Harald944

I checked the wiring diagram up to a model 88. As the above posters correctly stated, both the coolant level switch and the coolant pressure switch are switching ground if activated to the same common pin M11 on the central electrical system and from there it is directed to K21 and leading to pin 22 in the central information.

Starting 89 the wiring changed.
- Coolant pressure goes to the instrument cluster, plug 3, pin 6.
- Coolant level goes to the instrument cluster, plug 3, pin 7.

Both switches connect ground to pin 6 or 7 if they are activated.

Activating the coolant level switch triggers an alarm (cooling level low) in the cluster!
Activating the coolant pressure switch does .... nothing!

The instrument cluster on a model 89 and later contains an on board diagnosis software, if I activate this, I can reed out the position of the switches.
The diagnosis correctly indicates the position of the coolant level switch (#9 in the diagnosis software, level ok= open, level low = closed) and the coolant pressure switch (#10 in the diagnosis software). The coolant pressure switch should indicate open if pressure <= 0,1 bar, or closed if >= 0,1 bar (according to the manual)

So the switches are working or correctly simulated.

I see what the pressure switch does in a 88 model, but still have no idea what it does in an 89 model.

Harald

WallyP

My earlier post on this subject was wrong. I now have new info...

Without the pressure switch:
If the cold coolant level is low, the level switch will set the "low coolant" warning after twenty seconds. The driver can switch this warning off. As the engine and coolant warms, the coolant expands, and if the engine is stopped and restarted, there will be no low coolant warning, even though the cold level would be too low.

With the new pressure switch:
When the central warning system sees first a low coolant signal from the level swith, and then pressure from the pressure switch, the system will store that sequence, and will set the "low coolant" warning on subsequent starts, even though the level of the warm coolant is over the minimum.

If the pressure cap is removed, releasing the pressure, the central warning system assumes that coolant has been added, and the warning is removed until the next cold start begins the warning sequence again.

So, to repeat - the purpose of the pressure switch is to maintain the "low coolant" warning even on subsequent warm starts with expanded (but still low) coolant. It is a simple pressure switch - the central warning computer takes the level signal and the pressure signal and decides what to do with them.

Another example of the thoroughness of the design team...

Erik - Denmark

Wally,
Good find!
This explanation must be for the newer models (from S4) - Correct?

Harald944

Hallo WallyP,

Very good explanation,
thank you very much,
going to check that on my car

Thank you again

Harald

WallyP

Erik,

The explanation is from the Annual Service Info for the '84 model. This is from Disk 1 of the excellent set put out by Jim Morehouse ( email jim928@ptd.net ), which contains an unbelievable amount of 928 data and info.

Erik - Denmark

Wally,
Thank you, interesting info, but 1984 cannot be correct
Up to (incl) 1988 model, the two sensors are wired parallel and connected to P6 -> G6 -> Zentralinformator kl. 14
From 1989 model:
The level sensor is connected to E23 -> G13 -> 3/7 on combi instrument
The pressure sensor is connected to C25 -> D15 -> 3/6 on combi instrument
So the 'new' explanation is valid from the 1989 model.
I.e. both your explanations can be correct - I am not familiar with the newer models

Erik - Denmark

Harald,
Any news from your side, so we can get a conclusion of our discussion

Wally,
What was your comments to the above regarding the model year?

Regards from Erik in Denmark

Harald944

Checked the new pressure switch.
Switching occurs at about 0,15 bar.
switch open = unlimited resistance, switch closed = 500 ohms resistance.

So on my a car the switch is closed whenever there is only little pressure in the system, and Wally is correct for the explanation here.

Standing by for Eric's findings, because due to the wiring his switch should close at more than 1,0 bar.

But why are both switches before and after 89 the same part number?

Still not clear....

Harald

Erik - Denmark

Hey
I checked my (old) pressure sensor - It switch like yours, and are also the same number: 928 641 513 00
So we have a new mystically issue - How can it works?
Erik in Denmark

heinrich

I've had a coolant "disappearance" for a year or so now ... I knew it wasn;t going into the engine, but after having my radiator refurbished (I thought it was the culprit) I finally discovered that under pressure, the silly little plastic coolant pressure switch was the problem ... very slight leak. When I pulled it of, it fell into peices.

So if you're wondering where your coolant could be going, this is one possibility. EIGHTY DOLLARS!!!