I have an '86 f250 4x4 6.9l with a c-6 ... The problem is that in "D" it revs to around 2,500 before shifting and then it clunks ... If I start off in "2" it shifts from first to second just fine ... Then I can put it in "D" and third comes in fine too ... Fluid is clean and right on the mark ... Reverse is ok too ...

 

My truck is around the same as yours except i got the 460 and my tranny shifts like that all the time but its also has a shift kit...but i know some other guys with stock c-6 trannys that shift exactly like that so im wondering if its normal

 

Just wondering because 2500 rpm seems a bit high to me ... And the diff between having it in "D" and "2" seems wierd too ...

 

Set me straight here please.
You say the transmission shifts when placed in "2" position?
86 C6's have a select shift valve body.
That means when the lever is placed in 2 that's all there is, second gear, there ain't no mo.
D position has 3 speeds.

 

So as I understand you:

---If I put it in "1" that is first only as with almost any other tranny---

---When I put in "2" then I have no first but it starts out in second and only uses second like a stick ---

---When I put it in "D" then it starts in first, shifts to second then to third ...

Are these assumptions correct?

 

aaaaaaaaaaaaaaaaaaaaaaaaaayup

 

ok ... But I am still concerned that my diesel seems to want to wind up to around 2,500 rpm before shifting ... Looks like I might have to take the tranny apart and rebuild it ...

 

With a shifting problem check the vacumn system. The diesels have a vacumn pump on the fender and a vacumn regulator on the throttle lever at the injection pump. You need the right amount of vacumn at the transmission vacumn modulator for proper upshifts.

 

Right, I was about to suggest the vacuum pump.
Also check the vacuum lines for cracks etc.

 

Vacuum pump ... Is that the belt driven thingy below the alternator on the pass side?







*adds "DUMBY" to other names to call self later ...

 

REMOVAL


Disconnect battery ground cable and remove shield, if equipped.
Loosen vacuum pump adjustment bolt and the pivot bolt, then slide pump downward and remove drive belt from pulley. If vacuum pump drive belt is to be replaced, alternator drive belt must be removed.
Remove hose clamp and disconnect pump from hose on manifold vacuum outlet fitting.
Remove pivot and adjustment bolts and the bolts retaining the pump to the adjustment plate.
Remove vacuum pump and adjustment plate. Vacuum pump is serviced as a unit. Do not attempt to disassemble.
INSTALLATION


Install pump to adjustment plate attaching bolts, torquing bolts to 15---20 ft. lbs. on 2.2 liter engine, 11---18 ft. lbs. on 6.9 liter engine, or 10---20 ft. lbs. on 2.3 liter engine.
Position pump and plate on vacuum pump bracket and loosely install pivot and adjustment bolts.
Connect hose from manifold vacuum outlet fitting to pump and install hose clamp.
Place vacuum pump drive belt on pulley and adjust belt tension to specifications, torquing pivot and adjustment bolts to 15---20 ft. lbs. on 2.2 liter engine, 11---18 ft. lbs. on 6.9 liter engine, or 10---20 ft. lbs. on 2.3 liter engine. Alternator belt tension must be adjusted before vacuum pump belt tension is adjusted.
Install shield, if equipped, and connect battery ground cable.
Start engine and check operation of brake system. Brake light will remain on until vacuum builds up to normal level.

 

I infer by Stuart1's post that the vacuum pump effects both the tranny and brake operation ... Brakes work fine ... No fading or pulling ... Nice even stopping without having to floor the pedal ...

 

Some diesel equipped vehicles had a vacuum booster for stuff that needs vacuum and a hydraboost for the brake system.

 

Hydraboost? ... I know hydro means water ... Hydra mean fluid?

 

Yes.
The hydra boost system feeds power steering fluid and pressure to the brake booster.
It is a very good system.