Does it have a modulator

Tiny
SANITATION
  • MEMBER
  • 1991 GMC SIERRA
Sifts hard in to first gear
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Thursday, September 22nd, 2011 AT 1:33 AM

15 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
It isn't the old vacuum modulator from years ago, but it has a modulator for shifting. See attached schematic. This regards both the 700R4 and the 4L60. That is what is listed for your vehicle
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Thursday, September 22nd, 2011 AT 2:22 AM
Tiny
SANITATION
  • MEMBER
Is there part I need to replace
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Thursday, September 22nd, 2011 AT 2:38 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
The modulator is internal and functions on hydraulic pressure for shifts (not park to drive). Is the idle too high, or could it be a bad U-joint? Is the fluid clean and full?
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Thursday, September 22nd, 2011 AT 2:51 AM
Tiny
SANITATION
  • MEMBER
U-joint are good fluid is clean and full revs hi be for it shift
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Thursday, September 22nd, 2011 AT 3:09 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
That sounds like the IAC may be giving you problems.
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Thursday, September 22nd, 2011 AT 3:41 AM
Tiny
SANITATION
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What is iac
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Thursday, September 22nd, 2011 AT 3:45 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Idle air control valve. It maintains a steady idle. If the engine is reving when you shift, that will cause a hard shift.
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Thursday, September 22nd, 2011 AT 4:04 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Here is a better description to help you understand.

The engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, (called counts).

This movement controls air flow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a scan tool. Zero counts corresponds to a fully closed passage, while 140 or more counts (depending on the application) corresponds to full flow.

I have attached a picture so you can identify it.
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Thursday, September 22nd, 2011 AT 4:09 AM
Tiny
SANITATION
  • MEMBER
When I start off in drive it get a well be for it sift to the next gear it revs high a first
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Thursday, September 22nd, 2011 AT 4:21 AM
Tiny
KHLOW2008
  • EXPERT
That is an indication of slipping. Is flaring occurring for all upshifts or only between specific gears?
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Thursday, September 22nd, 2011 AT 4:34 PM
Tiny
SANITATION
  • MEMBER
Only from low to secound gear
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Thursday, September 22nd, 2011 AT 5:12 PM
Tiny
KHLOW2008
  • EXPERT
Which transmission do you have?
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Friday, September 23rd, 2011 AT 12:41 PM
Tiny
SANITATION
  • MEMBER
700R4 transmission
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Friday, September 23rd, 2011 AT 11:31 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
The same on the 700R4
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Saturday, September 24th, 2011 AT 2:06 AM
Tiny
KHLOW2008
  • EXPERT
These are possibel causes and you should start checking with external components and adjustment as offerred by Jacobandnickolas.

SLIPPING OR ROUGH 1-2 SHIFT

Throttle lever and bracket assembly damaged, not installed properly or T.V. Cable broken or binding. Valve body assembly throttle valve sticking. T.V. Bushing turned in bore. Defective valve body assembly. 1-2 shift valve train, line bias valve, accumulator valve or T.V. Limit valve stuck. Gaskets or spacer plate incorrect,
damaged or not installed properly. Body face not flat.

Defective 2-4 servo assembly. Apply pin incorrect length. Servo seals or "O" ring seals missing or damaged. Restricted or missing oil passages. Case servo bore damaged. Defective 2nd accumulator. Porosity in 1-2 accumulator housing or piston. Piston seal or groove damaged. Nicks or burrs in 1-2 accumulator housing. Missing or restricted oil passages or 2-4 band worn or not positioned properly. Oil pump assembly or case faces not flat.
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Saturday, September 24th, 2011 AT 11:46 AM

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