Are my fuses longer than yours?

http://forums.pelicanparts.com/uploa...1179964578.jpg

Regarding questions about fuses, fuse size and wire condition, here are a couple of pics with the new block installed. The block came with three new factory fuses, so I don't think the fuses are the problem. Got to be wire resistence or a bad system component. I'll try applying some solder to the distribution end coming out of the block. Generally speaking, all of the wires connected to the fuse block are almost black with tarnish.

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Resistance reading on A/C clutch
 

Using my multimeter I just took a resistance measurement on the compressor clutch and got a reading of 3.7 - 3.8 ohms.

On Griffiths website they state a normal resistance reading should be 2.8 - 3.2 depending on the make/model of the compressor.

Is 3.7 - 3.8 far enough out of range to be contributing to my overheating problem?

That's a pretty personal question for a public forum!

But seriously, Mark, are you sure you have the correct amp rated fuses?

Also, you might give Charlie Griffiths a call and discuss this with him. He's truly a guru when it comes to 911 a/c.

Hang in there...you'll figure it out.

Re: Resistance reading on A/C clutch
 

When you touch the leads together what do you read? Subtract that from the reading.

It is hard to measure low resistance with a multimeter. To get accurate readings you have to put a substantial amount of current thru it and measure the current and voltage and use ohms law (which is what your meter does on a smaller scale).

Now since we got the meter out measure the voltage from the copper buss bar on top to the copper wire on the bottom. Use your voltmeter probe to push back the insulation on that red wire on the bottom to get to shiny copper. This of course with the AC clutch and fan running.

3 ohms at 12 volts shoud yield 4 amps. You can't figure the current draw of a DC motor using an ohmeter due to the back EMF.

Re: Re: Resistance reading on A/C clutch
 

Rick, I didn't get much of a reading at all touching the posts of the meter together.........did look a Bentley's today and they state the resistance should read 3.8 so I'm on the money.

Did a voltage measurement following your instructions and got a measurement of 9.3v (measured several times and got 9.3 each time)

Re: Re: Re: Resistance reading on A/C clutch
 

Are you sure it is 9.3 volts? The meter didn't auto-range on you did it? I would expect that reading to be under 0.5 volts.

Edit: I wouldn't expect the AC clutch to pull in with only 5 volts across it. (13.5 - 9.3 = 4.2)

I'll recheck tomorrow

Thanks for the guidance!

If your are sure you read the meter correctly leave the one probe connected to the top buss barr and move the probe up the chain (terminal, fuse clip, fuse, top terminal etc). When the voltage drop goes away the previous junction is at fault.

Since everything else is new I'm betting on a corroded wire/crimp ferrule at the bottom.

note; if you put the black lead at the top buss bar the voltage should read negative.

I tried cleaning up the wire end on the bottom connection last night. Removed the crimp ferrule and naturally could not get if back on. Thought I would try cleaning an reconnecting before resorting to cutting the "bad" end off. Twisted the relatively loose wire end as tight as I could and coated in dielectric grease before reattaching to the lower post.

Started everything up and put my finger on the top jumper and got excited when I did not notice heat immediately. My euphoria whas short lived, however, as it did become too hot to touch in about 20 seconds.

I've got enough slack in the distribution wire to cut some off use some fresh (20 year old wire) from under the insulation and then try your recommendation.

I must admit, frustration is setting in and I've drawn a line in the sand. I stopped by the dealership this AM on the way to the office and made an appt to drop the car off on Monday AM.

So, I've got 2 more days to tinker with it. http://www.pelicanparts.com/support/...s/a_frusty.gif

Thanks!!!!!!!!!

Mark,

Did you tap off the compressor wiring for the relay that controls an extra fan on the rear condenser?

Did you try disconnecting all additional wiring?

Yes & Yes.

I tapped off the pos lead at the A/C compressor to trigger the fan relay....the fans are drawing current from fuse #3 in the engine compartment.

I have disconnected everything (rear condenser fans, front condenser fan and the footwell fan that I added) [BTW I changed out the 4" spal at the evap inlet for a smaller computer fan that draws only 0.45amps] .....and I still get overheating.

Not sure if its possible to isolate the compressor and the evap fan (i.e. not sure one works without the other).....but the fan blower is new and "unlikely" to be problem.

Needless to say my attention has been totally pulled of my tail light housing short in an effort to troubleshoot my A/C fire hazard....perhaps the most valuable piece of equip I should be carrying is a fire extinguisher! ( ....and, no I'm not runnig the A/C until I get this solved)

Have you tried unplugging the condenser relay and disconnecting the clutch.
See if only running the evap fan causes a problem?

http://forums.pelicanparts.com/uploa...1180142337.jpg

No, I'll give it a try in the AM.

Thanks!

If you read your VOLTmeter right you have found the problem. That is usually the hard part.

Use your meter to isolate the resistance and fix it.

Basic electronics

Say 10 amps is going into IN. Those are resistors below (or connections). The little symbols are 0.01 ohms and the big one is 1 ohm.

IN o-------/\/\----------/\/\---------/\/\/\/\---------/\/\----------o OUT

If you put your volt meter probes on IN (+) and OUT (-) you will read 10 volts.

If you put your volt meter probe (+) on either the junction between the two small resistors or the small resistor and the big resistor and OUT (-) you will read 10 volts.

If you put your volt meter probe (+) on the connection to the right of the big resistor and OUT (-) you will read 0.1 volts or zero.

If you put your volt meter probes on IN (+) and (-) on the connection to the left of the big resistor you will read 0.2 volts or zero.

Voltage = Current x Resitance

Just to run some numbers say that with the 10 volts you measured the fan/clutch is drawing 10 amps. P = VI or 100 Watts.

How long could you hold your hand on a 100 Watt light bulb?

OK, I just re-tested voltage across the fuse with one probe on the top bus and the other probe into the distribution wire just below the post.

Got dramatically different results of 0.13, so let me explain my meter settings. If I switch the probes I get a reading of -0.13

Used probes in VoMA (+) and COM (-) and set meter dial on DC Volt scale (mine goes from 200m, 2, 20, 200,600) on 2.

Further troubleshooting today:

1) Disconnected power to A/C clutch and ran evap blower motor only -still heats up

2) Removed all ground connections at driver side front strut and cleaned them up with sand paper - still heats up

3) Removed and cleaned all ground connections at Battery Neg to chassis. Repaired exposed fog light ground wire (from rubbing) at that location - still heats up.

4) Cut off end of distribution wire and stripped back insulation to expose fresh wire....coated in dielectric grease and re-attached - still heats up.

We could be golfing instead.

How could I concentrate???????????:p

Paul, just sent you a PM

Was the fan running and the clutch pulled in?

2 watts isn't going to get that copper block too hot.

Mark: When you pulled the power for the fan assembly you're running on the rear condenser, where did you tap the power from?

I'm pulling power for the condenser mounted fans from the engine fuse block #3 which is for the rear window defroster that I don't use because it no work.

Like this.....ignore comments below diagram as it has been rewired (properly) per recommendation from mystertrain.

Anyway, I'm off to the dealership this AM to see what they can find.

Thanks all for your ideas and suggestions ( and to you electrical experts - thanks for your patience with me)

Will follow-up with what they find.


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Problem Resolved: System had been overcharged causing the compressor to run at higher pressure & draw more amperage.

All electricals checked out OK. Dealer tech evacuated the system and then recharged with the proper amount of R-12.

He states that it is normal for the fuse block to get "warm" when the A/C is running full tilt, but it should not get "hot". Static pressure in system before he started was 80psi.

Thanks again for all your help!

"Problem Resolved: System had been overcharged causing the compressor to run at higher pressure & draw more amperage."

??

The compressor clutch is an on/off device, it's current draw isn't modulated by compressor load or pressure. There is nothing electrical in the mechanical part of the compressor beyond the clutch. If there is too much load the clutch may merely slip which will make frictional heat which can heat up the clutch coil but this will increase resistance and cause current to decrease. Clutch slipping can also be caused by an incorrectly set magnetic gap.

Compressor was running continuously or too long (with clutch slipping insufficient compression was occuring) and thereby heating the circuit too long or all the time. Correcting the refrigerant charge level likely solved the slipping problem but perhaps other problems remain:

Running voltage is too high (car's voltage regulator failing?) and this causes excess current draw for fixed resistance items like the compressor clutch coil.

Clutch coil has a internal damage (shorting) and resistance is too low causing it to draw excessive current.

Clutch magnetic gap is excessive (or amp-turns in coil reduced due to shorting causing less magnetic field to be produced) thereby preventing sufficient clamping to occur causing the clutch to slip under high load.

Jim, .....my layman's interpretation.......thanks for the clarification!

:confused: :confused:

Someone better explain this to me

How can this statement from page3 be true?

"1) Disconnected power to A/C clutch and ran evap blower motor only -still heats up"

In my case it turned out to be a bad voltage regulator. I had the same melting fuse problem, but after a replacing the regulator and having the alternator rebuilt I have been running the AC constantly for over 2 weeks. Temps are up to 105http://www.pelicanparts.com/support/smileys/vuur.gif . No more melting fuses.

No more problems here after getting the system evacuated and recharged with the correct amount of R-12 at the dealership! Good thing.....humid 95 degrees yesterday!:)

"system recharged with R12 at the dealership" ?????

Mark...
Do you mean R134? The only place to get R12 is on the black market. Its as rare as King Tuts treasure!!!!! Since it was a dealership (Ellis, Hennessey) it had to be R134.

My 1973.5T has factory AC and years ago and I recall the AC fuse being as hot as hell, but never to a point of melting. Rather then putting in a new fuse block I installed an inline fuse and it has been working great since. I also replaced the older relay in the smugglers box with an updated unit. New barrier hoses, R134 and a fan switch and it runs cold in this Atlanta heat.

Glad to hear your worries are over, BUT if by chance you did find a way to get R12, please let us know your source!!

Bob
73.5T Sepia of course :cool:

Bob, definitely R-12. Shouldn't be too hard to find - just pricey. Wendt's shop did the overcharge (1 can R-12) and Jim Ellis did the evac and recharge with R-12....since I had too much in the system, I guess they came out ahead on product. Han's over at ******** has also got it.....he worked on my system last year and we went through two full charges before getting me back on the road "leak free".

BTW, Wendt charged $45.00 for 1 12 oz can. My father-in-law just gave me a couple of cans that he bought years ago at Kmart for $0.98 each! My how times have changed!

.....well shiver me timbers.......!!!!

I had Bob Sanderson a few years ago (before Peter took the shop) charge me up with R12 at great expense only to see the stuff run out the bottom low pressure hose that night. After replacing all the hoses with new barrier hose (Auto Cool in Smryna) and a new drier, I decided that R134 will have to suffice based on the R12 prices. So far it has been adequate for these hot temperatures, but Porsche AC systems have never been known to be really cold; just enough to keep the sweat to a minimum on sweltering days!

Glad to hear Mark that all is well on your AC now. Very informative thread.

Bob
73.5T Sepia :cool:

Is that what happened here ?

http://forums.pelicanparts.com/porsche-cars-sale/815756-fs-1982-911sc-5000-a.html
quote: It ran fine before the air conditioner condenser fan shorted out and caught fire.