ESOF Vacuum System - Diesel Forum -
1999-2007 General Questions General questions related to 1999-2007 Super Duty trucks. If it doesn't fit the other categories, post it here. Gas engine discussion that pertains to all models is allowed. Specific gas engine questions should use the Gas Engines forum.

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post #1 of 6 (permalink) Old 02-20-2014, 09:39 PM Thread Starter
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ESOF Vacuum System

The frequency of questions about the ESOF vacuum system has prompted me to offer some thoughts on the topic. The points offered here are an accumulation of information from this forum and my personal experience, and are offered with all the cautions that common sense would dictate. I apologize early on for the length of the post, but as I wrote, it just seemed to keep getting longer. Sorry for that. Take from it as you will and feel free to offer corrections or expansions. So, here goes.

Be mindful of the “temporary “nature of a couple of the components, which can mislead you if you expect things to “work” all the time. The vacuum pump will turn off when the system reaches a certain vacuum strength. Don’t think it should run every time you turn on the key. The solenoid only transmits vacuum to the section of the system “down-stream” from it as instructed (by the PCM?). Don’t expect to find vacuum behind the solenoid except for a short time after turning the Mode Selection Switch (MSS) 2wd>4dw or 4wd>2wd.

Testing the solenoid for electric function was confusing to me until I learned that some of the electric components have full time feed and switched ground.

A vacuum tester like the Mityvac brand is invaluable for troubleshooting this system. Several parts stores rent these, but I found several of theirs to be in non-working condition, so prove that it works before driving home. Also, a couple of them did not have the attachments you would need to connect to the lines and nipples. You will need both male and female attachments of the proper sizes.

The function of the vacuum hubs has been compared to a ball-point pen in that the action to engage is almost identical to the action to disengage. There is a vacuum pulse (not full-time) applied to engage the hubs and then the vacuum shuts off. To disengage, a similar (but shorter) pulse is applied. Those who think that the vacuum “holds” the hubs in a locked position are misinformed.

Testing whether the hub is locked does not require the front wheel to be off of the ground. The u-joint immediately inboard from the hub is accessible from the ground and attempting to turn this u-joint by hand can tell whether the hub is locked (to the wheel.) If both hubs are unlocked, even if the transfer case is engaged, these u-joints can turn. Because of the nature of a differential, turning one wheel (hub) will result in the opposite wheel turning in the opposite direction. Similarly, if any two of the three components (two wheels, one drive shaft) are free, those two can turn freely, even with the third locked. Beware, though, if two are locked, the third will not turn either, even if it itself is not locked.

This might be a good time to remind about the manual operation of the hubs, since the marking on the hubs seems hard to read after a while. I use the axiom of “clock is locked.” On both the driver side and the passenger side, turning the hub handle clockwise locks the hub. Turning it counter-clockwise unlocks the hub. For those who are only familiar with digital watches, phone your Dad or Grand-dad.

If you think that your hubs are not “in sync” with each other (one locked and the other unlocked) due to applying vacuum to one while the other side is plugged (or some other action), you can manually unlock a vacuum-locked hub by turning the knob from “auto” to “locked” and back to “auto.”

The default position of the distribution system is “Defrost.” This is why folks suffering from vacuum leaks before the solenoid experience heat/air coming from the defroster even when the knob is set to “dash” and/or “floor.” If there is a leak after the solenoid, this same shift to defrost will probably be experienced as the MSS is switched (2wd>4wd or vise versa), but the air should return to normal control after the solenoid closes off the vacuum after the pulse time described below and sufficient time for the pump to re-establish sufficient vacuum strength.

Pulse duration, as controlled by the solenoid:
As switch to 4wd, approximately 40 seconds
As switch to 2wd, approximately 10-20 seconds

Vacuum strength, behind the solenoid: 15 “ Hg.

Time to evacuate a “full” canister : less than 60 seconds. (This is from my son’s truck. I am writing this as I trouble-shoot my truck. My pump can run in excess of two minutes, and I am considering a new pump, but will probably wait until it quits completely.)

Routing of vacuum line:
A) From the pump to the canister is a single line
B) Immediately after the canister is a tee to split into two lines
1) Single line to inside the cab for the air distribution control.
2) Single line to the pulse vacuum solenoid (one side of a double-line connector)
C) From the solenoid (in the second side of the double-line connector) routing across the top of the canister, where there is a connector just before the line turns down beside the motor. This connection is a good test point. Down-stream (towards the wheels) can shed light on the integrity of the system to the hubs. Up-stream can validate the lines from the pump through the canister, the function of the solenoid, and the line immediately after the solenoid.
D) Somewhere after the line goes hidden, it splits into two lines, one to serve each front wheel.
E) As the line approaches the wheel well, it goes through a mounting bracket, and as it goes thru this bracket, it has a connector to transition to flexible rubber lines.
F) The rubber lines run to the nipple on the hub, at which point the “line” ends and the rest of the system is inside the hub. This rubber line is a point of frequent failure, and my recommendation is this: if your hands turn black when touching this line, then that is a sure sign that the rubber is deteriorating and the lines should be replaced. I have heard recommendations to use fuel line instead of searching for vacuum line and I have had good results doing this. The inside diameter of this line is around inch, but I do not know the exact spec. Approximately 24” long for each side. Driving to the parts store with these lines removed will not hurt anything. If you wish to plug the rubber line which runs to the hub itself, a .22 caliber bullet works quite well.

Point : the lines in A) and B2)are short and rigid. This (in my mind) makes them fragile and brittle. (Please don’t ask how I came to this opinion.) These lines are not available from Ford, except as a part of the whole system. Be warned.

The nipple on the hub is the best place to test if the hub itself will hold vacuum. If it fails the test at this point, there is a problem with the hub, probably (A) the “big yellow” o-ring which gets a lot of comments but seems to be accessible only while removing the actual hub itself, or (B) the smaller black o-ring which seals the hub-lock unit inside the hub. Neither of the o-rings should be reused if its “housing “ is dis-assembled. Ford’s part number for the small one is 4C3Z-1K106-AA and runs around $45 for 2 o-rings and 1 new clip (that holds the hub-lock in place.) Guzzle’s number is 19-001 and goes for $11.27 as of this writing for 2 o-rings and 2 new clips. Guzzle has done an excellent job of describing the maintenance of the innards of the hubs, and I will not try to expand or improve on that.

As I offered above, please feel free to comment with corrections. I will plan to monitor for these and will probably amend the original text to avoid folks having to read the whole thread to find out that I was wrong about something. If you made it through to this point, “Thank You,” but you may need a different hobby.


2002 F250 4WD Lariat Crew Cab 7.3L 258 k AIH Delete, Door Seal Mod, DieselSite Boots, 203 Thermostat, Hutch Mod, Harpoon Mod

Last edited by Popgunner; 02-20-2014 at 09:45 PM.
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post #2 of 6 (permalink) Old 02-21-2014, 12:03 PM
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There is also a axle vacuum seal that seal the axle to the hub bearing unit. If this is bad the hub will not hold a vacuum either. This is the issue I currently have with mine. This is a great write up and I hope it will help people to find their vacuum leaks.
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post #3 of 6 (permalink) Old 02-18-2015, 09:11 PM
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ESOF Vacuum System

Lock your hubs manually. I relied on the ESOF thankfully my family is alive, the family i crashed into was not injured and the X was repairable. scariest moment of my life while driving a vehicle in the snow. i never had an issue before and did not know about this problem nor the defrost indicator to see if you have a leak. If i think im gonna need 4x4 that day i just lock the hubs in before I leave. i would never recommend relying on ESOF.

scared my wife so bad that if we leave so cal to go to mammoth she is locking them before we even leave! better safe than sorry and not worth it.

After my accident found out tons on the forums and people that live in the snow country lock em first snow and un lock them in spring. There is no issue with driving in 2wd with locked in hubs.

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post #4 of 6 (permalink) Old 05-26-2015, 01:07 PM
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I can't find out to remove the esof assembly in my front hub in my 2005 f250 4wd. I need to get the hub of and cant get past the esof assembly.

2005 F250 Super Duty Crew Cab 4x4-6.0 power Stroke diesel-Donahoe Racing 4.5 inch lift. Truxedo bed cover-35x12.5x17 BFG M/T tires. 9000 lb.winch in basket for front or rear receiver. Fab-Tec bumper & light bar
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post #5 of 6 (permalink) Old 05-28-2015, 04:55 PM
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Maybe this link will help you out, it's for an 02 but should be ok
Welcome to guzzle's 4x4 Needle Bearing Maintenance Web Page

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post #6 of 6 (permalink) Old 08-08-2016, 08:17 PM
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Question I am having an issue with mine not pulling vacuum for the HVAC system, my thought was to plug line going to front hubs and locking them manually when needed. ( if you are a truck guy from way back you will fine this nostalgic) then my understanding is the system will only be needed for the hvac system is this assumption correct?
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