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944 turbo auxiliary water pump circuit test method

Paul 290T

PCGB Member

Having spent time working out how to electrically test the water cooling pump for my car’s turbocharger I thought I’d share what I learned and help others more quickly test their car’s system.

The pump’s relay 951 618 149 00 is NLA (so says my local OPC), but a few new ones are still kicking around at circa £200 at third party retailers. Hence, I decided to test before parting with the cash.

First how to test the relay. Once you remove the relay from the fuse box socket you will see each of its relay pins clearly identified by the numbers referenced below:
1. Connect your test power supply +ve to pin 30 and -ve to pin 31.
2. Connect your multimeter or test light across pins 87 and 31 of the relay. When the relay changes state (to energise the auxiliary water pump when the relay is fitted to the car) the multimeter will show 12V/the test light illuminates.
3. Momentarily bridge pins TS and 31. This simulates the coolant temperature sensor exceeding 118°C and grounding itself. If the relay is operating correctly the meter should now show 12V/test lamp lights for 25 seconds. Ensure these pins are not bridged before moving to the next step.
4. This time bridge pins 30 and 15b. This simulates the engine running condition whereby the DME relay puts 12V onto pin 15b of the pump relay. The meter/test lamp stays de-energised as long as the bridge is in place.
5. Now remove the pin 30 to 15b bridge, this simulates the engine being shutdown whereby the DME relay drops its 12V signal to the pump relay. If the pump relay is operating correctly the meter should now show 12V/test lamp lights for 25 seconds.

Idle curiosity got me wondering if the aux water pump would kick in while the engine was running if the temperature switch detected high temperature. I thought maybe Porsche deemed this unnecessary as the engine’s main water pump would be performing the cooling duty on the turbo at this time. But I was wrong, with relay pins 30 and 15b bridged (i.e. engine running condition) I then simultaneously bridged pins TS and 31: my pump relay pulled in and started the auxiliary water pump for 25 seconds.

Now to test the relay socket in the Central Electrics/fuse box. With the engine switched off and relay removed I fixed my multimeter -ve probe to an earthing point on the car. The socket markings were too hard to read so I am going to refer to them by the name of the relay pin that inserts into the socket. NOTE: Using this naming strategy results in sockets 15b and 31 switching places compared to the view you would see if looking at the pin layout on the relay:
a) Socket 87 had continuity to ground, via the plug that connects the aux water pump to the engine harness. 0V reference to ground.
b) Sockets 15b and 31 had continuity to ground. 0V reference to ground.
c) Socket TS connects to the plug that fits onto the temperature sensor for the turbo cooling circuit. 0V reference to ground.
d) Socket 30 is not connected to ground, it showed 12V referenced to ground.
e) Putting a jumper between sockets 30 and 87 ran the pump as long as the jumper was in place.
f) Temperature sensor plug to relay socket TS measured 0.2ohms.
g) The plug of the pump’s electrical lead was separated from the engine harness plug to measure the resistance from the harness to the relevant fuse box relay socket, I can’t remember the results, but both were less than 1ohm.

Now I started the engine:
a) Socket 30 but this time also 15b showed 12V referenced to ground. 15b is fed from the DME relay, which energises when the engine is running.
b) The other sockets showed 0V referenced to ground.
c) After letting the engine get hot enough for the cooling fans to kick in I tested it to see if the temperature sensor had continuity with ground, it did not. Maybe the coolant still wasn’t hot enough or maybe the switch was not working. A new sensor 930 606 118 00 was £87 inc VAT, so I ordered one from the OPC.

After refitting the pump relay to the socket in the fuse box:
d) Touching the temperature sensor plug to ground ran the pump for 25 seconds.

If you have any questions, ask now, while I can still remember what I was up to at the weekend!
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