Powertrain, generic · Transmission

P0979 Shift Solenoid C Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Low-voltage fault, usually a short to ground, in the control circuit of shift solenoid C. GM uses it mainly on the Allison 1000 six-speed of its heavy pickups and chassis cabs: in 2016 a ground short of 0.22 ohm or less, stricter than the 0.42 ohm for SS1 and SS2, or an open of 200 kOhm or more counts after 10 samples (250 ms, Type A). From 2022 the open moves to P097C and P0979 needs an intrusive low-side test (120 ms, MIL after one trip). The Allison PS3 pressure switch test (P0872) also points to P0979 when its fault is electrical. Ford uses it for SSC short to ground beside P0760 open; on 6R80 and 6F, SSC drives the CB26 clutch. High side is P0980.

Likely causes

Solenoid C control wire shorted to ground
Internal short of solenoid C inside the valve body
Open coil on GM 2016 Allison calibrations, which report opens under this code
Faulty low-side driver in the control module

Symptoms

  • MIL on after the first faulty trip
  • Transmission holds a default gear
  • Rough or delayed shifts involving the affected clutch

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,800
Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

shift solenoid cwiring harnesstcm

Related codes

P0980P097CP0978P0760P0872

References: gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.