1983 Coolant Reservoir Sensor -

[JK McDonald]

I'm helping a college age friend sort out his intermittent "Coolant Low-Level Warning Light" that suddenly began indicating a problem at start up. The master warning caution light "!" can be cleared but the "Low Coolant Level Light" stayed on at times. The actual coolant level within the radiator reservoir turned out to be fine - evidently it is just that the float switch portion of it's sensor misreading the coolant level.

After removing a few hoses and easing the reservoir assembly out of it's mount - the coolant level sensor cap can be unscrewed and the sensor itself pulled out for further testing. I've determined that the magnetic ring float is still buoyant and follows an external water surface level in a glass jar - but an ohm meter shows an erratic change in resistance as the float is manually positioned. Sometimes the multi-meter will indicate a fluctuating open - sometimes it reads anywhere from 500 to 1200 ohms. Obviously the float switch internals are "Farfignewton"

Does anyone know what the true resistance readings should be at each end of float travel ? There doesn't seem to be any spec's around.... I think it is a simple reed type open/closed switch that is forced closed as the floating magnetic ring approaches. But - I'm not sure if there is any logic circuitry within the sensor that talks to the warning signal unit as well.

Also - is the "Coolant Level Sensor" in the radiator reservoir the same as the "Windshield Reservoir Tank Level Sensor" and/or they interchangeable ? They look identical but have a slightly different P/N.

Coolant Reservoir Level Sensor - 928 641 503 01
Windshield Washer Tank Level Sensor - 928 641 501 01

Initially the radiator reservoir appeared to be low and I thought this was going to be short and sweet. But then after a day spent trying to determine if we had a coolant hose leak, a cracked coolant reservoir, an external wiring problem, a float sensor problem or a faulty warning control unit - we were both pretty wrung out. His long suffering dad (who is footing the restoration project bill) has recommended that we unscrew the radiator cap and drive a Toyota under it.

Michael

[JK McDonald]

After a little more time spent troubleshooting the "Coolant Low Level" warning light, a failing Radiator Reservoir Low Coolant Sensor was found to be the sole source of the problem. As tentatively suspected earlier, the Coolant Low Level Sensor is simply an On/Off Reed switch. When it is working properly - (On = 0 Resistance / Off = Infinite Resistance)

1. Correct Coolant Level = Float Raised = Internal Reed Switch Magnetically Held Opened = Dash Warning Light Off

2. Low Coolant Level = Float Down = Internal Reed Switch Spring Loaded Closed = Dash Warning Light On

Because the Two Electrical Pins on the Float Switch are almost inaccessible, I had initially left the Connector installed and had simply scraped off a small section of insulation from both wires to probe their copper core with a Digital Ohm Meter. With the defective Float Sensor producing an intermittent OPEN + the two wires still connected back to the Warning Control Unit (WCU) - I kept seeing an unstable (feedback ?) 500 to 1200 ohm resistance on the Meter. This initially chased me around for a few laps not knowing what resistance value I was looking for.

It may seem to be a bit tough to accurately verify the Float Level Switch operation with the Reservoir in place but it can be done. You must leave the two pin CONNECTOR attached to the Float Sw but with both wires severed. The length of the remaining wire going to the connector should be left long enough to allow access for an Ohm Meter evaluation. I soldered on a pair of additional 10 inch wires for some working elbow room. To manually raise and lower the Reservoir Coolant Level and cycle the float switch during the troubleshooting process, you will need a small diameter siphon hose (Mighty Vac) to remove coolant from the reservoir, a funnel to add back coolant and an empty milk jug as a temporary storage container.

To replace a defective Coolant Level Sensor Assembly - just remove the two exposed 10mm Reservoir mounting bolts, for some additional space to maneuver things around - disconnect the black Plastic Emission Valve upper 5/8" Hose running along side the reservoir and then kind of ease the reservoir out from under the fender. With the top of the sensor worked past the hood lift cylinder base point and the Float Sensor Retaining Cap unscrewed, gently pull the float sensor switch assembly straight up and out.

One other note - for a comparison I also removed the Float Level Sensor mounted upside down in the lower corner of the Windshield Washer Tank for inspection. Although it looks exactly like the Reservoir Coolant Level Sensor, it is much shorter in over all length. Since the Windshield Washer Tank has it's Water Level Sensor inverted - the Warning Control Unit must be looking for the OPPOSITE Float Switch conditions to generate a signal that determines if the Windshield Washer Solution Tank is Full or Empty.

Michael

[JK McDonald]

With the cost of a new 928 Porsche Coolant Level Sensor a bit more than the dad wanted to shell out for his son's project-mobile, I offered to see if a substitute could be found. The Behr cooling system components are used as factory parts by lots of car manufactures.

After giving up on a new part - we headed for a local foreign car salvage yard. I was able to finally match up an alternate Coolant Level Sensor from an old Audi 5000 that worked fine. (I know - I know - Porsche Sacrilege) For $10 we got the Level Sensor, an almost new Sensor Cap with sealing ring, the correctly oriented two pin Electrical Connector Plug plus the reservoir mounting hardware and an assortment of surrounding metric hose clamps.

Although the length of the Audi Sensor Stem (photo) is slightly shorter than the original Porsche part, the amount of float travel is exactly the same. This just means the coolant level in the reservoir will be about 1/2" over the center mark and a bit higher than before. I stuff rags down around any opening when working so any dropped bolt or clamp doesn't have far to go. The reservoir and sensor are ready to be reinstalled back into position.



To test the operation of the new/used sensor, I gradually added coolant (along with a stiff dose of Audi Anti-Rejection serum) until the warning light extinguished. To verify there was enough expansion room still available as the coolant reached operating temps, we took her out for an enthusiastic rip snorting road test. When we returned I had to hear the dad's college days story again about his Toyota that never broke, was fast as a speeding bullet and how he had to drain off the extra gas every week......

Perhaps someone has found a new direct replacement Coolant Level Sensor for our 928's.....

Life in the fast Lane - Michael

[JK McDonald]

Because the level of maintenance skills of the 1983 928 owner is pretty low - I feel a little obligated to at least watch over this Porsche-Audi-Project-mobile long enough for him to put a few miles on it. Plus since I'd not seen where an alternate part had been used in place of the original Porsche Coolant Level Sensor - I want to keep an eye on the status of the Audi part. So far the "Coolant Level Warning Light" is still out and after a quick inspection of the reservoir coolant level (both when hot and cold) shows everything is fine.

I might also add that a running buddy found a discussion on trying to adapt a Chinese made BMW coolant level sensor into a 928. At this point I think there are some fit and finish problems. Here is a link that kind of drops you into the middle of the thread....

Possible cheap coolant level sensor replacement? - Page 3 - Rennlist Discussion Forums

When super dad started in on his old college Toyota story again - how it flew through the air, seduced women with only a touch of the door handle and a whiff of the glove box - I told him that sounds exactly like a 928 Porsche...........

Michael

[ice-hot]

Great info. alot of work...but fun and educational for the owner, now he really knows what he got into.
I hate to say this but I just bought the whole tank on evilbay with sensor (not sure it works yet) just got it, for $30.00 shipped! Didnt need the sensor, so if it is inop... Tank looks good and thats what I needed.

John

[JK McDonald]

With nothing to lose, I decided to take the defective Coolant Level Sensor apart to see what is inside. It turned out to be a pretty simple process. 1st remove the large plastic nut from the top of the sensor assembly to allow better access to the area around the two electrical pins. The pins were held in place as a unit by a deteriorating semi-hard epoxy. The reed switch, farther below in the barrel is held in place with a very soft (non-hardening ?) sealant. The pins can be removed together by slowly running around the inner ring of the center shell with the tip of an Exacto-knife. The semi-hard Epoxy was pretty crumbly and once you've worked the center area free, can be pried out with a small flat tip screwdriver. Just be gentle as it clears the upper edge of the sensor housing. At this point use a thin miniature pair of needle nose pliers positioned beneath the epoxied pins to completely draw out the reed switch.

A visual inspection of the electrical leads attached to the reed switch didn't provide any indication of the failure that I'd expected - but I ran a soldering iron with some fresh solder on the electrical connections just to be sure. I also had soldered the electrical wires directly to the input pins earlier to eliminate a possible connector problem. To properly Ops test the reed switch using the float "magnetic ring" , all the sealant must be removed from the barrel innerds within the sensor. A large drill bit was selected to slowly remove the sealant from the main body while a long metal wire with a flattened tip cleaned out the remaining sealant from the hollow stem area that the magnetic float slides over.



A quick assembly of the sensor using an ohm meter verified that the reed switch was now working as advertised. I noticed that the two electrical Epoxied pins sat much lower in their seat once everything had been cleaned up.

With the float raised = Reed Switch Open = No warning Light
With the float lowered = Reed Switch Closed = Warning Light




After filling the sensor barrel with some new non-hardening silicone sealant and the final reassembly complete I realized that the original failure was NOW OBVIOUS. The internal position of the reed switch over the last 33 years had gradually moved - crawled - climbed UPWARD out of the sensor housing as the hardened epoxy had deteriorated, melted and/or swelled. This expansion not only elevated the upper electrical pins out of the sensor body but took the internal electrical leads + the attached reed switch OUT of the magnetic reach of the float. All that was needed to fix the original intermittent sensor warning light problem was evidently just a large hammer.

Hey Ice - After about $900 worth of effort we saved $30 worth of Ebay parts.......

Michael

[ice-hot]

I just did the same thing with the sensor I received yesterday.
This one was so corroded it looked like you just pulled out a green stick, or something.
Could not tell the wire from the flat brass bar! All one big happy corrosion.
I put it in some baking soda paste and will leave it for today and see what comes out?
Wish I would have taken a pic. though.
Yea JK but it was only labor, and what else but tinker with the P car would you have done? LOL
John