No overdrive
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Junior User
Joined: Jun 2012
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No overdrive
I have a 99 SCAB 4X4 that has recently stopped shifting into overdrive. I have read where there is a clip on the overdrive servo that breaks and blocks the plunger. If I understand right this requires getting into the valve body so I figured I'd just disengage the overdrive and go 3 gears (1st - 3rd shift fine) until I could get into the tranny. That's when I noticed that the overdrive selector has no effect on the shifting. Is it possible that the overdrive selector malfunction could cause the no overdrive symptom?
Lead Driver
Joined: Sep 2006
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No for the following reason; at every engine restart the PCM turns the OD function on again.
If it does not work, it's either a trans fault or an electrical fault.
Note the Lock-up function is separate.
Under a no fault condition, if the OD is turned off (light on), Lock-up will occur in third gear unless the switch is not signaling the PCM.
The light and switch functions are actually separate but built into the switch form.
This is why there are 3 or 4 leads going to the switch assembly.
To add, if the Lock-up does not work, the trans will generate more heat from fluid shearing in the converter.
If it does not work, it's either a trans fault or an electrical fault.
Note the Lock-up function is separate.
Under a no fault condition, if the OD is turned off (light on), Lock-up will occur in third gear unless the switch is not signaling the PCM.
The light and switch functions are actually separate but built into the switch form.
This is why there are 3 or 4 leads going to the switch assembly.
To add, if the Lock-up does not work, the trans will generate more heat from fluid shearing in the converter.
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Junior User
Joined: Jun 2012
Posts: 57
Likes: 0
Changed the gear shift lever out, now the overdrive selector works. I can at least drive the truck now. I'm hopeful someone can point to something else that would cause only 4th/overdrive to slip. I really don't want to drop the valve body if I don't have to, if it can even be done without pully the whole tranny.
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