Auxiliary Turbo Pump
(Turbo Coolant Pump)
The Porsche 944 Turbo uses a small electrical pump to move coolant into the turbo. This helps keep the turbo cool. A faulty pump could possibly lead to your turbo running hot, which will cause premature wear of your turbo bearings. With a price of about $250, the turbo coolant pump is very expensive, but it is vital to your turbo.
Mercedes diesel cars of the 1980s use a very similar pump, they can be had for less than $100. Looking at the picture of the Mercedes pump, it looked that it just may be as simple as changing the electrical pigtail from the Mercedes pump to the Porsche pigtail.
I purchased my pump from HIRSCH INDS. You will have to search for the part number, which is MBZ037173. You have two options; a Bosch pump (more expensive) or a MTC/Ronak (cheaper). Both should pull up in the search results. I bought the Bosch pump because I am familiar with Bosch, but not with MTC/Ronak.
The following write up first begins with a detailed review of what the turbo pump does and how it works, a testing procedure, followed by the procedure to get the Mercedes pump to work on your car properly.
Turbo Pump Operation per TECHNIK 491 321
Turbocharger Cooling
The turbocharger in Porsche 944 cars is connected in the engine's coolant circuit.
The coolant circuit of the turbocharger consists of a temperature switch (14), electric circulation pump (12), as well as an additional thermostat (15) in the water pump housing and the pertinent coolant lines.
The electric pump is located in the line between the expansion tank and turbocharger. The return line between the turbocharger and expansion tank is connected on the inlet side of the water pump, via the additional thermostat.
The temperature switch is also installed in the return flow line, direct at the outlet of the turbocharger.
Description of Operation
The additional thermostat of a running engine will open at a temperature of approximately 82* C to make a connection between the engine coolant circuit and turbocharger coolant circuit. The water pump draws coolant out of the expansion tank, which flows through the turbocharger.
If the temperature of the coolant in the return line of a stopped engine rises to approximately 115* C, the electric pump will be switched on by a temperature switch and the coolant circuit will then be expansion tank --> turbocharger --> expansion tank. A check valve in the expansion tank prevents the coolant from flowing back to the turbocharger.
The electric pump is always switched on for 25 seconds each time the engine is stopped, regardless of how high the coolant return flow temperature is.
Testing the Turbo Pump
The buzzing sound you hear for about 30 seconds when you shut the engine off is the turbo pump. If for some reason you stop hearing this sound, you need to assess the problem as soon as possible to keep your turbo from failing prematurely. However, the problem could also be a faulty relay. To make sure it is not a relay, while your pump is connected to the pigtail, unplug the turbo coolant pump relay from G21 in central electric. Put one jumper inside of terminal 87 (12+ volts) and another jumper inside of terminal 31 (ground).
Give terminal 87 12+ volts and the jumper on terminal 31 ground. I always use a Power Probe (one of the greatest tools ever invented) when doing electrical diagnostic work like this. If you're using a Power Probe, hook the positive jumper to the needle of the Power Probe, the ground to the ground clamp, and push the button forward.
If you don't have a power probe, use whatever archaic method you wish to use to give 12+ volts and ground.
You should hear the pump start to whine.
If the pump does not whine, unplug the pump. Look inside the connection, and there will be two terminals. One is flush, so you'll need to stick a piece of wire inside. The other you can attach an alligator clamp to. If you look very closely inside the pigtail, you'll see one terminal labeled as #1, and one terminal labeled as #2.
Give #1 12+ volts and #2 ground. The pump should begin to spin, as you are now bypassing all your cars electrics and giving the pump straight 12+ volts.
NOTE: The pump will spin as long as it has 12+ volts and ground on either side. However, if it is given 12+ volts and ground at the wrong terminal, it will spin in the opposite direction.
If the pump does not spin with direct 12+ volts, the pump is definitely faulty and needs replacement. If the pump spins with direct 12+ volts but not when jumping the relay, there is another problem. It could be a bad relay or a bad electrical connection somewhere.
Installing Mercedes Pump
Here you can see the Mercedes pump (left) next to the Porsche pump (right). Obviously, you can see that the electrical connection is different. This will need replacement. Also note that the electrical wire comes out 180* different than the Porsche pump. No big deal, you'll just need a few more inches of wire compared to the Porsche pump to wrap around the back side of the pump when installing it to the mounting bracket.
After careful inspection, I noticed that both the pump suction and discharge nozzles are smaller on the Mercedes pump then on the Porsche pump. So, we're going to swap the impeller housing from your old Porsche pump to the new Mercedes pump.
Here are both pumps with the impeller housing removed. They are held in place with four small screws (removed). Be careful with these screws. They round off easy. Use a good, unworn, #1 Phillips screw driver. A #2 screw driver does not fit as well inside the screws, this could lead to the screws rounding off much easier.
Make sure the o-ring is not damaged when you have the impeller housing removed. A damaged o-ring is a potential coolant leak point.
NOTE: If your pump is operating but simply leaking; it could just be a damaged o-ring. Run down to the hardware store and match up an o-ring with the pump. It worked for me for months until a total pump failure.
Install the Porsche impeller housing onto the Mercedes pump. Don't worry. It only goes on one way. Make sure you make sure that the impeller housing is seated against the pump body. The screws are pretty hard to screw in. Put them in as tight as possible.
Cut about 5.5 inches off of the pigtail on the Mercedes pump.
Cut the pigtail off the Porsche pump very close to the pump body.
Connect the red wire on the Mercedes pump pigtail to the red wire on the Porsche pump pigtail. Connect the black wire on the Mercedes pump pigtail to the black wire on the Porsche pump pigtail. Use soldering if possible. Make sure that the electrical connection is covered up and protected from the environment as possible. You don't want any leaking hoses to short out the connection.
Congratulations! You have now replaced your turbo coolant pump, and you saved almost $200! Now, take the money you saved and send $10 of that to my pay pal account so I can buy a six pack of Carlsberg as thanks. :)
