Imagine the Army generals' feeling of suspense when the key senator they had been briefing all morning on the wonders of the Divad field gun climbed inside a tank to try it.

Sen. Alan J. Dixon (D-Ill.), a member of the Senate Armed Services Committee who describes himself as "hawkish," had flown to Fort Bliss, Tex., last Friday to decide whether to vote for the Army's fiscal 1986 request of $531.6 million for the trouble-plagued Divad (division artillery defense) gun.

Dixon squeezed into a modified M48 tank, which carries the two 40mm Divad barrels around the battlefield, and tried to sight the weapon on the carcass of an Army personnel carrier placed downrange. He said the maneuver was harder than he thought it should be.

With help from an Army sergeant, Dixon twisted the knobs and sighted the Divad. He said he made a mental note that Divad was "a busy gun" that no G.I. should be expected to master in the heat of battle, especially as it bounced along.

Dixon pulled the trigger.

Wham! Wham! Jam!

One of the Divad barrels jammed and was not to fire again during Dixon's visit to the testing range. As of yesterday morning, he said, the Army had not determined why the belt feeding ammunition into the Divad had jammed. The problem was one that the Army had said was corrected.

One barrel was still working, so a drone was flown over as Dixon's next target. The drone was to represent a Soviet helicopter that the Divad, also known as the Sgt. York, is supposed to be able to knock down.

Dixon said he found that the Divad radar, built for an F16 fighter plane but adapted for the field gun, did not work as advertised. It was not just a matter of locking the radar on the drone and letting high-technology take over but of continuously watching the target to keep the gun hitting the drone as he fired.

The senator said he fired 88 rounds.

"At the end," he said in an interview yesterday, "the drone helicopter came down, but I think the sergeant did it."

Even allowing for the possibility that the jamming was a freak occurrence, Dixon said, "we don't have a gun that will perform as advertised. It's not ready to use under battle conditions. We need a better gun than we have now, but this is not the gun."

President Reagan's fiscal 1986 defense budget calls for committing $479.5 million to produce 117 Divads at Ford Aerospace and Communications Corp. at Newport Beach, Calif., $43.2 million for research on the gun and $8.9 for associated field facilities.

The $479.5 million should not be spent, Dixon said, until the Army demonstrates that the Divad is production-ready or switches to a better field gun to combat helicopters, low-flying aircraft and thin-skinned armored vehicles.

Triggering Dixon's trip to Fort Bliss, he said, were doubts that he had expressed about the Divad at a closed meeting of the Senate Armed Services Committee.

Committee Chairman Barry Goldwater (R-Ariz.) had argued that Dixon should see the gun before objecting to backing its production. The Defense Department was happy to oblige.

"By noon," Dixon said of intensive Army briefings, "I was pretty much convinced that anything that had happened in the past with Divad had been corrected."

Defense Secretary Caspar W. Weinberger ordered a Divad review before the fiscal 1986 budget containing $531.6 million for it went to Congress. "We need the capability," Weinberger said of the Divad program, slated to cost $6 billion if completed.

Dixon said he will not vote for Divad production money. He said he will vote for research and development money to improve or replace the gun.