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GM "BAS" engine start/stop inactive

Monday, June 1, 2015 - 07:00
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A 2009 BAS (Belted Alternator Starter) equipped vehicle may have a complaint of the engine stop/start system being inactive and the Malfunction Indicator Lamp (MIL) is illuminated. Diagnostic Trouble Codes P1808 (Transmission Fluid Pressure Position Switch Stuck OFF) or P1809 (Transmission Fluid Pressure Position Switch Stuck ON) may set.

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The Transmission Fluid Pressure Switch Assembly (TFP)  for BAS equipped vehicles has an additional switch input to the Engine Control Module (ECM) which allows the ECM to know when proper line pressure is being maintained by the main pump or the auxiliary pump. This is called the pressure switch “A” switch signal (figure 1).

There is a 1st design level and a 2nd design level, the 1st design level has a Normally OPEN switch, the second design level has a Normally CLOSED switch. The 1st design level switch was prone to collecting sludge in the switch contact area which would result in preventing the switch from closing. The 2nd design level switch being normally closed would have a longer service life.

When the 1st design level switch would stick open DTC P1808 would set. Installing a 2nd design level switch assembly into a 1st design level vehicle will cause DTC P1809 to be set.

Figure 1 Figure 2

 

 

 

 

 

 

 

 

 

Use the transmission identification tag as seen in Figure 2, located on the transmission case to identify which pressure switch assembly design level belongs in which model transmission as follows:

2009 Transmission Models 9BAJ, 9BBJ or 9CKJ use the 1st design level switch assembly with the normally open line pressure switch “A” and must be serviced with the 1st design level switch assembly.

2009 Transmission Models 9CCJ, 9DAJ or 9CMJ use the 2nd design level switch assembly with the normally closed line pressure switch “A” and must be serviced with the 2nd design pressure switch assembly. All later built transmissions were equipped with the 2nd design level switch assembly.

1st Design Level Fluid Pressure Switch Assembly...............................................24233218

2nd Design Level Fluid Pressure Switch Assembly..............................................24249846

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