WallyP

The pressure switch was added during one of the upgrades to the central warning systems - the earlier cars, including your '79, do not have the switch.

"The level sensor measures open circuit (as it should) and the tank is at the proper level."

Are you certain that the sensor should be open? I haven't looked at the '79 system in quite a while, but I thought that it should be closed for no warning, open on low coolant. Try shorting the connectors at the cap connector, and see if the warning goes away. If so, you probably have a bad sensor.

heinrich

Wally where is the pressure switch?

Ron_H

I've been playing games with my coolant warning lights as well. Before I left for the Denver trip, it would come on when cold and I just started the car. But by the time I got to a water source, it was warm. So I made the mistake of filling it to just the mid tank mark; of course when it cooled it would be lower. Finally in a rare stroke of inspiration, I realized the situation and added coolant when cold. Then it went a couple of weeks with no warning. Then I left for Denver, and could put maybe 500 miles on the car with no warning, which leads me to believe it was not a leak. But no oil contamination. So I added maybe 1/8" of water. Off it went for another atypical period. In fact, it lasted all week. Then I was on the way back and it came on again when hot. I let it cool and added another minute amount and the light was off for the distance between Fernley, Nevada and Sunnyvale, CA. and then some. It took another few days but it came on again. I cussed it out. Didn't help. Put in a small amount of water. It went off for a week or so. Now it's back on again. Pisses me off. Guess I'll check the cap. No water in the oil. Car seems to have good compression because it still runs strongly.

AJK

Are you certain that the sensor should be open? I haven't looked at the '79 system in quite a while, but I thought that it should be closed for no warning, open on low coolant. Try shorting the connectors at the cap connector, and see if the warning goes away.


I did short the connector. The warning light stayed on.

littleball_s4

I rescue this old post because there are some unclear things to me yet. I need some help.

Summary: Coolant light on, level and temp ok, level sensor shows short circuit (as it should, if I understood properly) and pressure sensor shows open circuit when cap is open (idem). Cleaned contacts, reconnect, start engine, light is still there.

Shorted float connector, start engine, no change. Reconnect, shorted pressure conector ("it will do no harm to try") start engine, no change.

Then I measured ohms in the wiring loom instead. The level connector had som 500 ohm, the pressure connector, short circuit. Strange, isn't it?

So, before I look for the central warning brains and check there, I would ask smbdy to confirm (87 s4), please:

-level sensor is open if low, shorted ok.
-Pressure sensor is open if cap is open, shorted if pressure is present.

Perhaps somebody with this car and a ohmmeter handy, it takes 30 seconds and a flat screwdriver to check in the car.

Thanks a lot!

underdog928

I think the coolant level sensor is the same on an '85. I have a broken coolant level sensor. To make the dash light go off, I just disconnected the switch at the tank. That would mean that the switch is OPEN when the coolant level is high. The opposite from your assumption above. I don't know about the pressure switch, but both switches may be in parallel so that if either is CLOSED the warning light goes on.

So disconnect the coolant level switch and if the warning light goes out, you have a broken coolant level switch. If not, it's the pressure switch.

Hope that helps.

WICruiser

My '85 has the same coolant level warning problem regardless of the float or pressure switches being open or shorted. Looking at the '85 wiring diagram the two switches are in parallel with both connected to a common wire coming from Central Information Connector position 14 and both then going to ground. I have not had a chance to diagnose mine yet but I suspect either a broken wire or a bad connection at the Central Infromation Connector.

littleball_s4

I appreciate the answers. But I cannot imagine (me, short minded) how the system works with both switches in parallel.

Imagine the light was off, and everything is normal (level ok, pressure low). Light must remain off. Then the pressure builds, light is set?

Imagine it has a time after wich the brains ignore the warning to avoid the problem of the previous paragraph. So level ok, pressure high does not trigger the warning after, say, two minutes running. Then a stone makes a hole in the rad, level goes down, engine is toasted but light is still off. I don't think either. Or you stall, turn engine off and back on. Light is set with level ok pressure high.

Thanks anyway, I'll check myself

littleball_s4

All right, 87 s4 coolant level switch is open at normal level, closed on low level condition. Pressure switch is open when the cap is open, but I forgot to check when pressure was built.

The 3rd screw of the tank is symetrically located to the other two?. Mi madre!

RngTrtl

I had to run my '85 a few miles home with the radiator cap off b/c one of my coolent hoses split and i had to tape it up to hold collent until I got home. My coolent light did not come on due to any kind of pressure switch (I had no pressure since the rad cap was off). Just tossing that out there.

dr bob

The switch is looking for -high pressure- as the warning condition to illuminate the warning lamp. Switch closes on high pressure. The level switch looks for low coolant level as a warning condition. Low level closes the switch. With the two switches in parallel, either switch in the warning condition (high pressure -or- low level -or- both) will give you a warning on the dash.

So driving with the cap loose or off won't ever generate a pressure warning.