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

[dieseldonato]

Found this interesting as well

Fusing Guide | 12 Volt Planet

Read our guide to fuses. Why, where and when to use them and how to select the correct rating. Fuses are critical in any electrical system and are used to protect a circuit and its components from excessive current

www.12voltplanet.co.uk

Which just kinda muddues the water a bit more.

 

[OlyTDI]

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

Thanks for that! Interesting. The only confusing part of that slow blow article for me was saying that if you're blowing a fast burn fuse and want to go with a slow burn, that you need to double the amperage of the slow burn fuse....(actually it said halve the amperage of the slow burn when switching to a fast burn...).

 

[dieseldonato]

Thanks for that! Interesting. The only confusing part of that slow blow article for me was saying that if you're blowing a fast burn fuse and want to go with a slow burn, that you need to double the amperage of the slow burn fuse....(actually it said halve the amperage of the slow burn when switching to a fast burn...).

Yes, I found it all a bit confusing as well. I found a pdf from little fuse that said every fuse has a certain amount of over current delay built I to it, it's just a function of how many amps over the rating is how fast it bows. Guess what I took from it was you need to know what the amp draw actually is. Motors have a pretty nasty in rush to get them started, which I would think would need a fuse that would handle that, even if its just a few seconds. (At least from what I've gathered from the above articles.)

how many amps does it take to blow a 30 amp automotive fuse - Google Search

This is the search I used, scroll down a bit and the little fuse pdf is called fuseology

 

[Vince Waldon]

Have had "fast blow" fuses in all cars since my 69 Beetle with no issues related to the fuse itself... pretty sure the problem here is not fast vs slow fuses.

Put another way, if a specific circuit demanded a specific non-standard kind of fuse pretty sure the manufacturer would make a big deal about it in the owners manual, on the fuse box, etc... considering all the service calls they'd get. And... someone on here would have heard of it methinks.

I'd say: find the exact wiring diagram of what that fuse supplies, remove every single load, and test the naked circuit first. Then start adding loads back one at a time... this is a weird one as it's impacted by the ignition circuit(s) as well. Somehow?!

 

[Genesis]

Its been a good long while since I had my MkIV apart far enough to see the wiring travel on this, but what I remember is:

1. The fan fuse goes to the thermo-switch and the output of that is directly (no path through the FCM) runs the fans on high. That is, if you jump the switch even with the car turned OFF the fans will run on high.

2. That circuit ALSO feeds the FCM's primary power. The ECU has an input to the FCM to "gate" the clutch because it can load-shed it and also will lock it out if there is no pressure or the temperature is too low. The FCM can drive the fans on EITHER low or high as high pressure on the system will cause the fans to run on HIGH even though the thermoswitch is not closed.

You have jumped the thermoswitch (I assume) with the car off and the fans ran on high without blowing the fuse. You have also checked the clutch coil itself and it is not shorted, and if the car is NOT running the fuse does not blow. This implies that it is the gating to the clutch by the ECU into the FCM that results in wherever the short is being energized.

Since it is only blowing the fuse when the vehicle is running the short has to be in a wiring path that can be completed no other way. I suspect the FCM itself is responsible -- it might not be, but that is the most-likely place for the short.

 

[BobnOH]

Components, faulty or damaged, can short. But I would start with inspecting all the wires and connects involved.

 

[Genesis]

Yep. MOST of the time when I have seen an electrical fault on a car it is the wiring, not the modules. ESPECIALLY on anything over 10 years old and the odds go up a LOT if the vehicle is parked outside regularly (critters eat wire insulation; go figure.)

 

[OlyTDI]

Its been a good long while since I had my MkIV apart far enough to see the wiring travel on this, but what I remember is:

1. The fan fuse goes to the thermo-switch and the output of that is directly (no path through the FCM) runs the fans on high. That is, if you jump the switch even with the car turned OFF the fans will run on high.

2. That circuit ALSO feeds the FCM's primary power. The ECU has an input to the FCM to "gate" the clutch because it can load-shed it and also will lock it out if there is no pressure or the temperature is too low. The FCM can drive the fans on EITHER low or high as high pressure on the system will cause the fans to run on HIGH even though the thermoswitch is not closed.

You have jumped the thermoswitch (I assume) with the car off and the fans ran on high without blowing the fuse. You have also checked the clutch coil itself and it is not shorted, and if the car is NOT running the fuse does not blow. This implies that it is the gating to the clutch by the ECU into the FCM that results in wherever the short is being energized.

Since it is only blowing the fuse when the vehicle is running the short has to be in a wiring path that can be completed no other way. I suspect the FCM itself is responsible -- it might not be, but that is the most-likely place for the short.

Thank you a million for that insight, Genesis. I hadn't done that specifically but will tomorrow when I resume tackling this. I wish there was a way to test the FCM as they're expensive but getting it narrowed down might result in the risk of buying a new on a prudent one. I hadn't been testing things with the fused circuit because I ran out of fuses! I have more coming tomorrow!

 

[OlyTDI]

Components, faulty or damaged, can short. But I would start with inspecting all the wires and connects involved.

Yes, that's what I've been focused on. All wires "look" great and there is little a wire in that pathway can short to. Everything is plastic!

 

[OlyTDI]

Yep. MOST of the time when I have seen an electrical fault on a car it is the wiring, not the modules. ESPECIALLY on anything over 10 years old and the odds go up a LOT if the vehicle is parked outside regularly (critters eat wire insulation; go figure.)

I agree but I have see a LOT of posts on VWVortex and other sites of VWs (not necessarily TDIs or even MK 4 but some included) discussing FCMs going south and needing replacement. This, however, is usually associated with overheating and strange fan behavior. I cannot remember seeing a post about fuses blowing being attributed to the FCM.

 

[mittzlepick]

theres alot going on with the fan control module. ac clutch wont run with the blower off, fans run via ecm readings i presume both can be triggered for high and low speed depending on engine temp. ac runs them on high.
have you tried running ac with the ac clutch unplugged engine running?
i have some fcms in my hoard you could try for yard sale price?
initial draw is popping the fuse silly idea but try out with a highway speed as they may have a bit of a jumpstart spinning from the wind?
always blame the chinese quality.
vag com can test ac clutch maybe run that test with a meter on the battery to see voltage drop.
hows the battery? alternator?
try voltage at battery with all consumers at high see if alternator if acting freaky? all the other grounds good?

 

[OlyTDI]

theres alot going on with the fan control module. ac clutch wont run with the blower off, fans run via ecm readings i presume both can be triggered for high and low speed depending on engine temp. ac runs them on high.
have you tried running ac with the ac clutch unplugged engine running?

Yes, I did unplug the clutch coil and it still blew the fuse with engine running and then activating the blower/AC together.

i have some fcms in my hoard you could try for yard sale price?

Let's do that -- nothting to lose. I'd even send it back if you want if it turns out it's not the FCM.

initial draw is popping the fuse silly idea but try out with a highway speed as they may have a bit of a jumpstart spinning from the wind?
always blame the chinese quality.
vag com can test ac clutch maybe run that test with a meter on the battery to see voltage drop.
hows the battery? alternator?

I have no evidence that either are faulty -- battery is in good shape but I'll look at the alternator -- what anomaly would I be looking for?

try voltage at battery with all consumers at high see if alternator if acting freaky? all the other grounds good?

I'll have to do that minus the fuse-blowing circuit connected of course...

 

[mittzlepick]

as far as the batery and alternator just grabbing at straws, pm me with adress we can go from there

 

[OlyTDI]

as far as the batery and alternator just grabbing at straws, pm me with adress we can go from there

 

[BobnOH]

also grasping. Have you tried running it with both fans unplugged temporarily? Anything to isolate the issue.............

 

[OlyTDI]

I was able to jump the coolant temp switch and run power to the fans as Genesis suggested above. Both fans ran (presumably on high) with no problem. I then tried to start the engine (the battery is out so was using jumper cables) and there was not enough juice to start the engine.

The battery is a VW battery purchased last year and I've never had a problem with it so I wasn't thinking of that as a possible cause. It measured around 12.2 volts so I put it on my smart charger (4.3amp) and it's been charging for about 5 hours. Once it's charged, I will re-install and do an alternator and battery test with voltage drop measurement of the pos and ground. I wasn't focused at all on the battery or alternator but perhaps one of those is contributing a voltage drop significant enough to blow that s180 fuse(?). I don't ever remember replacing the alternator but I have replaced the alternator pulley in the past. As I mentioned previously, I also recently put in a larger ga. alternator cable to the fuse top box but don't know how that might play into things unless the alternator is spiking or something...

 

[tgray]

Did you try running the a/c with just one fan? Some of these problems are tough when you keep blowing a fuse just to try something. What I do is buy a 30 amp auto rest circuit breaker and connect that where the fuse plugs in. The auto stores should carry them. You have a short in the larger secondary wires or a bad fan is pulling too much. If you can run on one fan see if something is heating up. Both fans should turn on without the engine running if you turn the key on and engine off with the A/c button pushed. Does this work? and then does it blow the fuse when you start the engine?

 

[OlyTDI]

I'll try one fan later this morning.

Yep, everything works fine with the engine not running.

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.

Both fans work when jumped.

This morning I'm checking:

the alternator and battery,
going to try to run the engine and AC without the FCM connected and see if what that does,

 

[OlyTDI]

Well I am utterly baffled. After doing all sorts of continued diagnostics, the AC is this morning working again without blowing fuse s180.

I checked the alternator today and it was putting out 14.5 to the battery. No voltage drop across that cable or on the ground side of the battery to frame.

The battery seems fine. It was depleted and took about 8 hours to charge it last night. Not sure why it was so depleted. And after monkeying around for the last 4 hours, it's on the charger again. I'm not sure what is draining the battery but it's concerning. I haven't had a depleted battery issue until I started troubleshooting this problem.

A really bizarre development is that all along, both of my fans were running on low with engine off as in the standard "check your condenser fans NOW" thread but then weren't this morning. Now when I do that test, both run but I think they are on high, not low. Also, when I did DanG's test for low speed fan operation by jumping the plug to the coolant switch, I got both fans running but again I think on high, not low.

I'm going to button it up and see what happens over the next few days. A member is sending me a salvaged FCM so I'll have that to substitute if things continue to go haywire.

I have no idea what's going on here.

Addendum: Took the vehicle out for a 1/2 hr drive. Worked the AC off/on. No hiccups. Air at the vents 39F on blower setting 1, 42F on 2, 44F on 3, and 46F on High. Ambient was 80F. AC is working great. No fuses blew.

I feel like I got lost in the wilderness for a week, came out hungry, thirsty, and all scratched up only to find myself at the beginning of the trailhead again.

 

[J_dude]

You’ll be able to tell if they’re running on high, they spin darn fast and make fan noise, on low they just sorta spin.

 

[OlyTDI]

You’ll be able to tell if they’re running on high, they spin darn fast and make fan noise, on low they just sorta spin.

Yes, that's what it was, fast and fan noisy. Not low-key or pokey.

 

[Genesis]

@OlyTDI if the fans run on HIGH with the key on and button in either the thermoswitch is shorted (it is outside the FCM circuit and in fact is active even with the key OFF), about the only possible causes are the FCM being boned or the fan(s) are internally shorted across the dropping resistor. The odds of BOTH fans suffering that fault at once approximate zero. If you can put power manually to the fans from the unplugged connector and get both low and high speed then it obviously is not the fans themselves.

 

[mrGutWrench]

From a cold start, turn the fans on with the A/c and feel the wires at the connection points. If it is a bad connection it will get warm right away. (SNIP)

I had a leak at the cabin filter cover a few years ago. At the same time, I noticed that the condensate drain was not dropping any water when the A/C was on (central NC is humid in the summer) so I investigated and found something like a small acorn stuck in the drain /go figger!!/ -- no wonder my wife had wet feet. A few months ago, my H/AC blower started running slow and making noise, so I bought a new one and put it in. All's OK now but connector terminals for the blower plug behind the glove box were really skanky. I cleaned them with terminal cleaner (the old ones were corroded, too, but of course they came out with the blower). and gave them a light scraping with a tiny file. That seemed to work well.
My Jetta Wagon is 20 years old, too. The terminals are probably something to check if you need to be in there anyway.

 

[mrGutWrench]

Note on Fan Control Module --
About this time last year, I "lost" all A/C. I'd switch it on normally and everything would seem to work; the blower would come on at all four speeds as usual, the A/C "button" light illuminated, and the temp control worked; the only thing was that the temp would vary from warm-outside-air to heater-air -- NO cooling at all. What was not normal was that there was no "click" of the A/C compressor clutch engaging and no little blip in the idle as usual when the clutch engages. When I lifted the hood and looked, it was apparent that the clutch was not engaging and there was no evidence that the clutch was seeing any power in any condition of engine running, switch position, etc.

I went through everything. There was power where power should be and no power where there shouldn't be. The radiator fans worked normally in the "switch-on" test but they were both noisy (gosh, they were something like 10 years/250K miles old) so I switched them out - no difference, of course, but it gave me confidence that they were not likely to be part of the problem .

I rigged up a little jumper to the compressor clutch connection. With the engine idling and A/C switched on, there would only be hot air coming from the vents but when I attached the jumper at the battery, the compressor clutch would "Click" and in just a few seconds, there would be cold air at the vents.

At that point, I couldn't think of any issue that would be causing trouble except the Fan Control Module function. I had a friend move the A/C control switches from "off" to "on", change fan speeds, turn fan to "off" position, etc. -- there was never any power at the wire to the compressor clutch from the FCM under any circumstance. I decided to change out the FCM.
(Note: Dealer quoted "Genuine VW Replacement Part; $741.00"; our Trusted TDI Parts Online source offered "Asian 'pattern' part", $90.00; "OEM" supplier part "$189,00. Since I wanted this issue "FIXED - NOW", I went for the online OEM part although it was double the cost of the "pattern" part.)
The change out was simple and easy. There was no apparent problem with the old FCM (original to the car from new) - no overheating signs, no burned connectors, nothing; it just didn't seem to provide power to the A/C clutch. Once the new part was in, my A/C worked as usual and continues to do so since.

At no time during my A/C problems did any fuse blow nor was there any sign of malfunction or damage to any other part. Since your symptoms are very different from mine, I *suspect* that if your issue involves the FCM, you have a different problem than mine, but I wanted to let you know my experience with a bad FCM and how the problems and fixes presented at that time.

 

[OlyTDI]

Well, I'm right back where I started! After driving around for a while yesterday with the AC working fine, I took a 20 minute trip into town this morning and while driving, the s180 fuse blew as soon as I activated the AC button (with the cabin blower on low). UUUGGGGHHHHH!

 

[OlyTDI]

@OlyTDI if the fans run on HIGH with the key on and button in either the thermoswitch is shorted (it is outside the FCM circuit and in fact is active even with the key OFF), about the only possible causes are the FCM being boned or the fan(s) are internally shorted across the dropping resistor. The odds of BOTH fans suffering that fault at once approximate zero. If you can put power manually to the fans from the unplugged connector and get both low and high speed then it obviously is not the fans themselves.

Roger that, Genesis -- thanks for that bit of clarity! As soon as I can muster the motivation to start tackling this all over again, I will do that test first.

 

[OlyTDI]

Note on Fan Control Module --
About this time last year, I "lost" all A/C. I'd switch it on normally and everything would seem to work; the blower would come on at all four speeds as usual, the A/C "button" light illuminated, and the temp control worked; the only thing was that the temp would vary from warm-outside-air to heater-air -- NO cooling at all. What was not normal was that there was no "click" of the A/C compressor clutch engaging and no little blip in the idle as usual when the clutch engages. When I lifted the hood and looked, it was apparent that the clutch was not engaging and there was no evidence that the clutch was seeing any power in any condition of engine running, switch position, etc.

I went through everything. There was power where power should be and no power where there shouldn't be. The radiator fans worked normally in the "switch-on" test but they were both noisy (gosh, they were something like 10 years/250K miles old) so I switched them out - no difference, of course, but it gave me confidence that they were not likely to be part of the problem .

I rigged up a little jumper to the compressor clutch connection. With the engine idling and A/C switched on, there would only be hot air coming from the vents but when I attached the jumper at the battery, the compressor clutch would "Click" and in just a few seconds, there would be cold air at the vents.

At that point, I couldn't think of any issue that would be causing trouble except the Fan Control Module function. I had a friend move the A/C control switches from "off" to "on", change fan speeds, turn fan to "off" position, etc. -- there was never any power at the wire to the compressor clutch from the FCM under any circumstance. I decided to change out the FCM.
(Note: Dealer quoted "Genuine VW Replacement Part; $741.00"; our Trusted TDI Parts Online source offered "Asian 'pattern' part", $90.00; "OEM" supplier part "$189,00. Since I wanted this issue "FIXED - NOW", I went for the online OEM part although it was double the cost of the "pattern" part.)
The change out was simple and easy. There was no apparent problem with the old FCM (original to the car from new) - no overheating signs, no burned connectors, nothing; it just didn't seem to provide power to the A/C clutch. Once the new part was in, my A/C worked as usual and continues to do so since.

At no time during my A/C problems did any fuse blow nor was there any sign of malfunction or damage to any other part. Since your symptoms are very different from mine, I *suspect* that if your issue involves the FCM, you have a different problem than mine, but I wanted to let you know my experience with a bad FCM and how the problems and fixes presented at that time.

Yes, different from the problem I'm having but your story adds value to the idea that the FCM can indeed go south and wreak havoc.

 

[tgray]

In all my thousands of miles in these cars on many cars it seems I have had these FCMs are at the top of my replacement list. Even if you don't find it is not the problem it is good to have a good working spare.

 

[OlyTDI]

I'm looking forward to putting in a replacement as at this point, it's the least laborious aspect of continuing to try and figure this out!

 

[OlyTDI]

Update before I end the thread: It was indeed the FCM. Member Mittzlepick sent me a salvaged one and it solved the problem. I don't know how I could have figured that out alternatively to replacing it. But it's an FYI to anyone having a similar issue!

Thanks very much to everyone who assisted!