New A/C Problem - melting fuses???

[rick-l]

Are my fuses longer than yours?

[mthomas58]

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.

[mthomas58]

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?

[scottb]

Originally posted by rick-l
Are my fuses longer than yours?

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.

[rick-l]

Originally posted by mthomas58
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?

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.

[mthomas58]

Originally posted by rick-l
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.

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)

[rick-l]

Originally posted by mthomas58
Did a voltage measurement following your instructions and got a measurement of 9.3v (measured several times and got 9.3 each time)

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)

[mthomas58]

I'll recheck tomorrow

Thanks for the guidance!

[rick-l]

Originally posted by mthomas58
I'll recheck tomorrow

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.

[mthomas58]

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.

Thanks!!!!!!!!!

[derkpitt]

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?

[mthomas58]

Originally posted by derkpitt
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)

[derkpitt]

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

[mthomas58]

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

Thanks!

[rick-l]

Originally posted by mthomas58
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.

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.

[rick-l]

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

[rick-l]

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?

[mthomas58]

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.

[carnutzzz]

We could be golfing instead.

[mthomas58]

How could I concentrate???????????