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Coolant pressure switch 928.641.513.00
Can someone tell me how to test this?
Is it simply a normally open switch which goes to closed circuit with pressure?
Written on it is '0.1+ -0.1 Bar 6-12 volt'. That is very low pressure.
It shares the same wire into the central warning module as the low coolant level switch.
Presumably if either goes closed circuit, they switch to earth and trigger the low coolant level?
This switch was add starting with the 1984 models, and is explained (not too clearly) in the booklet for the changes by year for 1984.
If the coolant level of a cold engine is too low, this will switch on the coolant level warning light after a delay of approximately 20 seconds. Coolant will expand as the engine warms up and the coolant level will rise. If the engine is stopped and started again the warning will go out. This happens even though the cold coolant level is too low.
To avoid unnecessary troubleshooting, starting in 1984 models, the central warning system has a pressure switch located in the return line to the expansion tank which now works in conjunction with the coolant level switch. The pressure switch senses the pressure in the cooling system and sends this information to the central warning unit.
Now, if there had first been a warning from the coolant level switch and afterwards from the pressure switch, the central warning system will store this signal sequence and maintain the warning, even after the coolant has expanded and the coolant level switch has an opened circuit.
In other words, when starting the engine again, the warning lamp remains on, even if the coolant level appears correct.
If the cap at the expansion tank is removed (to add coolant), pressure escapes and the pressure switch circuit opens. The central warning system, with this indication, assumes coolant has been added, and turns the warning lamp out.
Thanks Wally.
Does this mean that with the switch in hand my ohm meter will show open circuit and go to closed circuit if I apply pressure to the switch pipes?
Mike
This switch was add starting with the 1984 models, and is explained (not too clearly) in the booklet for the changes by year for 1984.
If the coolant level of a cold engine is too low, this will switch on the coolant level warning light after a delay of approximately 20 seconds. Coolant will expand as the engine warms up and the coolant level will rise. If the engine is stopped and started again the warning will go out. This happens even though the cold coolant level is too low.
To avoid unnecessary troubleshooting, starting in 1984 models, the central warning system has a pressure switch located in the return line to the expansion tank which now works in conjunction with the coolant level switch. The pressure switch senses the pressure in the cooling system and sends this information to the central warning unit.
Now, if there had first been a warning from the coolant level switch and afterwards from the pressure switch, the central warning system will store this signal sequence and maintain the warning, even after the coolant has expanded and the coolant level switch has an opened circuit.
In other words, when starting the engine again, the warning lamp remains on, even if the coolant level appears correct.
If the cap at the expansion tank is removed (to add coolant), pressure escapes and the pressure switch circuit opens. The central warning system, with this indication, assumes coolant has been added, and turns the warning lamp out.
So Wally, if I understand correctly, if the pressure switch is going bad I could be getting a low coolant warning even if the coolant level is correct or high? I have checked the coolant level over the past few days when I have been getting a level warning and it has been correct to high (new cap and tank too). When I overfill it it pukes out the overflow at the cap (like it is supposed to do). Perhaps I should just replace the pressure switch (original) and see what happens since the coolant level sensor is NLA? Other than that part, all the rubber and every seal in the car is new or less than 2 years old so I know I don't have a leak or anything (oil and coolant are keeping in their respective places like they should).
So Wally, if I understand correctly, if the pressure switch is going bad I could be getting a low coolant warning even if the coolant level is correct or high? I have checked the coolant level over the past few days when I have been getting a level warning and it has been correct to high (new cap and tank too). When I overfill it it pukes out the overflow at the cap (like it is supposed to do). Perhaps I should just replace the pressure switch (original) and see what happens since the coolant level sensor is NLA? Other than that part, all the rubber and every seal in the car is new or less than 2 years old so I know I don't have a leak or anything (oil and coolant are keeping in their respective places like they should).
Jeff,
As I understand it you would have to have a low level alarm condition triggered to start with to get the alarm. The switch needs minimal pressure to make a contact which is fine considering liquid expands when warm.
During my recent TB/WP adventure I discovered my switch was blocked with some muddy crud [a bit worrying] - when I "looked" at the switch it fell apart on the body seam. I cleaned it all up, used RTV to re-seal it and wired the body together just in case it wanted to let go. This was just a trial that is holding- the best advice I got was probably to remove it altogether I suspect. Just another over complication of dubious value I feel.
This reminds me- I must check how it is holding up!
Hmmm. I still have the original installed with no leaks, even after a couple hose changes. I think I sourced a spare one several years ago when they were NLA for a while.
Some of the replacement plastic parts in the coolant system, like these, heater control valves, etc, seem to be somewhat lower quality than what came in the car. I replaced my HCV as PM about 5 years into my stewardship period, and the replacement is still doing its job nicely. Yet we hear about so many recent purchases that leak. Even Porsche is sourcing low-cost replacements I guess. And maybe not from ISO-90xx-qualified suppliers it seems. Part of their manufacturing and sourcing systems overhaul that started in the late 1990's I guess, when young business-school grads took over the checkbook from the engineers and car guys.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Thread bump, I was replacing coolant hose today and noticed a potential short on the harness side of the sensor plug wiring. In that wires exposed and appear to be contacting each other...
As far as I know the system working fine, no warnings, etc.
Does the actual plug come apart to service the wire?
And how does one see if sensor still in spec?
Not real motivated to replace, especially if we are talking about a non-essential (read, over engineered) component...
I pried open the harness connector, and swapped in a set of new contacts pulled out of a seat directional switch. Some mods, then some assembly....
As we review:
Alan cringes...
Bob shakes his head, but is too nice to say anything....
Sean laughs, and says, hey that will probably work dude.
You could hear the rattle all the way over there? Wow...
Fix is good, considering all the dubious alternatives. Not so sure about the color choices for the shrink sleeves, acknowledging that it's getting harder to find good blue-with-orange-tracer shrink tube in the smaller sizes.
Another thread bump. OK, I'm going the other way on this system -- rather than getting rid of the coolant pressure sender, I have the chance to ADD it to an earlier car (1983 US -- the year just before it was first introduced in 1984!)
I'm retrofitting in a 1984 front harness to my '83. This harness feeds the washer pumps, headlights, ignition control module, 14-pin to the engine harness and much more. Why go with the '84 harness? Because the '83 harness is no longer made, and the '84 version is the closest available.
The new harness was just delivered. The only difference that I've discovered with the new harness vs. my stock '83 is the addition of the plug for the coolant pressure sender and the ganging of an additional blue/red wire to one of the pins on connector block "P".
Because it's my understanding from this and other threads that the coolant level sender and the coolant pressure sender are wired in parallel (although the pressure sender is apparently either open or 500 ohms), I'm wondering: What would the effect be of adding in the pressure sender to my '83?
Would the pressure sender still work/cause a dash alarm if an underpressure situation is detected? (Is that even its function?)
I'm not sure I fully understand the principle of its operation above, but I am guessing/suspecting that with the older central warning system, it would not properly differentiate between the two signals, it wouldn't give the twenty minute delay or have memory (both of these situations I am OK with), but most importantly I'm wondering if it might trigger an alarm when the engine is cold until adequate pressure is built up in the system?
On the other hand, if it would properly give me an underpressure warning (with or without a low coolant level warning), that to me would be worth tossing one in. PS I know from other discussions that the pressure senders sometimes leak, but I only want to focus this question on whether it would function properly.
04-10-2014, 05:57 AM
Coolant pressure switch 928.641.513.00
