On a 1991 7.3 IDI, It's mounted on the pass fender, about the size of a tennis ball, one vacuum line in, 3 wires out. Looks like a transducer, probably feeding the ECU a manifold vacuum reading, maybe for cruise controller? Searched the forums here, found a picture of it in an 85 IDI, but not labeled. What's the nomenclature and what's the purpose?

 

Not completely sure but I seem to recall reading somewhere it is some type of "low vacuum warning sensor/switch" no idea what it does either. It seems like half the stuff they put on these trucks in the 80s can be deleted, at least on the carbureted gassers. I know removed a ton of extra wiring to the engine harness on my 84 F150 when I was going through it for the frame off.

 

You can scratch the ECU and manifold vacuum, our engines have neither.

 

It's the low vacuum sensor. It SHOULD turn on the red BRAKE (!) failure warning light on your dash when there's insufficient vacuum from the vacuum pump.

To elaborate on post #3, diesel engines do not produce vacuum. But SOME of the TRUCKS they go in still have vacuum-operated systems external to the engine. In our case, it's the brake boost, the cruise control if so equipped, and the HVAC controls if equipped with A/C. Ford could have replaced all of those with non-vacuum alternatives, but instead equipped the engine with an accessory belt-driven vacuum pump. If that pump fails, you lose brake boost and the brake pedal becomes REAL hard. The light acts as a warning.

Re. the ECU, our IDIs don't have one because there's nothing electronic to control. It's a completely mechanically-injected engine. There's an electromechanical (not electronic) solenoid that shuts off fuel in the injection pump when you turn the key off; that's the ONLY electrical thing our engines need to keep running. It's one of the things that makes owning and maintaining these trucks so popular.

 

Copy all that. This rig has a brand new belt driven Motorcraft vacuum pump that runs all of the above: vacuum brake booster, cruise control, A/C, HVAC, and the mystery widget which must be the low vac warning switch. The good news is all those vacuum circuits hold 20 inches with no drop from the vacuum distribution manifold, so no money required yet. Thanks for the collective wisdom.

 

I wouldnt ditch it, vacuum canister and low vacuum sensor as mention. Handy to know before you touch the brakes that you arent going to have any.

 

True that. Mine didn't work and it was a damn good thing I happened to be pulling a trailer with electric brakes the night my vacuum pump died on a busy Phoenix off ramp.

 

Just finished testing vacuum readings to diagnose brake behavior. New Motorcraft vacuum pump and new Motorcraft "zero loss" booster do get to 20-21 inches and holds, until you actually use the brakes, at which point each pedal press and release drops 5 (more) inches down. So the third pedal press (in quick succession, parked, in neutral, idling or revving steadily at 1800 rpm) only has single digit vacuum available and doesn't really stop the truck. The 4th press goes rock hard. It takes the pump more than 10 seconds to recover.

Oh yeah, no low vacuum idiot light comes on. Yet another problem to diagnose.

This is going to absolutely suck in a crowded parking lot filled with soccer moms in minivans who won't let you out of your parking space as you attempt to reverse out. Can't give these keys to the wife without a million dollar insurance policy...

I will start a new thread with a video of the vacuum gauge documenting pump behavior. From what I've heard, this is fairly common, thus the infatuation with hydroboost, or whatever Ford calls it.

 

If you park it and let it run until it reads 21 inches then shut it off and get out without touching the brake, will it hold 21 overnight?

 

A warning.. if one installs a larger vacuum storage tank.. you have more presses of the brake but>> it takes longer to build vacuum again.. so two tennis ball size storage vessels will double the time needed to build vacuum...ect

 

Why is this a problem? Do you pump the brakes as when coming to a stop?