30A fuse for fans blowing after replacing fuse box and alternator cable

[OlyTDI]

07/03 Update:

Well I’m not sure how to proceed from here. To recap:

Engine off, cabin blower on, AC on = both fans work.

Engine on, cabin blower on, AC on = S180 (30A) fuse blows immediately upon switching on AC.

Engine on, cabin blower on, AC on, compressor coil harness disconnected = S180 (30A) fuse blows immediately upon switching on AC.

The (red) wire that leaves that S180 (30A) fuse slot location goes directly to the Coolant Fan Control (FC) Thermal Switch (F18 if looking at Bentley). That switch is mounted in the radiator and has three wires that plug-in to it. I tested for continuity between where the red wire exits the S180 fuse and where it goes into the Coolant Fan Control (FC) Thermal Switch plug connector and there was continuity without resistance or any anomalies when vigorously moving the harness around.

I’m not sure how to further test as I don’t know where that red wire circuit from S180 goes from there. Can a coolant thermal switch short? If yes, why only with the engine running?

What seems like the most important clue is that the fuse blows only when the engine IS running (blower/AC on) but not when the engine is NOT running (blower/AC on).

My battery, battery box, ductwork, is all pulled out and am reluctant to put it all back together without fixing this first. Hoping for a breakthrough! Thanks all for helping and please keep forwarding your ideas and suggestions! Not sure where to go from here.

 

[OlyTDI]

Even if they were bad, they would blow the 5A fuse by the driver's door, not the 30A under the hood. AC switch is just an input to the FCM which does the high current switching.

Thanks for that! How would one go about testing/troubleshooting the FCM? What is it about the engine running that would cause a short in that S180 fused line?

 

[Fahrvegnugen]

What did you do with old big alternator wire? I replaced mine and have been driving around with the old one because it’s attached in a loom. I just bent it out of the way but when driving it apparently touches the alternator causing arcing which blows my radiator fans 5 amp fuse.

 

[OlyTDI]

What did you do with old big alternator wire? I replaced mine and have been driving around with the old one because it’s attached in a loom. I just bent it out of the way but when driving it apparently touches the alternator causing arcing which blows my radiator fans 5 amp fuse.

Hmmm. I disconnected it from both ends (the alternator side and the fuse block side) and just zip tied off the two loose ends so that they're out of the way of anything. I did that for the same reason you did -- it's in a loom -- but mine is not connected to anything on either end. Just the new, larger gauge cable now goes from alternator to fuse box.

 

[Mike_04GolfTDI]

Does the same circuit with the blowing fuse also supply power to the A/C compressor clutch?

I recently had to replace the PTO clutch on a tractor because it would blow as soon as it was engaged. It’s similar to an A/C compressor clutch, just bigger.

Use an ohmmeter to check the resistance of the compressor clutch and slowly turn the clutch around by hand. You may find points in the rotation where the resistance goes very low, or to zero. You could do the same with the fans. It may not be consistent, which is why you have to rotate them.

Over time, heat and vibration wears out the coating on the wire windings of the electromagnets in the motors or clutch, and then you get lots of points where they touch and short out.

In the case of that PTO clutch, resistance should have been around 3 or 4 ohms, but instead it varied between 1 and 0 ohms as the clutch was rotated. Using Ohm’s Law, it was clear that too much current would flow through such a low resistance.

I bet you have something similar going on.

 

[J_dude]

Does the same circuit with the blowing fuse also supply power to the A/C compressor clutch?

I recently had to replace the PTO clutch on a tractor because it would blow as soon as it was engaged. It’s similar to an A/C compressor clutch, just bigger.

Use an ohmmeter to check the resistance of the compressor clutch and slowly turn the clutch around by hand. You may find points in the rotation where the resistance goes very low, or to zero. You could do the same with the fans. It may not be consistent, which is why you have to rotate them.

Over time, heat and vibration wears out the coating on the wire windings of the electromagnets in the motors or clutch, and then you get lots of points where they touch and short out.

In the case of that PTO clutch, resistance should have been around 3 or 4 ohms, but instead it varied between 1 and 0 ohms as the clutch was rotated. Using Ohm’s Law, it was clear that too much current would flow through such a low resistance.

I bet you have something similar going on.

I believe he said it blows the fuse even with the clutch coil disconnected, so that’s probably not the case.

 

[OlyTDI]

Yes, thanks Mike, I appreciate your thoughts, but J dude is correct about the fuse blowing with that coil disconnected. It's an only-with-engine-running-problem, which would include activation of the clutch but that part of the circuit has been excluded.

 

[Genesis]

Only the coil itself has been excluded (the short piece of wire from the connector to it.) There IS a dead short somewhere in the wiring fed by that 30A fuse.

 

[Fahrvegnugen]

Maybe try with car off again only this time wiggle the green 30 amp fuse to see if that has a good connection?

 

[OlyTDI]

Only the coil itself has been excluded (the short piece of wire from the connector to it.) There IS a dead short somewhere in the wiring fed by that 30A fuse.

Yes! The section that remains in question is from that connector back through the Fan Control Module and Coolant Fan Control Thermal Switch. From the thermal switch to the fuse box is good best that I can tell. I don't know how to test the FCM. The problem with testing for a short is that you need the battery and I've pulled all of that out to access the wiring and harnesses. I'll be very surprised if it's a shorted wire in one of the looms as everything looks so good...including the connectors. When I pulled the Fan Control Module out, all of the pins and sockets gleam. No corrosion anywhere that I can see.

 

[OlyTDI]

Maybe try with car off again only this time wiggle the green 30 amp fuse to see if that has a good connection?

Yes, I'll try that once I get everything back together. I don't want to put it all back together to then have to tear it all apart again! That fuse box is brand new from IDParts.

 

[OlyTDI]

I just stumbled on an old thread from 2011 whereby someone was having a similar problem with the S180 fuse blowing after putting in VW replacement fans and having fixed their AC system:

S180 AC / Fans fuse keeps burning out

I thought I was done with AC problems when I installed a pair of new WV rad fans. Originaly I replaced the fans with aftermarket ones from rock auto but they were wired for single speed only internaly and would pop the S180 instantly. Up to that point the car (02 Beetle) had the original 30A...

forums.tdiclub.com

In that thread, one of the posters indicated that they too, had this problem had to get 30 amp fuses from the VW dealer because they were slow burn as opposed to the the generic off the shelf 30 amp fuses from auto parts store which are fast burn.

The OP of that thread posted this in response:

"Finally got to the Dealer and picked up some S180 type 30 amp fuses. They look a bit different than the originals with clear plastic over the top green body. I put a new one in and bingo, the AC has been running with no burn out. r72, you got it right that in no way can you run a generic 30amp fuse in that S180 spot."

I would love to hear any responses/insight to this!

I didn't know that there was perhaps, a different fuse type for that slot.

 

[Zak99b5]

Definitely worth trying a slow-blow fuse in there.

You can use jumper cables between the battery (on the ground out of the car) and the battery terminals so you can test with access to the FCM and the wiring you need to look at.

 

[OlyTDI]

07/04 Update:

With regard to finding the dead short in my system that keeps blowing fuse s180, I chose to work from that fuse at the battery top fuse box downstream given that both of my fans seem to work fine as evidenced by the key on/engine off (blower and AC button on) test. If you recall, the fuse blows when this test is done with the engine running -- no fuse blowing behavior with the engine off and fans running. So I started dissecting and isolating the hot wire from from the fuse box down, one leg at a time. What a mess.

Well, I ended up with short to ground all along the pathway to the fans which disappeared when I unplugged the fans. So working backward from the fans, I found that both fan harnesses seem to be shorted across all of their pins -- each fan independently -- which I find confusing.

Each fan has its own 3 pin plug -- presumably ground, low, and hi. On both fans, the 3 connector pins in their plugs show continuity between each of them, that is, testing across pins 1 and 2; 2 and 3; and 1 and 3 shows continuity -- all the wires in the fan are somehow connected together in the fan. I found this on both fans.

Additionally, with no fans plugged-in to their harnesses, I get no short-to-ground in the wire path from where the fans plug-in all the way upstream. BUT...if I plug-in one fan, I get the short to ground upstream, and if I then unplug that fan and plug-in the other fan , I also get a short to ground upstream from the plug for that other fan as well! These are two completely isolated fans when unplugged, Both shorting-to-ground independently and each then (presumably) causing the s180 fuse to blow due to short-to-ground. But, again, when both are unplugged there is no short-to-ground upstream anywhere in that hot wire path.

Can it really be that both fans are shorted?

Can it really be that they did so at the same time?

How can the fans be running if they're shorted?

What am I missing here? What am I not understanding?

Grateful for any insight and help, and thankful to those who've chimed-in. This is starting to drive me crazy!

Happy 4th!

 

[Vince Waldon]

There's a "short".... 0 ohms... and then there's "low resistance"... a few ohms.... which is what you'd see measuring directly across the fan leads... since they are fairly high current motors.

Which are you seeing, and how are you measuring?

 

[OlyTDI]

Thanks for weighing-in, Vince - appreciated.

Well, I've been using a multi-meter on ohms but have been using it with the beep function so that I can crawl around without looking at the device. So when I touch the leads together, I get a steady beep indicating continuity. With regard to short-to-ground testing, I've been testing the wire by placing one lead onto that wire and the other to ground to see if that leg of the hot wire "beeps" when I complete to ground. With regard to the fan connection pins, I was testing across pins. So any two pins I connected to had continuity.

Am I messing up my testing protocol? I don't have a 12v test light and only two fuses left so I've been looking for what is/isn't connected and what positive wires are/are not completing a circuit to ground.

I do find this confusing and challenging, though and clearly don't know what I'm doing!

 

[Vince Waldon]

Yup, the beep test will not be able to distinguish between low ohms and zero ohms, so it will beep when across the fan leads... and from the hot fan lead to ground.. normally. Every connection tested this way will beep, unfortunately.

If you put your multi-meter on ohms (no beep) what does it read when you simply short the two probes together?

 

[OlyTDI]

I get 0.1 to 0.3 ohms mostly settling on 0.2 to 0.3 on both auto mode and audible mode when touching those leads together.

 

[Vince Waldon]

OK great, it can read small values of ohms. I'd suggest leaving it in auto going forward, as the beep can be confusing.

If you now measure across the large fan's terminals you should be able to accurately measure the fan's windings... which should be more than 0.3 ohms.. maybe even an ohm or two. If you measure 0.3 ohms you have an actual short...same reading as shorted leads.

 

[dieseldonato]

Make sure you're not touching either of the probes. You'll throw your readings off.

 

[OlyTDI]

OK great, it can read small values of ohms. I'd suggest leaving it in auto going forward, as the beep can be confusing.

If you now measure across the large fan's terminals you should be able to accurately measure the fan's windings... which should be more than 0.3 ohms.. maybe even an ohm or two. If you measure 0.3 ohms you have an actual short...same reading as shorted leads.

Thanks Vince, I will do that first thing in the morning and report. But with regard to testing short to ground, any reading reading of continuity between the hot wire and the engine or frame ground is indeed a short, correct? Otherwise would read OL?

 

[OlyTDI]

Make sure you're not touching either of the probes. You'll throw your readings off.

Yes! Thank you!

 

[Vince Waldon]

Thanks Vince, I will do that first thing in the morning and report. But with regard to testing short to ground, any reading reading of continuity between the hot wire and the engine or frame ground is indeed a short, correct? Otherwise would read OL?

If the hot wire has any loads attached to it (like the fans, or relays, or bulbs) your ohm-meter will read the resistance of the load thru to ground...and if there are several bigger loads in parallel on that same hot wire the combined parallel resistance will look pretty small.

The hot wire in question is protected with a 30A fuse... so the loads are in fact going to be big and the combined resistance may look darn close to a dead short. Ohms Law R = V / I = 14.5 volts / 30 amps or about half an ohm or so combined (just as an outside ballpark... of course the circuit is not running at 30 amps normally). However, you get the idea... if it's running at 15 amps normally your ohm-meter will read around an ohm, and unless it's a very expensive meter it may or may not be able to tell the difference between that and shorted leads.

Have you been able to track down a wiring diagram of what exactly is powered by that 30A fuse? Ideally this would tell you what loads to disconnect, before checking each load (and then the circuit itself) for a dead short to ground.

 

[Zak99b5]

If the fuse runs the fans fine without the engine running, I think you should start looking elsewhere. Where that is, I do not know.

But it’s starting or running the engine that seems to blow the fuse. This doesn’t really make sense to me, but you’re lucky in that this is a hard fault. I guess first thing to figure out is if it’s blowing because of the starter or is it just when the motor is running. You could start the car without the fuse, then put one in, A/C off. If It doesn’t blow, then turn on A/C and see what happens. If it doesn’t blow, then shut off A/C and the car, and restart.

 

[dieseldonato]

Does this 30 amp fuse power the blower motor as well?

 

[OlyTDI]

If the fuse runs the fans fine without the engine running, I think you should start looking elsewhere. Where that is, I do not know.

But it’s starting or running the engine that seems to blow the fuse. This doesn’t really make sense to me, but you’re lucky in that this is a hard fault. I guess first thing to figure out is if it’s blowing because of the starter or is it just when the motor is running. You could start the car without the fuse, then put one in, A/C off. If It doesn’t blow, then turn on A/C and see what happens. If it doesn’t blow, then shut off A/C and the car, and restart.

Well, not exactly. It's when the AC is activated (which requires the engine running, the blower on, and the AC switch on) that the fuse blow. The fuse does not blow on engine start or with engine running with either the blower on and AC off or with AC on and blower off (presumably because these two functions are required for AC to actually run the compressor).

But...the fuse certainly would blow if the blower and AC switch both were already on upon startup.

So to recap: if I start the engine with the either the blower motor off, or the AC switch off, (or both off), the fuse does not blow. Furthermore, the engine runs fine and the fuse doesn't blow while running with the blower on, OR the AC switch on, but not both. HOWEVER, once running, if I turn the blower fan on AND then activate the AC switch, the fuse blows. Or, once running, if I turn on the AC switch, AND then turn on the blower, the fuse blows -- doesn't matter which is done first or last. The common denominator seems to be that the fuse blows on actual physical activation of the AC.

With the engine off, I can activate the AC switch and turn the cabin blower fan on and the fuse will not blow and both fans operate as normally indicated in the "check your radiator/condenser fan operation NOW!" thread. So I would think that the fans are fine. They wouldn't run if they were shorted. I was testing that wrong I think.

 

[OlyTDI]

Does this 30 amp fuse power the blower motor as well?

I'll have to look in Bentley but I don't think so...

 

[OlyTDI]

If the hot wire has any loads attached to it (like the fans, or relays, or bulbs) your ohm-meter will read the resistance of the load thru to ground...and if there are several bigger loads in parallel on that same hot wire the combined parallel resistance will look pretty small.

The hot wire in question is protected with a 30A fuse... so the loads are in fact going to be big and the combined resistance may look darn close to a dead short. Ohms Law R = V / I = 14.5 volts / 30 amps or about half an ohm or so combined (just as an outside ballpark... of course the circuit is not running at 30 amps normally). However, you get the idea... if it's running at 15 amps normally your ohm-meter will read around an ohm, and unless it's a very expensive meter it may or may not be able to tell the difference between that and shorted leads.

Have you been able to track down a wiring diagram of what exactly is powered by that 30A fuse? Ideally this would tell you what loads to disconnect, before checking each load (and then the circuit itself) for a dead short to ground.

So that 30 amp wire goes to the radiator thermal switch then on to the fan control module and finally to the fans.

With the whole system normally connected, I have continuity (my meter beeps) between the output side of the 30 amp fuse holder and ground. Am I correct that this indicates a short to ground?

I may be testing that hot wire completely wrong.

 

[OlyTDI]

Does anyone have any opinions on the idea in post #42 regarding a slow blow fuse? Could this simply be a startup surge issue that isn't a threat to the circuitry ongoing but exceeds 30amps for a brief moment on startup of the AC? I know that if it's blowing fuses, there is a problem but that post is intriguing as it appears to discuss this very problem.

I'd really be interested in knowing if the factory fuse in that box was a slow blow...

 

[dieseldonato]

A regular 30 amp fuse is rated something like 150% it's rated capacity before it will bow out..

How many amps does it REALLY take to blow a fuse

too damn many! today after work i had a few beers and decided to smoke test some fuses. using my high current amp meter and labscope i recorded exactly how many amps, and how long it takes to pop a few different fuses. basically i just took jumper leads straight from a 12v battery to the...

www.southbayriders.com

This guy did some testing on different fuses. Decent read.
Edit, upon some further googling I found this as well, suggesting a slow burn fuse is used in many European vehicles, vs northamerican cars use the fast acting type... so possibly you're on to something?

Moss Tech 101: Don’t Blow a Fuse Over Your Fuses

2017 Issue 2, Main

mossmotoring.com