in trying to diagnose the late shift problem with my 88 superduty diesel 4X4, i think i have found the problem. i only have 17 inches of vacuum from the pump. somewhere i remember hearing that it should be more like 23-25 inches, and if this is true, then it might be why i only have 2 inches at the trans modulator instead of 5-6 the book calls for. does this sound rite, and am i correct that the vacuum pump should be putting out 25 ??

 

Do a vacuum test....... go here for the test.

28385&cat=500&thumb=1

 

i did do a test, and get 17 inches at idle, 2-3 at wot. but it is suposed to be 5-6 at wot.

 

try adjusting the modulater valve on the ip also check the one on the trans.

 

The Modulator valve is on the trans the (VRV) Vacuum Regulating Valve is on the IP...

How to adjust VRV

You don't have the engine running for the test, you need to apply 20" vacuum to upper VRV port and attach vacuum gauge to lower VRV port. The lower port is the one that goes to tranny, the upper to vacuum pump.
You have to maintain 20" througout this test. If it drops off you need to pump it back up.
Remove throttle return spring, cycle throttle lever 5 times from idle to WOT. Make sure 20" vacuum is maintained on upper port side.
Move throttle so it is 0.515" (33/64") from wide open (they make a tool for this). Vacuum gauge should read from 6-8" of vacuum. If incorrect adjust VRV to 7".
Again apply 20" vacuum, cycle throttle from idle to WOT 5 times. Return to idle. Gauge should read at least 13".

It says if you can't get the 7" or the 13" to replace the VRV. If VRV checks out maybe the diaphragm (modulator), line from VRV goes to the vacuum diaphragm I believe. Can't find any adjustment for the diaphragm but I think there is a little.

Basically all you need for the test is a vacuum pump (handheld type), vacuum gauge and a ruler to measure 1/2" or a 33/64" drill bit in lieu of the .515" tool. And a screwdriver for any adjustment needed.

 

"How to adjust VRV"
i have done the adjustment 3 times and still come up with the same problem

on wot, the vrv gives 2 inches of vacuum. the vrv is new. it replaced a new vrv. the modulator is new, it replaced a new modulator. i am only getting 17 inches of vacuum to the vrv, and at wot it goes instantly to 2 inches. no adjustment of the vrv will increase the vacuum, it will only close off vacuum earlier, and if not adjusted to full, it will give 0 vacuum. this is what has me puzzled enough to think i do not have enough vacuum from the pump. that is why i am looking to find the amount of vacuum the pump should be drawing

 

Do this test again........

Page 1 http:///showphoto.php?photo=10952&papass=&sort=1&thecat=50 0

Page 2 http:///showphoto.php?photo=10951&si...t=1&thecat=500

Check for air leaks in lines and especially under the reservoir (coffee can) on the pax fender..........

 

so basically that is telling me that i should have 21 inches of vacuum, and that my pump is bad??

 

Either the pump is bad or you have vacuum leaks.....check everything. Even disconnect the heater vacuum line to the interior and plug it for the check. Any loose connections inside can lower vacuum too......

 

try removing the spring to the diaphragm and make it about 1/16” shorter +/-. This will cause VRV to pull more vacuum at idle&WOT.

 

I thought it was weird that @tjc transport was asking about a vacuum pump. I think he's gotten the problem fixed in the past 20 years. His 88 probably has 250k+ miles on it now.

Here's a good thread on VRVs if anyone digs up this thread and needs more info.

https://www.ford-trucks.com/forums/1...tion-pump.html

 

Wow!! 20 year old thread. And the problem was fixed 20 years ago after putting a new vacuum pump on

 

you are off by a little bit. I think it was around 200k miles when this issue started after swapping the C 6 in when the ransfer case blew up. Since the DMF was also bad and trans was giving me grief, it was cheaper to do an auto swap as i needed the truck within a week of it going down.
It now has a little over 496,000 miles on it.

 

I think it’s awesome how after 20 years the OP is still here to post on it, with the same truck that still runs and drives. Some posters come and go, some of us stay around. These forums are great in many different ways.


Reading it I was a little amused how you were merely asking what a healthy vac pump should be pulling, but nobody directly answered that lol

 

things were different 20 years ago. we had computers yes, but the information stored in those computers was not what it is today.