Checking my radiator/condensor fan and A/C doesn't engage.Wires above battery shorted

2003 Jetta TDI. So, the A/C hasn't been working. In searching the related threads I saw the sticky about checking the radiator/condensor fan as well. I had the car off, key in on position, a/c selected on (lighted up switch), blower on 1 position. This is after it was driven home from work. I popped the hood and both fans in the front are not spinning. I then went to check the fuse number 16 in troubleshooting the A/C as well. As soon as I pulled the number 16 fuse, the passenger side fan started spinning and did so for five minutes. Is this to be expected? Also, a wire or wires on top of the battery case, 1 black and four red wires are shorted/overheated it seems (the black cable (50A) seems to be the overheating one and the red one right next to it is discolored) and melted the plastic on top of the casing. SEE PICS below. Boy, I could use some advice on where to proceed next. Thanks for any tidbits, shaming advice, helpful threads.... Have a good evening.

Ok that's basically a fire in the fusible link area. Disconnect the battery, disconnect hose and clean 'em up. If the cable is compromised (probably) replace it.

If ONE fan runs with Fuse 16 out the OTHER one is dead. Both fans either run or none do on command; they're wired in parallel. So right now you have a wiring problem AND you have a dead fan, guaranteed. Fix both and then continue on; there's more because the fans should run with the button in, fan switch on "1" and key on (and since you know ONE fan works the other should run too) but fix what you know is broken and then continue looking for the rest.

The FCM may be implicated here but until you get the wiring and fans themselves taken care of changing modules is getting ahead of yourself. Incidentally the pax side fan for reasons I don't understand is MUCH less likely to fail on low; it's almost always the driver side one that goes first. The driver side one MAY be shorted internally (which is why the resistor popped on "low"); if it is then it's VERY IMPORTANT to replace it first otherwise if you replace the FCM you risk immediately blowing up the new one. Electronics do NOT like dead shorts at all and fuses frequently do not blow fast enough to prevent destruction of Power FETs under dead-short conditions.

I'd pull the battery and check the grounds under the tray given that in the above-battery box. Good bet you have problems down there too.

Thank you for the reply. It seems the blower motor is out too. Works for a few seconds to a minute then goes out completely. Unless you think that could be related to the wiring or FCM problems??? My ultimate goal was to get the A/C clutch to engage as it clearly wasn't and I've discovered a multitude of other problems it seems... This is no longer my daily driver and the problems weren't reported to me. Yay!

Take on one thing at a time that you know is broken, then re-assess.

You have -- at minimum -- a serious wiring problem (near-fire) and a dead fan. Those HAVE to be fixed. The driver-side fan is not causing the blower to stop, but electrical problems could quite-easily be involved in that.

For the ECU to send the A/C clutch power the button must be in and the fan switch in other than "OFF" (button will be lit; if it's not then the signal is not being sent that you have selected A/C to the ECU), the engine must be running and not at WOT, the ECU must believe the coolant is not excessively hot (engine is not overheating), the outside air temp sensor must not be showing that it's below about 40F and the pressure sensor must be showing a pressure in the normal range (neither too low or extremely high.)

That's a lot of inputs and they all have to be valid. Electrical faults can play hell with any or all of them. While the usual is that the driver-side fan fails open through the resistor but not on the high side (that is, it will run when the thermoswitch is closed in the radiator or FCM commands high, but not at all on low) they CAN fail shorted and if it does the low resistor will likely burn up and go open immediately. If the system calls for high in that situation you now have a dead short through the FCM; whether the fusing is fast enough for the FCM to survive that is uncertain but I wouldn't take the bet that it is.

Since you know the driver fan is dead change it and fix the wiring damage; those are both guaranteed to need repair so take care of it and while you're doing the top side pull the battery and check the grounds; I bet you find that they need attention as well.

That black wire with the 50 Amp fuse is the power lead from the Alternator. It appears the connection was not good resulting in a lot of resistance which generated the heat. The AC issue(s) alone did not cause the heat damage.

Genesis is advising you properly as how to sort out the AC issues.

A good place to get a new fuse box:

https://www.tunemyeuro.com/clearanc...beetle-1-8l-2-0l-1-9-tdi-fuse-box-1j0937617d/

I pulled the battery and most of the box, not the very bottom, last night tracing the black wire towards the alternator. Are the battery grounds the bolts on the bottom of the battery box or under the bottom of the battery box with wires on them??? I'll look it up in the Bentley I suppose, but forum advice is great and direct! I ordered a fan, an alternator wire (the insulation is falling off), a fuse box, and a blower motor from IDparts for under 4 bills shipped. I'll install this all and troubleshoot from there. I'm looking for obvious signs of electrical damage above and from underneath as well. There's a small nail in the back tire too that slow leaks as well that needs to get done today ('thanks for finally telling me honey, and driving it on 4psi!'). I will update this post when I get all this done and let you know what happens. Thanks again, Club!

If the connection between the black alternator wire and the crimp terminal overheated, and the 50A fuse did not pop, the crimp connection was bad. That connection should easily be able to withstand what the fuse can.

mandrake slink said:

I pulled the battery and most of the box, not the very bottom, last night tracing the black wire towards the alternator. Are the battery grounds the bolts on the bottom of the battery box or under the bottom of the battery box with wires on them???

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99% sure it's the bolts under the battery tray, not the 4 holding down the battery tray.

Yep -- they're under the tray. Bad grounds cause all sorts of wonky things to happen.

The picture looks to me like the nut was loose.
While your at it take the heavy ground wire off the trans and clean the connections.

wonneber said:

The picture looks to me like the nut was loose.
While your at it take the heavy ground wire off the trans and clean the connections.

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It looks like that (at a weird angle and to the right of where its supposed to be) because the black plastic fuse box melted and contorted. It was still a snug nut, with some clean contact, but its dirty and I'm replacing it. It's a miracle it isn't touching the red wire next to it, but there's enough melted black plastic between the two still, yay, disaster averted. I'll be looking at the nearby grounds and cleaning and protecting them as well, thanks. Found a decent list of all the grounds on the Mk4 here.


Edit: I guess if you're looking at the post height exposed compared to the other ones, yeah I suppose the nut does look a little high, loose. It felt tight when I took it off, strange though.

Alright, just got the blower motor in, vents inside blow air nicely now. I cleaned all the grounds on the frame and the one on the transmission and used battery terminal protector on them just for ****s. They looked fine to me when I took them off. Replaced the large radiator fan, the alternator cable, and the battery-mounted fuse box as well. Both fans spin now when you pull fuse 16 which is great. Strange thing, the alternator cable connection says 110A whereas the one on the old fuse box says 150A?? No worries I guess, just wondering whats up? Anyhow, the A/C is still a no go. You turn the car on, turn the blower to 1 and press the A/C button and there is no dip in RPM as would be expected when the clutch engages. The A/C button does light up. I guess at this point I need to hot wire the compressor to test that the clutch actually functions? Or should I be troubleshooting the fan control module? Any ideas about where to go from here are greatly appreciated. LOL, I ended up with an extra 11mm nut at the end (***), but then just realized the FrostHeater was still dangling down below the battery box!

mandrake slink said:

I guess at this point I need to hot wire the compressor to test that the clutch actually functions? Or should I be troubleshooting the fan control module?

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Disconnect the power feed to the compressor and check resistance of the clutch coil, should be close to 0 ohms. They often fail open, especially if the system has been operated while the large fan is bad. Coil can be replaced without opening the system. If the coil reads good, check that the ECU is feeding it 12v when the system is on.

Simon

P2B said:

Disconnect the power feed to the compressor and check resistance of the clutch coil, should be close to 0 ohms. They often fail open, especially if the system has been operated while the large fan is bad. Coil can be replaced without opening the system. If the coil reads good, check that the ECU is feeding it 12v when the system is on.

Simon

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Yeah, it's 3.8 ohms across the two leads at the compressor when the unit is disconnected, obviously. I need a new A/C clutch coil then?

Well, after watching this guy's video with a broken and then new clutch coil, it would seem my coil is fine as the reading is essentially the same. Correct me if I'm wrong here?

Coil is fine, is the ECU sending 12v to engage the clutch?

Okay. I ran the vehicle while up on ramps and probed the connector coming to the compressor with the A/C on full blast. It says 4.14 volts. That seems odd no? Shouldn't it be 12 volts?

Stop!

First things first, because your last post of "after fixes" didn't include this.

Put ignition switch ON, push AC button, fan switch to "1".
Inside fan should run, AC light is ON and are both fans running at low speed on the radiator?

They have to be. If they're not then you still have a fan control problem and the clutch will not come on.

That both fans run with fuse 16 out means they're both electrically good BUT the other test still has to pass. Let's assume for the moment that test passes.

3.8 ohms is almost-certainly good in terms of the clutch coil. But your voltage is low coming from the ECU, but not zero -- so the wiring is PROBABLY good, but the ECU is not happy and not sending power.

Again (from my previous post) the ECU has to see (1) the cowl temperature sensor has to be indicating OVER 40F, (2) the coolant temperature must NOT be too high (overheating) and (3) the HVAC pressure transducer must NOT be either too low or too high. If the wiring to #1 or #3 is open, or either of those senders is bad, then the clutch will not pull in.

You've resolved both the physical fans and potential electrical issue at the top fuse box, plus checked grounds. Now you have to chase the other electrical signals and make sure they're good. It appears you have power to the coil from the ECU IF it wants to send it, AND the clutch coil itself is good. This points at one or more of the "gating" factors that the ECU uses to determine whether the clutch can be engaged remain invalid.

With key ON, fans set to 1, and A/C on and button lit up the fans ARE NOT spinning on the radiator. They are not moving at all. Fan control module then? If so, can that be tested with a meter?

Not necessarily.

If I remember correctly when I replaced the compressor in my '03 after the system was evacuated and as it was being filled (e.g. engine *running*, fan on HIGH, button in) the fans did NOT come on (nor did the compressor engage) until the pressure came up as it was being filled.

If the ECU senses overheat it will run the fans on high and disable the compressor, so that's not likely implicated (since the fans are not running at all with the button in and ignition on.)

The pressure transducer and OAT sensor and wiring to them need to be checked; both are sensors, not switches, so you can't jumper them like you can the radiator mounted switch. The transducer is a PWM signal; not sure what the OAT sensor is. I don't recall if you can read either or both off VCDS and right now my kid has her car at work, so I can't go look through the measuring blocks to see if they show up where you can see them or not.

Well, I looked at the ambient temperature sensor under the cowl. There was a fair bit of leaf litter in there which I vacuumed up. Sensor looked like the one shown here. The wires were intact and not eaten or broken. Not sure if its a switch or thermistor, but I took it out and ohmetered it; it read 870 ohms across the 2 pins.

I do not know if the cowl sensor is a thermistor or a PWM sensor; I've not had one go bad on any of the ALH-engined cars I've worked on thus far. I know the pressure transducer on the '03s is a PWN transducer (one of those I HAVE had to replace on a friend's car); you cannot check it with an ohmmeter. An oscilloscope will be effective or a meter that can read pulse duty-cycle will work, but not a straight ohm measurement. The one I replaced for a friend (there's only one sensor in the A/C plumbing, on the high side) was returning no signal at all and the A/C was completely inop.

You could try putting the cowl sensor in the fridge for a while, take it out and then stick an ohmmeter on it and see if it's value is different and changes as it warms up. That would be a pretty decent indication it's a simple thermistor, although being a 2-pin I bet it is since a PWM or other digital sensor typically has three pins -- V+, Ground and Signal.

The CTS on these cars has two independent circuits; one is the input for the dash gauge and the second goes to the ECU. I've had to change those every ~5 years or so on my car -- my typical experience is that it fails with low dash gauge readings even though the car is fully warmed up -- but I don't know if the ECU uses that as a gate for the clutch (that is, if it shows sub-freezing coolant temps due to being defective if that's one of the gates the ECU has to pass for the A/C system) or if that sensor is only used for injection timing and glow-plug duration purposes.

Here's another observation: I just drove to the store and back, not far, but its warm out here, 85F, and the coolant got to 180-190ish degrees according to the dash gauge. When I got home, I decided to look at the fans under the hood while car is running and at full operating temperature. Neither fan is turning. To me, this would seem to suggest the FCM might be implicated in this whole mess, no? The fans or at least the big one generally cycle on when the car is hot in the summer? I've replaced the large fan and it still doesn't seem to actually cycle on during use at least not without fuse 16 pulled.

Not necessarily.

There is no separate fan switching -- either both run or neither runs. Same for speed - either both run on high or neither does; there is no "split" so one can run on high and the other low. They're just not wired that way.

The dash gauge will read "straight up" over a WIDE range; it's basically useless to tell you what's going on. All it tells you when it's in the middle is that the engine is in the "normal operating" temperature range. If you have VCDS, however, it reads actual coolant temperature (as does a scan tool such as Torque if you have one running on your phone with a bluetooth transmitter plugged into the OBD port.) These engines don't warm up much without a fair bit of load on them and when moving there's plenty of airflow under nearly all conditions with the A/C off to cool them without the fans running at all.

Fuse 8 (green plastic) above the battery and its wiring has to be good for the FCM to work and low speed fan operation. Fuse 3, which is a bare metal strip above the battery, has to work for high speed fan operation. Fuse 5 and 25 on the cabin panels ALSO have to be good.

Here's the entire fan testing document -- walk through it STEP BY STEP.

http://pics.tdiclub.com/data/516/A4_Coolant_Fan_testing_Rev_7.pdf

Throwing parts at the problem is likely going to lead to replacing things that aren't broken.

If and only if all the electrical tests pass THEN I would suspect the PWM pressure sensor on the A/C high side, since accurately diagnosing that requires a meter that can read duty cycle or a 'scope IMHO you want to run down the fan control electrical side first and make sure that is working correctly -- which requires only a couple of jumper wires, a voltmeter and test light.

Thank you again, kind sir for the thoughtful, succinct response on this sunny Sunday afternoon. Back to work for me, I suppose. May you be blessed with 500k more miles.

I had no refrigerant in the system. PSI is 0 in the high and low side, locking out the comp and the fans. There's a leak somewhere, but I can't see it above or below with a UV pen. I'm positive there's fluorescein dye in there too. I'm thinking of doing as many o-rings as I can, the txv, and the receiver/drier, evacuate and fill and hope for the best. I put a little bit of refrigerant in and the comp and the fans all turned on. I am going to recover this soon before servicing, I know... Any thoughts on this? Should I be doing a N2 fill test before/after servicing to find out the difference maybe?

mandrake slink said:

I had no refrigerant in the system. PSI is 0 in the high and low side, locking out the comp and the fans. There's a leak somewhere, but I can't see it above or below with a UV pen. I'm positive there's fluorescein dye in there too. I'm thinking of doing as many o-rings as I can, the txv, and the receiver/drier, evacuate and fill and hope for the best. I put a little bit of refrigerant in and the comp and the fans all turned on. I am going to recover this soon before servicing, I know... Any thoughts on this? Should I be doing a N2 fill test before/after servicing to find out the difference maybe?

Click to expand...

I would do the other rings with the N2 fill test but wait to do the drier. No sense in switching the drier only to find you have a leaky evaporator, then have to buy yet another drier.