I brought the 1987
S4 out of storage today. It had been there for about a year, hadn't planned to
leave it that long, things just happened and I've had the Euro at my house.
Anyway, on the way home I was stuck in a School zone, just sitting there, not moving for 10 to 15 minutes. I finally get through the school zone, with in 1/2 to 1 mile, the Red "!" dash light came on. I noticed the Red light on the top of the Engine Temp gauge was ON, but the temp gauge never got above 1/2 way and had cooled down already. I was only 2 miles from home on a back road, I shut the engine off and coasted for awhile. When it slowed down to 15 Mph or so, I started it again. Drove a little bit to my neighborhood turn in.
I shut the car off again, and coasted, I did this a couple of times. Each time within a few seconds, the light would come on again, but the gauge still showed normal. There were no strange noises or smells to indicate it was overheating.
What may cause this?
Check if the fans run full speed. That will really confirm a high engine temp. If the lamp goes on without fans running, it is likely a signal problem or so. If the fans go on and the engine is really hot, you may have a gauge that gets stuck mechanically?
The coolant sensor is just above the waterpump, to the driverside, and provides the coolant temperature information. Separate sensor. The fans are triggered independently by the temp sensor at the radiator ( more sensors make them fans run). The alarm is triggered by the central warning unit which takes the signal from the same coolant temp sensor. Before going in deeper, check that coolant sensor and connections.
1992 Porsche 928 GTS Midnight blue
It sounds like you have not yet determined if the car is indeed over heating. I suspect it is not overheating from your description - especially since the temp gauge is reading right and you see no steam coming from under the hood.
The gauge is not connected to the over heat light. They are two separate circuits. The sensor is the one on the water manifold with regular lugs.
One side of the sensor is variable and connects to the gauge. The other is a limit switch and connects to the light.
It sounds like from the symptoms - what with the car sitting a while - that you have an electrical gremlin. I would start by measuring continuity on the sensor for the light side. See if it is really the sensor triggering the light or if there is some grounding problem that is causing the problem.
Many of the inputs to the error computer work by grounding the signal lead.
A quick continuity test of the sensor when you see the light will tell if the sensor is bad. You can try disconnecting the wire for the light from the sensor and see what happens. If the light still comes on when driving the car than you have either a wiring short or a bad error computer - yes the error computers do crap out and in odd ways. Mine kept telling me my brake pads were bad.
Dan The Pod Guy
Now that you mention this.... you're correct. They changed the temp sensor design in 1989 along with the digital dash. Sam has a 1987.
The analog instrument cluster version has a variable resister to show coolant temp, and contact to ground to trigger the alarm. 3 prong operation. The sensor has 2 contacts, one is 4.5mm (temp switch) and one is 6.3mm (NTC sensor) Both act against chassis-ground.
The digital instrument cluster version has only a variable resister to show coolant temp. 2 prong operation, no ground.
So Sam's problem could well be a sensor issue. I don't remember if it is making contact or breaking contact that triggers the alarm. You'd have to check.
Anyone know for sure or post a sensor reading here?
If it works like breaking contact, a poor connection at the sensor or at the CEB would trigger the alarm as well and that is good. So maybe it is just a bad connection.
1992 Porsche 928 GTS Midnight blue