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I have a 1988 S4. My transmission cooler hoses were leaking so I decided to replace them. To get better access I drained the radiator fluid. Today I finished installing the oil hoses and filled the rad. I took the car for a drive, saw the red light coming on and noticed the coolant temp approaching the red line. I got the car home and found I had lost coolant from the reservoir. I let the car cool down, filled the reservoir with coolant. I started the car and as the temp rose the fans would not come on and the rad air flow flaps stayed closed (the flaps always worked). When I turn on the AC switch the fans come on. I checked every fuse, all fuses are good. In the spring I replaced the thermostat and the PO replaced the water pump 2-3 years ago. As the temp increases I noticed the reservoir level doesn't change.....stuck thermostat?
Well I tested a couple things: hood switch functions fine (14 ohms open and infinite ohms when closed. I removed the electrical connectors from the intake air temp sensor and shorted them together. With the ignition on the fans run. I also removed the electrical connector from the drivers side FWD lower radiator switch and with the ignition on the fans run.
The hood switch serves only to stop the fans from working when the bonnet is open [a safety feature]. The Temp 2 is there to tell the engine management system how to fuel the motor in transient conditions like cold start etc and has nothing to do with the cooling programme. The intake air temp switch is used when the motor is shutdown and the output is used to determine how long the fans run on after the motor is turned off. As I am aware the a/c control system overrides the cooling logic and drives both fans irrespective of coolant temperature, same for the auto box temperature switch. The temperature as shown on the dash and as understood by the LH/EZ computers is taken from the sensors on the water bridge and thus the temperature indicated on the dash panel is no inferential indication that the thermostat is opening. If the alarm annunciator lights up I assume that is driven by one of the sensors on the water bridge but I am not 100% sure of that.
The sender located on the front bottom driver's side of the radiator controls the cooling programme and thus the logic derived from this output signal modulates the fan programme and the flap programme. The only thing I would do with the flaps is remove them but then I have lived with the 928 in a very hot climate for the last 20 years. There is no feedback I am aware of that tells the driver what temperature it is recording but its function can be checked by comparing resistance to output.
When the motor is cold the flaps are open and when the ignition is switched on with a cold motor [i.e. below 79C] the flaps should then close. The fans will turn on and the flaps will open only when the temperature in the coolant reaches the setpoint of the programme when the coolant leaving the radiator reaches 79C, and then operation commences as per the logic contained in the extract described in the attached image. Generally speaking both fans operate simultaneously at the fan speed stated in the description but as I understand, the logic checks that both fans are taking current draw and if it detects a problem with one fan the remaining fan is driven to full load. The second attachment is the logic diagram, the graphics are a little tricky to follow but on that diagram it also shows the resistance values of the radiator mounted sender [i.e. not Temp2].
If you cannot find what is not working then maybe the water pump impeller has taken a dump. If you get the motor fully warmed and the temperature indicated rises beyond that normally seen, check the temperature of the return hose- if it feels cool [temperature less than 79C on the driver's side radiator end tank] then coolant is not circulating correctly and the issue becomes one of thermostat loop or water pump. When the pump fails one of two things can happen- the bearing goes and with it the seal so you lose coolant. The other possibility is the plastic impeller falls of the pump shaft- this can happen if the bearing fails and mechanical contact is made or it can happen even if the bearing does not fail [as has happened to me twice over the years].
Finally when you changed the thermostat did you change the rear seal? The thermostat does two things- it progressively opens a flow path to the radiator and it progressively cuts off the internal circulation. If the rear seal is not working correctly water flows preferentially around the internal circulation loop.
Thanks for the reply. When I changed the thermostat I did replace the rear seal. It sounds like the sender on the drivers side front rad is most likely the culprit. A failed water pump wouldn't affect the flaps.
Thanks for the reply. When I changed the thermostat I did replace the rear seal. It sounds like the sender on the drivers side front rad is most likely the culprit. A failed water pump wouldn't affect the flaps.
Steven,
By all means check everything but loss of coolant is not explained by an instrumentation issue. If you emptied the coolant system then getting it fully filled and vented correctly can be tricky and it is quite normal to have the level seemingly drop after an initial run. Theoretically the system is self venting but this does not always happen. After draining my system I invariably run the engine until it reaches normal temperature without the reservoir cap and burping the hoses in the interim. Some believe this does no good but I find it does. Also, whilst warming up set the heater to full hot to ensure coolant is circulating in that loop.
The loss of coolant is because the car overheated, the coolant was coming out of the coolant reservoir. If the temp sensor at the front of the rad stopped working the fans wouldn't come on and the flaps would stay closed. I would assume that would cause the car to overheat?
Expanding some on Fred's excellent description, and in spite of folks who suggest the the system is self-bleeding... I'm about 50/50 on getting the system burped successfully without some help. Things that work for me:
-- Car sits a little nose-down while filling the system. The reservoir needs to be the highest spot in the circulating system while filling and burping.
-- Keep The Reservoir FULL as you warm the engine initially. Like to-the-tippy-top full.
-- I sometimes disconnect the vent hose on the radiator to allow it to fill completely. Use care because the plastic nipple on the radiator can be fragile. Remove the hose to vent vapor, restore it when liquid is venting out.
-- As the engine warms, rapidly squeeze the top radiator hose to make sure it's full of water. More importantly, it will help move air from the top of the water bridge.
-- Keep the temp slider on the HVAC panel at full-hot, so there's circulation through the heater and at least the right side cylinder head.
-- Blip the throttle to encourage circulation via the water pump.
---
The thermostat MUST OPEN and allow circulation through the radiator before the sensor for the fan (lower left front of radiator, opposite the lower radiator hose nozzle) sees heat and runs the fans. The thermostat requires actual liquid circulation to see engine heat, so the challenge seems to be breaking the air/vapor bubble that forms in the water bridge. The lower hose will be cold while the upper hose gets hot, until the thermostat actually opens. Then suddenly the temp will drop a bit on the gauge, the lower hose gets hot, and the fans start. OK to squeeze that lower hose too, but be aware that the fans will run as soon as circulation starts so keep your hands clear of them.
Also: The thermostat has a small vent hole in the disk to allow vapor/air to clear the lower hose. The thermostat must be installed with that vent hole on top, or risk vapor-lock at the top of the hose at the nozzle. Once the system is full ad circulating this is less of a worry, but it will slow the initial burping effort a lot. The vapor alone isn't enough to heat the wax cartridge in the thermostat to cause it to open. That vapor will of course end up in the water bridge so it's a continuous effort while filling and purging to maintain liquid circulation there.
It's quite normal to fill the system initially, drive the car, then find that it needs another quart or more of coolant to bring the reservoir level to normal. On my car, "normal" is a little above the seam in the reservoir, less than an inch. The coolant will find that "normal" level once all the air is burped out. That may take a little bit of driving. I carry the last gallon of distilled water with me on that first drive so I can top the reservoir as needed.
Housekeeping stuff: The two vapor vent lines at the front of the engine and the top of the radiator must slope up continuously towards the reservoir. There is not enough differential pressure in those lines initially to push vapor bubbles through even a minor low spot that's filled with liquid. More than a few clinic cars have passed through with the lines looped down as they pass the shock tower rather than over it as they should. having the car a little nose-down helps some with this while filling. The slope is much less of an issue once the system is circulating under pressure, but initially it can challenge the filling and burping effort.
The loss of coolant is because the car overheated, the coolant was coming out of the coolant reservoir. If the temp sensor at the front of the rad stopped working the fans wouldn't come on and the flaps would stay closed. I would assume that would cause the car to overheat?
Steven,
If the flaps are not opening then air cannot flow across the radiators as it should do. If the flaps are open when the engine is cold and not running, they subsequently close when the engine starts and then do not open when the temperature rises then such would suggest a problem with the measured temperature signal. Thus I would suggest you pull the connector from the sender, measure the resistance with the motor cold, run the motor some until the motor gets to the normal operating temperature. Shut the motor down and then measure the resistance. At normal operating conditions the coolant should be in the region of 85C. You have the resistance values on the diagram I posted and even better if you can measure the temperature of the end tank with an I/R gun.
So.....I removed the electrical connector off the Rad temp switch and checked resistance with the engine cold 2140 ohms, engine at normal operating temp I checked again and 2050 ohms and with the temp indicator at the top white band (above normal operating engine Temp) 2005 ohms. I filled the reservoir to the top and burped the upper Rad hose. I also put on full interior heat (HVAC). I noticed the lower Rad hose never got hot, the fans didn’t come on and this time the air flaps didn’t stay closed, they were full open. I think it’s safe to say the Rad temp sensor and the thermostat need replacing. As for the previous question the sensor I shorted was the air temp sensor on top of the intake, that made the fans come on. The Rad temp sensor I removed the electrical connector and that also made the fans come on with the ignition on. Thank you so much everyone for your help, being new to this car I really appreciate the help.
My assumption for the Rad temp sensor needing to be replaced is not much of a difference in resistance from the coolant being cold and hot. The thermostat is stuck closed because the lower Rad hose never got hot.
I believe you're overthinking this. You drained the coolant then when you refilled it you most likely got a air trap in the system. Like the others said try to raise the rear of the car leave the rad cap off turn the heat on full temp the run the engine all the while feeling the large rad hose from the bottom connection. When that hose get's warm your system is bled.
09-07-2018, 01:31 AM
Overheated, fans not coming on.
