1992 GMC Suburban not shifting into OD, Transmission Modulator?

Not a wire. Your transmission shift cable may be disconnected from the throttle. Or if you have a vacuum modulator the vacuum tube may be disconnected. Check your fluid level.

Sounds like a vacuum leak probably at the modulator valve on or close to the transmission.

check vacum hose running to back of trans for vacum if has good vacum replace modulator valve on transmission(where vacum hose ends).its a easy fix.

There is a vacuum line that is connected to a vacuum modulator on the transmission. The vacuum modulator looks like a tiny inline fuel filter on the right rear of the transmission. Remove the vacuum line from the modulator, if transmission fluid spills out when the rubber hose is removed the modulator is punctured. The engine will **** your transmission fluid through the vacuum line and into the intake system and burn it through combustion. If the modulator is punctured you must have it replaced.

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

Check for a vacuum leak to the transmission modulator. the modulator is on the trans abd has a vacuum hose on it.

check for vacuum leak replace modulator on transmission

Ck your trans modulator., and if not already done, you pan filter and fluids

Check that the OD solenoid on the torque converter is good still. When the R4 wont go into OD you have no more gears than 1st an 2nd. If the OD clutch drum was relaced if it was burnt, that will make a problem also. You have to have OD in order for these babies to work right.

That trans is either an RE4FO2A or V or RE4FO4A orV.Things to check when no 4th gear are:Inhibitor switch or control linkage misadjusted,no revolution sensor,speed sensor,or fluid temperature sensor signal,damaged overdrive switch,shift solenoid A,or brake band.