$800 later, still blowing AC fuses......

The 30 amp fuse gets hot and blows after several minutes. This shuts down the entire AC system. Had shop put in a new fuse holder and upgraded wiring. Had motor out, cleaned and checked for shorts, and wear, but all looked ok. Didn't check AMP draw. Why would this all of a sudden happen? Had the car for years in the Miami heat and AC has been fine. The option of buying the fan with the assembly for $7k, is out of the question. Should I send the motor out for a rebuild? I didn't go with the Jumbo fuse assembly, but just a new standard fuse holder. Is it the motor that has too much draw, or the circuit can't handle the amp draw, causing the heat? What about wiring the fan direct to the battery, with a fuse of course, and then triggering it with the old circuit via a relay. This would eliminate having to trace the problem circuit....Suggestions? Help, it's hot as hell in Miami!


David

 

Have the motor tested to see if the amperage draw is higher than what the specs say it should be. The older an electric motor gets the less efficient it gets and I have seen many that draw more and more amperage as time goes by.

 

Reversed Polarity would be my knee-jerk response - but, they went through the whole she-bang, huh... perhaps they paid attention to the electrical issues only... and maybe, just maybe, the refrigerant needs to be replaced. If that's low, then everything's working overtime, and hard.

 

+1!

 

I'm not good with electrical but Blowing fuses in my book means there is a shirt to ground. So something 12+ is touching something ground. That could be a wire with a bad cover touching another wire or ground via metal on the body of the car. So I would start by 1) do you have 12+ at the fusebox going to the A/C? should be yes 2) do you have 12+v power going to the A/C fuse block which should be yes. But then your fuse blows. So then your short/bad wire is from the fuse block to the last thing that is powered that evenually goes to ground. So start at this last thing and take it out of the loop until your fuse stops blowing. That will narrow the problem down to a wire or motor for example. sounds simple it isn't. It is a real pain to find.

 

I was thinking of a short too FBB, BUT..............the fuse doesnt blow straight away. Shorts in electrical systems blow fuses straight away normally.

So I dont think he has a short where a power wire is touching metal in the car.

I still think the a/c fan motor is building up too much resistance when he turns in on and after a few minutes the fuse pops as it exceeds the limit of the designated fuse.

If he puts an amp meter on the a/c fan motor, then he will know how many amps is drawing. If its only drawing a few more amps than what it should be, he could just put a bigger fuse in there. Not the right way to do it, but it CAN be done.

If its drawing A LOT more amps than what the fuse can handle, then you know there is a serious issue there with the fan motor.

But I could be wrong also. Electrical gremlins can be VERY hard to diagnose sometimes!

 

Mine was doing this, started blowing the hidden fuse shortly after I got the car, found the a/c relay looked shinny new so someone had problems before, then melted the fuse and holder so I replaced it and started tracing with an oms meter and found mine was a intermittent short inside the a/c clutch assembly. have you tried running just the fan without the a/c on? If the fan runs then check wire resistance and like mine a/c clutch, if even the fan on makes the fuse blow I would look at fan draw or wire resistance to the fuse block. I'm not much of a mechanic, but I have slept at a Holiday Inn before.

 

Good idea! Will check fan without A/C running to eliminate clutch or freon issues. I am thinking it is the fan though. If it is the motor, I plan to send the it to a rebuilder, since it is not available separately. Anybody know of a small electric motor shop?

DMG

 

I had Tim Stanford do a single bypass surgery on the fan motor for the A/C in my 348. They skipped the 3-4 relays and just put an inline self resetting breaker. I was told there have been a few fires from the fuse overheating. I was told it overheats due to the resistance of all the relays in the circuit.

BT

 

I forgot about Tim Stanford. He seems to be the guy to go to in the area, and would like to go around the stock old and weak wiring. Do you have a number for him? I get an answering machine and no call back from the number i find online.


David

 

That is not how electrical motors work. Electrical motors (DC especially) have a low impedance at zero RPMs so the inrush of current creates a big TQ moment and the thing starts rotating. As the motor rotates faster and faster, the inductance of the windings increase its impeadance and the current decreases to a lower sustainable level.

Now, if the motor does not get up to its normal speed, the current remains high and the fuse will blow. This can be caused by a host of things, bad bearings, dust/dirt on the fan baldes, leaves,... It can ALSO be caused by a short in the windings where the current never gets down low enough and the fuse eventually dies.

 

Yes Mitch. Totally agree. When I made that post, I was talking about when the motor is ****ed. Not when its working fine.

Yep. You're right there mate. Have seen it all I reckon.

 

I went over to:

http://www.srbymichael.com/manufacturers/spal/fans.shtml

and read:

http://www.srbymichael.com/manufacturers/spal/images/Dual%20Fan.pdf

In this article is a table of current draw versus air flow. I looked for the data on the correct fan to no avail. If we scale this dual 12" fan at its air flow to the 16 high powered fan at its air flow, we get a first order look at how much current should be going through our fans when they are turned on.

When we scale this dual fan into its 16" compatriots we see even with unpreturbed airflow the fan is operating near 29A with the rated fuse at 30A. It just takes a little restriction in the airflow for the current demands to exceed 30A and slowly blow the fuse. Additional sources of drag just exasterbate the problem...

We could have a perfectly fine fuse, perfect wiring, fan in brand new condition, and everything on the car in working order, and a leaf of paper in font of the radiator could blow the fuse. So, let us considier that this 16" fan is more efficient (for some reason) than the dual 12" fan we have data on. I can see this taking the current down to 27A maybe even 25A. But still a good (bad actually) restriction in the air flow could raise the current draw and blow a perfectly fine fuse.

Assuming the theory is reasonable.....

And remember the startup currents are even higher....

restrict Air Flow Amps Power
0.0 2360 29.2 379.6
0.1 2228 29.9 388.3
0.2 2142 31.4 407.7
0.3 2005 31.6 410.6
0.4 1868 32.3 420.3
0.5 1717 32.9 428.0
0.6 1541 33.4 433.9
0.7 1388 34.2 444.5
0.8 1240 34.1 443.5
0.9 1093 35.2 457.1
1.0 947 36.2 470.7