Question about 1999 GMC Jimmy
There is no vacuum modulator to your transmission on you Jimmy. The transmission you have is electronically controlled. Going to take a guess on your problem is delayed shift points, since your asking about the Modulator. 1-2 shift does not happen at WOT until you let off the gas: Best case:
try replacing the TPS. Middle case: leak in the 2nd gear apply circuit
(servo assembly or 1-2 accumulator). Double check by using the pressure
gauge and watch for a big drop when the PCM commands 2nd gear. Worst
case: poor line pressure rise.
1-2 Shift shudder at WOT; delayed or abnormal 1-2 shift; There's a problem ONLY on the 1-2 shift: 1-2 accumulator piston cracked or stuck cocked in the bore. Check the yellow spring inside the accum housing for breakage. Also, if the accumulator housing walls are scored, the housing must be replaced. Easy fix in the pan.
1-2 shift is delayed and harsh, may not shift into OD: Check TPS for smooth and linear electrical response over the entire range of motion. If not, replace.
Soft shifting, gradual performance degradation: Poor line pressure rise due to leaking boost valve, clogged EPC filter screen, failing EPC solenoid, or worst case: leaky seals throughout. Transgo HD2-C kit fixes first two without removing trans. Seal restorer may fix last problem, but probably R&R. Also try a transmission flush with BG brand products (Firestone stocks it).
Good luck and keep me posted on what the problem is and be glad to help you get your Jimmy running 100% again soon
Posted on Nov 16, 2009
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Posted on Jan 02, 2017
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