonable?

WallyP

There are a couple of possibilities for a problem with the coolant valve...

If the valve is installed backwards, the pump pressure can force coolant thru the valve even though it is fully functional as far as opening/closing. The black side should be closer to the cylinder head.

The seal between the operating flap and the outlet tube can become displaced, resulting in constant leakage even when the valve appears to be closed. A visual inspection (looking into the inlet side of the valve) is the easiest check for this condition.

ROG100

Most of the available heater valves are now made in the Republic of China and are known to fail/leak. Make sure you only replace with a German made original Mercedes unit