What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Where Can I Find Photo Diagram of Vacume Hoses for My 1986 GMC 3/4 Ton 4WD?

D

Dave1

Registered Member
Joined
Jan 4, 2002
Posts
82
Reaction score
0
Location
Winter Springs, (Central) Florida
Just had a fresh rebuilt 350 engine installed and now my T-400 transmission shifts hard and will not drop down when accelerator is depressed. Doesn't seem to have any timing advance either. I am guessing the vacume lines may not be run correctly. Hope that is all it is.

Don't know if this is related but when at a stop and engine idling, the engine throbs and shakes. What would cause this? Carb??

Dave
[email protected]
 
Shifting hard is likely the modulator not being hooked up. TH400 in my car I couldn't tell the difference one way or another, but I know others that certainly felt the harder shifting when the same happened to them. Does it take off with ahuge hesitation? If so, could be vacuum leak from disconnected modulator line. Just needs manifold vacuum if the line/fitting was removed from your truck.

Not downshifting is going to be related to the kickdown solenoid. Probably didn't get hooked back up.

Sounds like you had a shop do it, be prepared for broken or cut wiring or connector(s). Hope they just didn't plug it back in. :(
 
Yes, a shop did it. The problem that I discovered too late was they had a fellow working on it that was a trainee that had no experience and he also had little supervision throughout the job. I found many issues resulting from his lack of know how.

I refuse to take my truck back to them and am now faced with trying to run down the problems myself. I suspected a vacume line problem and need to know where the various lines need to be run and connected to what.

Any help will be appreciated.

Thanks, Dave
[email protected]
 
I thought there was a sticker under the hood on these trucks. Like on the upper radiator support.
 
Junkyard!

DITTO!---If your diagram is missing,try a junkyard and look for another truck the same year...or a local library might have a chiltons or motors manual that has only vacuum diagrams and electrical wiring...I use the library a lot for their reference books,I'm luck the one in the next town over has a well stocked library,because a state college is located nearby... :crazy:
 
I haven't found a vacume line diagram for my 86 GMC 3/4 4WD truck yet. I was hoping someone could direct me to one on the internet or send me a diagram or picture or info of how the vacume lines should be run/connected. I have determined that mine are completely wrong.

Last night I noticed a large hose end type vacume fitting of about 5/16" diameter screwed into the intake manifold right in front of the distributor that was left open with no hoses attached to it. I capped it and the idle slowed down and smoothed out alot. I am guessing something is supposed to be attached there. Maybe that is why my transmission is not shifting correctly.

I just need to re-run all the lines and make sure they are connected correctly to the right vacume sources on the intake manifold and the carb.

Thanks for any info/help.

Dave
[email protected]
 
I'll try..

Well,I can walk you thru the basic hoses that need to be hooked up...

First,the PVC valve hose usually goes on the large spout in the front center of the carb(stock Q-jet)--if its an aftermarket carb,like an edelbrock,it also has a large nipple in the front center for it--on a Holley or other carb you might have a nipple,or you may have to hook it directly to the manifold somehow...it needs manifold vacuum...(constant vaccum at idle)..

The power brake booster hose threads into the rear of a stock Q-jet--again,if your carb has been swapped,you'll have to get manifold vacuum to that somehow as well..

The vacuum advance hose needs "ported" vacuum on most engines,that means no vacuum at idle,but it will have vacuum off idle and slightly above--the fitting is on the carb--you'll have to run the engine and find the right nipple,a vacuum gauge would be handy to test this,but your finger will work too---however some engines run better with it hooked to manifold vacuum--so you'll have to try both and see which seems better...

The missing fitting you found sounds like the port in the manifiold between the carb and the distributor--thats where the transmission's vacuum modulator's hose normally hooks up--it needs manifold vacuum to operate properly...that fitting usually has a few nipples for other hoses as well,some trucks use all of them,some trucks have the "extra" ports capped off...

There are lots of other hoses,like ones for the EGR vale,EFE valve,air conditioning vent doors,etc..but hooking up the PCV valve,power brake booster,tranny modultor,and spark advance hoses should get you in the ball park--the others you can figure out eventually when you find a diagram,or look at another similar truck,or find one with the diagram intact at a junkyard-
its important not to leave any open fittings--vacuum leaks will raise havoc with idle,fuel mileage,and could even burn a valve due to the lean mixture from too much air getting in the motor...plug any leftover ports..--good luck!.. :crazy:
 
Thanks for the reply and feedback.

My carb is stock and the smog pumps and hardware have been removed. There is an after market cruise control on it that has a vacume line attached to it but I don't know what that line should connect to.

The vacume line that is connected to the transmission modulator is a small 1/8" type rubber line and has a mid-section of hard copper tubing in it. I guess the copper tubing is used in long sections to prevent colapse. That copper tubing is not secured/attached to the engine block or firewall, just hanging there, and has a short section of 1/8" rubber tubing on each end of it. The open port that is in the intake manifold between the carb and distributor is fairly large at about 1/4" or 5/16" so to make a connection to the small transmission line, I will have to find/buy a reducing coupling of some sort. There is alot of vacume in that manifiold port.

The rubber line that goes to the distributor advance is a 2' long 1/8" hose so I am asumming it slips onto one of the tiny vacume ports that stick out of the base of the carb. Does it matter which one? I think I put it on the one in the front of the carb.

I don't remember seeing a large port on the rear of the carb you mentioned for the power brake booster. I'll look for it tonight.

Thanks, Dave
[email protected]
 
Diagrams are on the way.

I must have sent these pictures out about 100 times by now. You would think that more service manuals would have them.

I have Diagrams from a lot of earlier Vacuum routings if you would like to do some modifications to your routing.
 
I finally found the vacume diagrams on my truck. They were on the air breather but were on the back side and I hadn't seen them. The factory diagram was not easy to decipher. It has many components and abreviations that I don't have a clue what is.

Apparently my truck had smog pumps and such at one time and those have been since removed. There are some vac lines that apparently went to some of that stuff.

I found another truck just like mine and looked at how his vac lines were arranged and that helped some. Not sure how many of the vac lines and devices under the hood "need" to be connected but maybe I have enough of them run right now that everything will function properly.

I ran the transmission vac line to the large manifold port and that straightened out the transmision shifting problem. That was a relief to learn I did NOT have trans trouble. The transmission had been leaking some fluid and that seemed to have stopped also. Don't know if that was coincidense or not, but glad it stopped.

Not sure about the rest of the vac lines but everything except my aftermarket cruise control seems to be working. Will keep working on this problem and see where I go with it.

Thanks for all the info.

Dave
 
heres the abbreviations

AIR air injection reactor
DVTR diverter valve
EFE early fuel evaporation
EGR exhaust gas recirculation
PCV positive crankcase ventilation
TRC throttle return control
TVS thermal vacuum switch
 
You must log in or register to reply here.

Latest Posts

Top Bottom