The 944 Turbo has uses two computers to manage the engine, these are known simply as the DME and the KLR. The 944 NA uses only the DME.
If you're troubleshooting engine problems of the 944 Turbo (aka 951), then you should start by taking a peek at Porsche's test plan for troubleshooting. It is pretty thorough and fairly easy to follow. There are sections of it that require special test equipment but most section use basic shop equipment and provide a detailed look at what you are testing. Here is a link to the Porsche test plan for the 944 Turbo DME/KLR equipment. Basically this plan addresses each input and output of the DME/KLR and shows us how to verify that those inputs and outputs are working.
Many of the troubleshooting suggestions that you read about on the various forums come from this test plan. Those suggestions are well intentioned and often known shortcuts to isolating a problem. The test plan is comprehensive but easier to follow than you might expect. Here's the link, check it out.
Porsche's test plan for troubleshooting the 944 Turbo DME/KLR.
In the engineering world there are a variety of disciplines. Two of these that are frequently overlooked are the Maintainability and Reliability people. These guys sit in on every design meeting and review to ensure that products are created that can survive real world repair cycles. They create a overarching maintenance plan and within it are test plans that address every detail. The maintenance plan defines things like level of repair, needed inspections, rebuild vs. discard of parts. The test plan has the specifics of how to test and determine faulty components. These two plans are important because they force the overall design to have a detailed vision of how problems will be addressed through out the lifecycle of the car.
So ,when you look at Porsche design and ask, "What were they thinking?" The test plan is part of the answer that question.
01 - Ignition pulses on ign. coil - 1
02 - Throttle switch, Idle contact - 2
03 - Full load signal from KLR
04 - Start signal (ign. lock - 50)
05 - Ground and shielding from reference mark sensor
06 - Air flow sensor - 4 (NTC I)
07 - Air flow sensor - 2 (sensor plate potentiometer)
08 - Speed sensor
09 - Air flow sensor - 3
10 - Performance data change (to -1 for CH, S, AUS, NZL, Cal., Japan)
11 - Conn. point 4-pin term 1 (economy control)
12 - Test socket (A) (USA / unleaded gasoline engines)
13 - NTC II
14 - ti-signals, injectors 3+4
15 - ti-signals, injectors 1+2
16 - Ground
17 - Ground
18 - Power supply relay G 5 - 87
19 - Ground
20 - Ground activation relay G 5 - 85b
21 - Conn. point 4-pin term 2 (tachometer - 1)
22 - Air flow sensor - 1 (NTC I)
23 - Shielding (speed sensor / oxygen sensor)
24 - Oxygen sensor (USA / unleaded gasoline engines)
25 - Reference mark sensor
26 - Reference mark sensor
27 - Speed sensor
28 - Ground
29 - A/C compressor (+)
30 - Altitude correction unit (USA / unleaded gasoline engines)
31 - Trigger signal for KR
32 - TD* input final stage from KR
33 - Idle positioner - 3
34 - Idle positioner - 5
There are a variety of aftermarket chips out there for use in your DME/KLR. One of them is made my Rogue Tuning. It requires a few supporitn changes.