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Low trans. line pressure

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 74Chev, Feb 5, 2003.

  1. 74Chev

    74Chev 1/2 ton status

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    Just installed an aux. tranny cooler and while verifying fluid flow direction noticed that there was very little flow from the factory cooler line at idle. (I could pee harder.) This tranny is also very sluggish to go into reverse when cold in the morning, but will kick in if you give it some gas. The tranny was supposedly rebuilt shortly before I got it, but who knows??? Am I correct in assuming the poor line pressure is due to a defective front pump, also causing the delay into reverse when it is cold?
     
  2. Highlander

    Highlander 1/2 ton status

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    I would agree with that but what the hell do I know. /forums/images/graemlins/ignore.gif
    When I put Synthetic ATF in my 700r I did notice that the Fluid did flow quite well. I wonder if a shift kit would help you out.


    Eric
     
  3. ratlover

    ratlover 1/2 ton status

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    What kind of tranny? Sounds like you have a 700r4 and the TV cable aint adjusted right. Dont drive it if it is. Low line pressure=puked tranny. If its just dribbling out you have a problem. If its running it shouldnt neccisarily be coming out at the rate of a fire house though. any other problems? Every one pees at a different rate at different times so its kinda hard to diagnose from your discription /forums/images/graemlins/tongue.gif Heck, get a few brewskis in me and I wiz with enough pressure to opperate small hydralics /forums/images/graemlins/crazy.gif
     
  4. the professor

    the professor 1/2 ton status

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    Are you sure the line-pressure is low..??
    --
    This is for all transmissions, but I will assume a TH350
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    What color is the trans fluid..??? Red or brown (or worse black)
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    Did you ever change it..??
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    If it is just brown, not black and does not smell burnt...
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    Try a fluid and filter change...
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    Use Type F trans fluid, and when the pan is down, look for flakes of black...(burnt clutch)
    --
    Recent overhauls DO NOT rule out plugging filters...if the overhauler used ANY RAGS to wipe out the case or parts..you can plug the filer with lint...crap from the converter, if re-used, also can plug the filter
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    Change the fluid a few times, you can only get 3.5 qts of the 10 to 12 at a time, and use Type F...it is slightly thicker (Dextron 7.5wt, Type F 10wt)..and has fewer "friction modifiers", that make the trans slip...Prefer NOT to use FA, it is for "lock-up" converters, and again, softens shifts..(it is also 10wt)
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    If its black, and/or smells burnt...figure an overhaul /forums/images/graemlins/crazy.gif
    --
    --
    This is the first step
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    --
    The oil flow for the cooler goes this way...it leaves the converter, goes to the cooler, then back into the trans for "lubrication"...
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    There is a "COOLER BY-PASS VALVE", located in the front pump, which "could" be the problem...
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    OR IT COULD BE A PLUGGING FILTER
    -
    Sluggish in the AM...just in foreward...your possibly loosing the lo/reverse clutch, or maby the foreward clutch... Just in reverse, and the 2/3 shift is good, then again, the lo/reverse clutch..and maby the foreward clutch...
    --
    OR IT MAY BE A PLUGGING FILTER, or loose valve body...
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    Beg or borrow a transmission pressure tester, and do a pressure test on the trans... (the tap is on the pass side, near the accumulator.)
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    Need somewhere around 60-155 in D1/2/3 @ 85 psi in L/2 ...the big issue is Reverse, NEED 240psi or better...less is an indication of problems..
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    OR SIMPLY A LOOSE VALVE BODY... (use an inch lb torque wrench, 100in lbs)
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    a sticking or weak pressure regulator spring (in the valve body on a TH350) or if the reverse pressure is lo, a sticking "reverse boost valve" could be the problem...ALL accessed in the valve body...
    /forums/images/graemlins/truck.gif
     
  5. 74Chev

    74Chev 1/2 ton status

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    Thanks Professor,

    Update: Pulled the pan last night, found a little bit of clutch material but no more than on any other pan I have ever pulled. The fluid is clean. It had the felt type filter but I installed the screen type (I had an extra at my shop.) The filter was slightly plugged. I have a transmission flush machine which can monitor cooler line pressure but I don't know what it should run at so I haven't bothered to hook it up to it. I know it's not worth flushing becasue the fluid is really clean. I checked the cooler line pressure it isn't much just a dribble at idle, I know my 74 shoots it out pretty quick even at idle.
    Thanks for the pressure specs on main pressure I will check that next. It seems to work a little better now but still sluggish on engaging reverse or drive in the morning. The tranny works so good and shifts hard I hate to pull it out for a rebuild if it is just a problem with the valve body. Anyone got any other ideas-maybe I should just break down and take it to a tranny shop (I hate not knowing more about automatic trans). Suggestions??? /forums/images/graemlins/confused.gif
     
  6. the professor

    the professor 1/2 ton status

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    Three things I left out last night...one regarding the cold shift...and three the cooler
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    COLD SHIFT:

    Does the trans shift firmly, and at the right points...??
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    The wrong vacuum modulator, or a universal adjustable will have an effect on the line-pressure...particularly at idle..
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    The "universals" are usualy adjustable, with a small screw driver the vaccum line port..screw it in a couple turns and try it...
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    TH350 &400's use vaccum mods, but the effect on line-pressure is more pronounced with a TH350..I have even used a small screw in the trans end of the modulator, ...to increase the pressure...(must be big enough to not fall into the valve..take the valve out with a magnet and notice how they interface)
    --
    --
    /forums/images/graemlins/confused.gif
    Depending on how "sluggish" this initial engagement is, it may not indicate any serious problem..TH350's tend to "drain back" the torque converters into the trans when they sit, about 6 qts, 1/2 the converter...this can take a few seconds to fill, particularly when cold...
    -
    /forums/images/graemlins/ooo.gif
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    COOLER:

    FIRST: /forums/images/graemlins/blush.gif
    I missed what you said the first time...you said "you had lo flow at idle"..DOES IT INCREASE WITH RPM....???

    If if DOES you HAVE NO PROBLEM... /forums/images/graemlins/grin.gif the cooler by-pass valve is probably not seating properly, and will probably fix itself, in any case it still will be no problem..hook you cooler up, drive it a little hard, and check the cooler gets warm...


    If it DOES NOT you may have a problem... /forums/images/graemlins/frown.gif

    Like I said, and I checked my Motors trans manual, flow charts to verify my memory,

    the oil goes from the pressure regulator in the valve body,...through the hub of the front pump housing,...to the converter, ..and back out of the converter again to the front pump housing,.. where it is routed out of the case to the cooler.

    through the cooler, ...then back into the case,.... through the front pump and out the hub of the pump to the inside of Direct clutch hub to lube the shaft...
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    the COOLER BYPASS VALVE, is located between the cooler outlet and cooler inlet lines IN THE FRONT PUMP...
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    If the trans functions properly otherwise..THE PROBLEM MUST BE THE COOLER BYPASS VALVE
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    --
    Reason;

    if the line pressure was so low that there was NO CONVERTER return...the trans would slip or not work at all

    if there was some blockage, or massive internal leak, you would loose converter pressure (very, very high stall speed)

    and you would burn up the bushings and planetary gearsets fairly quickly with NO LUBRICATION

    i.e. YOU ARE GETTING LUBRICATION, YOU ARE GETTIN CONVERTER PRESSURE....so the assumption is that the COOLER BY-PASS..is stuck open,.... or the spring broke....
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    The trans will work normaly otherwise, but you will have no cooler....it is being by-passed internaly..eventualy you will fry the trans from overheating...

    /forums/images/graemlins/eek.gifDrag race guys plug the cooler lines, and remove the by-pass valve completely...the trans doesn't get hot enough in 1/4 mile to need a cooler...

    /forums/images/graemlins/cool.gifI do transmissions for a living, presently on dissablilty...ASE Master, Ford EEC and driveline spec , GM CCC and trans specialist...Ca EA smog certified...Honda master tech..doing my best to help you by remote control
    /forums/images/graemlins/truck.gif
     
  7. 74Chev

    74Chev 1/2 ton status

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    Prof,

    You definitely the man when when it comes to autos. Little more info. By sluggish I mean 2 minutes to engage when its cold. It will make good shifts and no slipping though once its warm, and I run a healthy 400 sbc. I doubt it has an adjustable modulator but I will check. Found a receipt from previous owner, tranny purchased for $100 from boneyard. The inlet line to the cooler does seem to get warm after driving. I never winged the throttle way up when the cooler was disconnected only ran to about 900 rpm, not much flow, a fast dribble. By the way when I was pulling my boat over a grade this summer tranny overheated big time and was puking out the vent tube. I have put about 5000 mi on the Jimmy since I got it and the fluid isn't getting burned and the tranny is otherwise functioning normally. Does this help your diagnosis any? Thanks for all the help so far.
     
  8. the professor

    the professor 1/2 ton status

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    This ones a toughie with out hands on "diagnosis", and even then, it might require a tear-down to find the problem
    --
    One problem at a time...
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    You are going to have to do a pressure test on the trans...WHEN COLD, ...
    --
    --
    See the oil flow to the front pump is simple,
    >it goes through the filter bolted to the valve body,
    >then directly through the valve body to the case,
    >a channel routes it to the front of the case,
    >where it connects with the front pump cover and is routed to the pump...

    (if you pull the TQ/flex plate cover, and look at the iron thing bolted with 5/16 bolts to the front of the case, you can see two "bulges", about 2" wide, going from the edge up..these are the pump feed and return)....

    >the return comes back out,
    >then goes to through the case to the pressure regulator in the valve body...
    >it comes out of the valve body, back into the case, and a channel goes to the side of the case,
    >where there is a "tap-plug" to check the pressure...
    >i don't want to get too involved yet, but the "tap-plug" reads the line-pressure at the accumulator...
    --
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    THING that COULD BE wrong that can be fixed in the truck..
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    IF the pressure DOES NOT come up
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    Possibly a valve body / or separator plate gasket is leaking (loose valve body, overheated hardened gaskets)
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    IT COULD be a sticking pressure regulator, but that seems unlikely
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    --
    THE FIX HERE is to pull the VALVE BODY and SEPARATOR PLATE, REPLACE THE GASKETS and SOAK the VALVE BODY in PAINT THINNER
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    This job is not hard, just tricky...there are six check balls to replace.and gasket alignment is critical (smear Vaseline all over both sides of the plate and line up the holes...2 holes are smaller, use the bolts to align)..A B&M shift kit, would include the gaskets and all directions...and some modifications..USE an INCH LB torque wrench..and Vaseline to hold the balls in place.
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    IF the line-pressure COMES UP, or comes up partialy, but it the truck does not go..the problem MIGHT still be in the valve body gaskets, or a sticking shift valve ..you can always hope..but it is possibly a hardened seal, losing pressure...the lo/reverse piston comes to mind...YOU WILL HAVE TO DECIDE IF YOU WANT TO TRY and take the chance with the valve body FIX
    ---
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    Now if you do this, and still have problem...the repair will need to remove the trans...
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    Probably piston seals, or the front pump or both

    See, in the front pump housing (the cast iron cover on the front of the trans), is where the "cooler by-pass valve" AND the "converter primer valve" are located...
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    I have seen problems (broken springs, bad seating, sticking) with the primer valve...
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    Its function is to hold oil in the pump itself, so that when the oil drains out of the pump housing when the vehicle sits, it has oil to start the "suction" to pull oil up out of the pan...
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    also, a severly worn pump can do the same thing...
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    --
    However, I am suspicious, that both of these problems occur together...they SHOULD NOT be related, if the trans works OK otherwise...since this trans came from a yard, possibly some grit or dirt got into the front pump, causing both the valves to stick...
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    COOLER FLOW
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    To check cooler flow, your going to have to rev it up some...it should pick up ALOT...if it doesn't, it is probably/possibly the by-pass
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    Try this...block the cooler-outlet and inlet with a plug (temporary)...take the truck for a ride, and rev it through the gears...this MAY wash out what ever is in the cooler by-pass
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    --
    Trans over-heating when towing isn't unusual...particularly with a 400 and the trans probably has an 11" 2000prm+ converter...lots of heat...GM had an aux air cooler as an option..I had one in 1977, just for towing...and boiling the fluid won't necessarly hurt it...but it can harden the seals and gaskets...Unless it SLIPS it won't burn the clutches...

    /forums/images/graemlins/truck.gif
     

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