Re: [Chrysler300] 1961 300 G A/C Fuse
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Re: [Chrysler300] 1961 300 G A/C Fuse

Hi Keith , Arc cannot increase a current , basic physics of it . The heat loss etc or arc is in series with motor power , so that is a voltage drop like the speed resistor which also then drops the current . Like speed resistor. So the current in the fuse drops with bad contacts , which also burn .  Motor slows . I do not question what you see but explanation of blown fuse is not a contact arc . AC induction motors draw more current on low volts might be where you get this ? But that is not here . 
As a guess , some intermittant problem in blower motor ( sticks mechanically at start ( bearings) or burned brushes ) or as Don said a short to ground . ( possibly erratic) The only way current can rise above normal is shorted coils or stalled motor or a short to Ground in a control switch or wire . Someone said clutch is on separate fuse , and my comment on clutch with this fuse might be wrong .. As Don said various control schemes over the years . Follow that 25-30A wire on FS diagram , look at every  place it goes . Put ammeter at motor see what it draws or if it jumps around .
Also hi currents in SAE 25 or 30 fuses can melt the fuse if clips are loose or corroded and bending them generally makes it worse . This usually takes 20 sec or more . I have had to wire in an in line fuse in every pre 1990 Dodge Truck heater fan I ever owned , end of the fuse clips melt loose . 300F heater switch too .
If it blows instantly = usually a short .. That can come and go . If motor jammed for 5 sec maybe it blows . When the contacts close . 
I think it is the repetitive motor start up process that you are eliminating by shorting the contact ? 
All this is from very far away but an ammeter ( or the one on your dash ) is next . Watch if it jumps way high at start . Once free and running the current will drop to normal . But if stays up a few seconds motor Is  having a hard time getting going . That would blow fuse randomly at start up of fan ? 
Hope this helps ! 

Sent from my iPhone

On Aug 14, 2016, at 1:55 AM, Keith Boonstra <kboonstra.zeegroup@xxxxxxxxx> wrote:

Hi John,

You come up with so much sharp stuff that I hesitate to get in a discussion with you - especially re EE, in which am a complete rube. But am I wrong to understand that the arc we refer to, and the resistance it creates, may somewhat lower the voltage, but increase the current (amps) to something crazy high? Enough certainly to blow a little 30A fuse? Everything I've read on it says that's how a CV arc works.

If this is not correct, how can we explain that I don't blow fuses if I simply shunt out the stat where I suspect the arcing is occurring?


On Sat, Aug 13, 2016 at 4:32 PM, John Grady <jkg@xxxxxxxxxxxxxxxxx> wrote:
Hi Keith  , arcing in the contacts cannot increase current , unless arc goes to ground . In fact it lowers current . ( acts as resistance ) . 
My bet is clutch coil or wire to it at compressor .. Which also cycles with stat. 

Sent from my iPhone

On Aug 13, 2016, at 3:09 PM, Keith Boonstra kboonstra@xxxxxxxxxxxx [Chrysler300] <> wrote:

Hi Val, 

I chased the exact same problem on my AC for years with no luck. Went through a couple dozen 30A fuses, and yet the AC rarely would keep working for more than a few minutes to a few hours. I replaced the fan-speed control and the fan motor, and checked all my amp draws up and down the line. Could never read more than 8 amps total and yet the fuses kept blowing. Really frustrating.

Then Jeff Carter suggested I look for the cause to be in the thermostatic control, and that I should gently abrade the points. I did that and it seemed to improve it, but a couple hours of use later I was back to blowing fuses again. I still thought he was probably right and that arcing in that stat unit was where the problem lay. I figured what might be happening was that the points were closing very slowly as the ether in the thermocouple expanded instead of snapping closed quickly - causing a brief (but too long) arcing across the points and a draw over 30 amps just long enough to blow a fuse.

To check the theory, I put in yet another fuse and hit the road again in 95 degree weather for 3 hours. Only this time I operated it by turning the stat to either full OFF or full COLD, and would only make that change with the AC system itself shut off for a moment. I did not set the stat at any partial call for cooling on this trip, and therefore the points did not cycle and could not arc as before. The bottom line is that it worked like a charm and that's the last fuse I've had to put in it. I know now that arcing in the stat is the only explanation.

What I cannot explain is why this would not have been a big problem with ACs back in the day - or what they did about it if is was. But my solution is that this week I am going to put in a relay after the stat. The stat will then only be a low amp signal to the relay, and the relay can do the heavy work. Jeff says there is a 20A breaker on the back of the wiper switch I can tap for the relay power source. 

So if my problem sounds like yours, give this test a try. Don't let the system cycle through the stat on a partial-cooling setting and see if the 25A fuse holds.

Good luck.

Keith Boonstra

On Sat, Aug 13, 2016 at 1:27 PM, Val Jeffers edward1108@xxxxxxxxx [Chrysler300] <> wrote:

  Hi All,

             Just had a new receiver/drier installed yesterday. The mechanic advised the air was nice and cold. Now the 25 amp fuse keeps blowing every time I snap in a new one ! Any thoughts as to the reason ?


                                                                                            Val Jeffers




Posted by: John Grady <jkg@xxxxxxxxxxxxxxxxx>

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