Five-8

Hi All

So after a replacement Remanufactured Denso Compressor/High Pressure switch/Receiver Dryer/and the two main hoses to the compressor all installed i have A/C..cool if not cold air...but i can live with that.

Went out today for a short toot, (first one really after the work noted above),and noticed two things which suggest all is not well.First, while engaging the A/C while cruising i can feel a "substantial" knock down in power(almost feels like an ever so slight tap on the brake pedal).Turning the A/C off while cruising results in the converse...like you just threw some significant ballast out the window :-)

Second, clearly evident was a drain on the battery guage, which was consistent and was worsening the longer i drove with the A/C on, to the point where the guage was heading towards the red (10) and much more evident in start/stop conditions...turn it off and the guage returned fairly quickly to its normal reading.

Would appreciate your suspicions as to the culprit(s).
Did a search but could not find related info, but methinks i am a bit rubbish at searching.

Many Thanks In Advance

Cheers,

Adrian

Speedtoys

1) AC compressor adds load to the engine, thats normal
2) AC starts up the fans..thats..normal..the health of your charging system and/or pod voltage wiring, could be suspect.

Whats it do on the high way?

WallyP

First thing to check is that the alternator belt is really tight...

SeanR

Also, you can't rely on the dash gauge as they are rarely accurate. Get a volt meter and test it at the jump post when this happens.

terry gt

AC uses about 10 HP , all through the rpm , I tryed it on a dyno . So at cruise or light exceleration the AC make a big change + 1 to what speedtoys said .

Five-8

i understand that there is additional load when both the compressor and condenser fan engage, but what i'm getting/feeling in terms of immediate power falloff is beyond what anyone would consider acceptable... its not subtle at all,in fact it's quite disconcerting.
I only did a few miles at highway speed (50 ish) and it was just as noticeable at the higher revs, when engaging the A/C.
I did read while searching, that the guage is likely to be inaccurate, but the guage IS suggesting drain of some magnitude because it reads higher again when the A/C is turned off...will test it at the jump post though.
Will check the alternator belt too !

Thanks Fellas!

Cheers,

Adrian

Tails

In conjunction of all the above, have you checked the discharge pressure for excess pressure which would put excessive load on the compressor which will be transferred to the engine when the compressor clutch is engaged.

No sure whether your vehicle is fitted with an excess pressure cut out, if it is then you may also have a fault with this excess pressure cut out.

Tails 1990 928 S4 Auto

Speedtoys

Take note..that @highway speed cruising, that 10Hp compressor MAY be 12% of your engine output at the time, or more.

Cruising doesnt take much HP...ya, you'll feel it.

griffiths

A) Excessive HP loss with AC on could be an 'overcharged'.
B) Battery drain with AC on in conjunction with HP loss,
could mean clutch coil is draining more amps than normal,
possible coil shorting out if excessive heat is transferring from
compressor body to coil.

1) Check high side pressure at idle if excessive then remove some
refrigerant to lower high side pressure to normal.
2) Recheck if you added sufficient qty of refrigerant oil to system.
and
3) Don't run the system, with the exception of trouble shooting, until you fix the issue.

86'928S MeteorGrey

As for cold air, hows the air flow? Are your condenser fins clean? The HP draw from my car didn't seem too significant, but I could feel a very slight drop in power when engaged...

What kind of coolant are you running? R12, 134a, F12?

Check current draw of the compressor coil. Should be 55watts or 4.16A @ 12V.

Check all body grounds. Check for shorted wire in the freeze switch, low pressure switch, compressor coil circuit.

Five-8

You guys are fantastic! I will chase down ALL your suggestions and hopefully come up with a remedy!

Air Flow? well, i don't have anything to compare it to, but the air flow seems a bit anemic with the fan on high, currently the air is flowing out the defrost vents not the centre vent(another thing to fix), but the air coming out the door vents is cold but not comparable to more current vehicles, refrigerant is R134 and i understand it is less cold than the original refrigerant.
Condenser fins could be cleaner for sure but aren't overly fouled i dont' think.

I will not use the system as griffiths suggests, thank you sir for the warning!

Will report back after completing this to do list!

Many Thanks

Cheers,

Adrian

Cosmo Kramer

When you had the system charged how much R134 did they put in? Porsche says 950 grams for R12 but you need to take 20-25% off that for R134. If they put in 950 grams of 134 you will get the symptoms you describe due to excessive pressure.

Bill Ball

I had a compressor in one 928 that I worked on that apparently was partially seized and drew a lot of horsepower and current, eventually burning out the AC clutch. I replaced the clutch only to have it burn out (melt the epoxy) and fail again before I realized the compressor was the problem after it finally seized enough to make a racket.

The effect of the compressor kicking on in my car is barely noticeable from an engine output perspective. Yours appears to be much more dramatic. I suppose it could be due to overcharging, but then initial pressures when it kicks on are fairly low.