billdrah

My 1978 shows 40 PIS on the low side with ac off and vacuum when the compressor is engaged. I don't know what on the high side since I have yet to find that port but I assume really high. This leads me to think the expansion valve is clogged which in previous experience makes me think the compressor is dying. I see the old style compressor cost a mint, has anyone switched compressor types?

SeanR

Where are you hooking your gauges up? Please share a picture.

Mrmerlin

After you answer Sean I recently performed this repair to a 1978 machine.
This is what i did.
you can replace the original Bosch compressor and upgrade to the new style compressor that was used up to the 86 models,
You might consider Griffiths for the lines and the compressor and adjusting bracket,
you need a
new/ used Block mounting plate, with the proper bolts the compressor mounts to this plate.
The new/ used compressor belt adjusting bracket,
new compressor belt
the new/used compressor,
the 2 new hoses, suction and discharge
the 2 hose manifolds, these are not included with a new compressor, the new hoses attach to these
used parts can be purchased at 928 intl.
a new drier,
and expansion valve.
The proper compressor oil
Mineral if your staying with R12
That said a plugged ex valve is common on systems that have not been used in many years and have lost the charge,
thus moisture causes corrosion at the location of the valve.

NOTE Greg Brown makes a replacement hi pressure hose to replace the HP line that runs above the computer under the hot post
this requires silver soldering new fittings to the line where the old swages have been removed.
Look to see if the hose is leaking by seeing greasy areas where dirt has attached to the fender well
removal of the HP line to the EX valve is the best way to replace the fitting,
its time consuming to get the line out but since its all apart its not too bad of a job.

billdrah

Here is where I am checking what I think must be the low side, have yet to spot high side port.

SeanR

That is indeed the low side. High pressure side is located on the drier, right in front of the condenser.

griffiths

shows 40 PIS on the low side with ac off and vacuum when the compressor is engaged.

When you are seeing 40 psi on the low side, how long has the AC been 'off', and what is the ambient temperature around the car?
If you don't recall. Run the same pressure test again on the low side, however make sure the low side has been off for a good time, say 2 hours,
and report the low side pressure and the ambient temperature when you do it

dr bob

^^ What Griff said. Take the static reading with the engine off and car cold. Look at your gauge and see what the corresponding temperature is for the pressure you see. If pressure is higher than corresponding temperature tells you for your ambient, you likely have air drawn (sucked) into the system. Trying to diagnose the compressor from just one measurement on the low side is impossible. If as you suspect you have very low pressure on the suction side and high pressure on the high side while running, congrats your compressor is wonderful. If low on both sides the charge is likely low. Not sure why you'd assume the high side is really high. What are the air temps when you are doing your testing?

Before you get into the refrigeration side looking for causes of odd pressure and temperature symptoms, make darn sure that all the air-handling stuff is working correctly. The airflow and where the heat is taken out play a huge roll in what the refrigeration-side symptoms look like.