1988 Porsche 944S Gear wont shift when engine is running.

you forgot to say MANUAL transmission????????
and step #1 on all M/T box clutch pedal fails (say pedal see?
is the fluid for THE MC to slave cylinder is dry.
check fluid under hood for cutch MC, master cylinder above
the CLUTCH PEDAL it is.
if dry or very low, and goes lower the MC or SLAVE is bad.
i replace them both for faster longer service life./
the clutch bad can be variable until all fluid is lost
one key clue,is
is can not start the engine unless in Neutral.
or can not shift to 1st gear to move car. ,from neutral engine running
or all gears grind as you shift.
or the canary gear REVERSE loves to grind first is early warning of clutch failures.

a new MC and slave is cheap, do not buy a new transmission until MC and slave are proven good. first.

You can test the clutch hypothesis. Carefully put the car in gear for a safe direction, then depress the clutch and turn over the engine. If the car lurches, then the clutch is not working. In this case, check the linkage to the clutch, or if yours is hydraulic, check the fluid level in the clutch master cylinder. If the cylinder is low, then don't stop by just filling it, you want to find out where the fluid went. Look for leaks - especially around the clutch slave cylinder.
If on the other hand your engine it starts with the transmission in gear and the clutch down without lurching, then try letting the clutch out slowly. With any luck you have it running and in gear. See what happens next. Good Luck!

bad pressure plate or stuck throw out bearing.

no fluid in the clutch reservoir due to a failed clutch slave cylinder
time to replace both the clutch slave and master cylinder

Get the clutch slave cylinder and master cylinder looked at.One of the 2 has failed.If the pedal goes to the floor or is very loose,this is very likely the cause.Not an expensive fix.Hope this helps

Was the fluid low? Does the pedal have any resistance? does it feel normal? Did it happen all of a sudden? Does it shift into gear with the engine off? You say it doesnt shift into gear, but does the shifter move into gear or not? Or it does and the truck wont move? Sounds like maybe the SLAVE CYL isnt operating? leaking? theres more questions to answer.

i would check the throwout bearing, if this is the problem try starting in first go for a while than try up shifting to second. after a while your transaxle should warm up making an easier gear transition. ( this only partains to bad throwout bearing )

If hydraulic actuated mechanism, loss of fluid, defective master cylinder or slave cylinder at the clutch. Missing pin or other part between slave cylinder and clutch arm. Loose or broken slave cylinder mounting. Missing pin or broken part between pedal and master cylinder. If mechanical linkage, look for broken components or missing pins at moving joints. If you tried to shift into gear with engine running, you may have damaged the transmission gears.

check make sure the clutch master cylinder working properly along with slave cylinder. both master and slave don't have to leak fluid to be faulty, the rubber seal inside the unit could be worned and when apply pressure the fluid blow by or seep between the rubber seal causing the the unit not to work properly giving you a weak feel and pressure at the pedal. both unit have to work properly in sync. if one are weak you will have the same issue. clutch should have a good tight feel not sloppy or weak. pressure plate will also play important role, weak pressure plate will also give you a weak pressure feedback from the foot pedal and also will cause the synchronizer in the transmission not to engage and work properly. check this basic item fist before messing with your tranny. clutch master and slave cyc are fairly cheap to fix and replace.

Hi mate. I'm no Porsche expert but the most common diagnosis would be clutch failure. If you can shift (even some) gears with the engine off then this would be plausible. Before looking at the complete clutch, try the master and/or slave cylinder, it's more likely to be the master. Pop the bonnet and check the level of brake fluid in the reservoir, there is no "clutch fluid", it's just brake fluid. If it's below the minimum line, top it up then see if it engages gear. Get it checked out, but tell the mechanic your theory, he'll soon confirm it if it is.