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So called ALDL (assembly line diagnostics link) diagnostics connector which resided around your left knee is there for a service tool called Scanner. With Scanner you can communicate with your transmission control computer. Control computer measures and maintains over 40 parameters and stores fault codes if something goes wrong. Scanner is an expensive service shop tool.
FIELD SERVICE MODE
You can read fault codes without any service tool if you short pins 5 and 6 of that connector. When pins 5-6 shorted check trans lamp gives the code with flash sequence. First comes 12 and then possible fault code. Codes are shown in sequence. E.g..12 is flash, pause, flash, flash, long pause and same again. Be careful playing with connector.
Connector pin order is:
from left upper corner: 1=np, 2=?, 3= np, 4=chassis ground, 5=signal ground, 6=enable PCM diagnostics, 7=np, 8= np
from left lower corner: 9=serial data, 10=np, 11=np, 12=np, 13=np, 14=np, 15=np, 16=unswitched battery 12V
np = no pin at the connector
I have verified this with my N/A diesel. I should work also with Turbo. In case of turbo you should be able to read also engine codes with this. Play with your own risk.
Juha Niinikoski HMC4 97 NA Diesel
This procedure will work fine with the 6.5L (Y) N.A. Diesel but not with the 6.5L (Z)Turbo Diesel engine. The Turbo uses an OBD II compliant Class 2 interface with an Alpha-numeric code structure eg. PO251. Grounding the DLC will not produce flash codes on the Turbo application.
I found a discrepency in the manual which affects the below procedure: the pin number on the TCM may be either D7 or D8. One page says D7, another says D8. I find more references to D8 than to D7. Look for a dark blue wire (not black).
OK, this is for a 95 NA diesel. Other models may vary in pin numbers, color codes, etc.
The TPS (Throttle Position Sender) takes a +5V and Ground, and returns a value between between .5 and 4.5 volts depending on throttle position. With the ignition on, the engine not running, but warm, cold advance/fast idle not set, the specified voltage is .67V. Opening the throttle will increase the voltage.
To check the TPS, at the computer probe the pin D7 (or D8), which should have a dark blue wire attached (if D7 has a black wire, or no wire, then look at pin D8). With ignition on, engine not running, the idle voltage should be about 0.5V, depressing the accelerator will cause the voltage to *smoothly* increase to a maximum of 4.5V. Checking at the computer also tests the wireing harness, as well.
The D connector is the larger of the two connectors on the computer.
Probing under the hood is possible but difficult, you should not pierce the insulation in an attempt to measure the voltage as wire and connector damage will result. Instead you must use an adapter made to test the TPS. You can make an adapter with two connectors that match those on the TPS (standard GM connectors) if you want to check under the hood.
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