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Discussion Starter #1
For several years, I have had a relay that turns rapidly on and off. The relay is somewhere under the dash near the steering column. This relay seems to affect the auxiliary cooling fan, sometimes, but not always, turning the fan on and off randomly, which appears to have wrecked the fan clutch on the original electric fan.

The bad behavior is not consistent. I can go months with the relay behaving correctly, no issues. Or, it will act up for only part of the time on a single drive. Or, I could go on several drives over a period of weeks or months with the relay switching rapidly on and off crazily for the entire time the car is on.

At one point in the past, it seemed that cleaning the battery terminals would fix the problem. Lately, there doesn't seem to be any connection between the malfunction and the condition of the battery terminals. The battery is a bit old, but a multimeter says it makes 12.6V engine off, 13.6V engine running.

I had a competent mechanic replace the original electric fan a few months ago. I had mentioned the malfunctioning relay to the mechanic, but at that time, the relay was not misbehaving, so I guess he could not replicate and diagnose.

Today, at only 87F, which just isn't that hot, I was running the AC most of the time. The relay was going completely berserk whether the AC was on or not. After about 45 minutes of driving, the relay stopped misbehaving. Around that time, I noticed the electric fan was on all of the time, even with the AC off. I went into a store. After about 8 minutes in the store with the engine off, when I came back out the electric fan was still running, which I don't recall the car ever doing for more than about a minute or so when the car was new. There is plenty of coolant. I drove the car around some more, gently, no turbo boost, with the AC off and the heater on full blast. The heater was dumping plenty of heat into the cabin, but the fan stayed on. I turned the car off. The fan stayed on. When I got back to my driveway, heater still on full-blast, fan still running, I turned the car off, the fan continued to run. I didn't wait long this time. I disconnected the battery. When I re-connected the battery a few moments later, the fan started up immediately. By the time I was done cleaning the battery terminals, when I reconnected the battery, the fan did not turn on again, but the relay was now chattering crazily again, sometimes, but not always, turning the fan rapidly on and off.

I guess the original fan clutch was broken so long I do not remember how long it is normal for the electric fan to stay on after turning the car off. 8 minutes on while I was inside the store seems like the fan was stuck on and would have eventually drained the battery.

Thoughts?

1999 B5 AEB 1.8T / 5MT. 205K.
 

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The 1.8T fan control relay is at the rear upper relay panel. This is behind the main relay panel and a pain to reach. Apart from replacing it, I would check the wiring for corrosion as it's near the ECU plenum. Disconnect the battery, remove the lower dash, and depending on if you are a contortionist or not, it may be easier to drop the fuse panel and lower relay panel.
The fuse panel is easy, just a couple of bolts and clips to get it out. The main relay panel (it's actually 2 parts) is another 2 bolts.
100031


If you get to this part, you've gone too far, might as well change the heater core then!
100032
 
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Discussion Starter #3
Thank-you, I appreciate. Is it fuse 384?

I needed a new battery anyway, so I just bought one to see if that will help. Probably not, because when the auxiliary fan comes on now it will not shut off. After turning the engine off, that fan will run at least 30 minutes or until it depletes the battery. At the 30 minute mark, when I finally disconnected the old battery, the old battery voltage was down to 10.5V, and at that point the dashboard temperature needle was noticeably to the left of (below) the 190F center mark. If the temp being below the 190F center mark means anything, there is no reason for that fan to still be on after 30 minutes. Frankly, I don't even remember that fan continuing to run with the engine off when the car was young. I only remember that fan coming on with the AC.
 

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Note that this applies only to the 1.8T engine.
The two fan control relays on the rear upper relay panel (8-position) are, counting from left:
Position 2, second speed coolant fan control, may be labeled V7. This relay runs the fan at high speed, directly on battery voltage.
Position 5, first speed coolant fan control, may be labeled V7 or V35. This relay runs the fan at low speed via a resistor out under the hood on a frame rail. This relay pulls in whenever the A/C is turned on, so it gets a lot of use. Given the OPs symptoms, I would probably replace this relay if only for good measure. I suspect you'll find it's contacts pitted and sticking together ("welded").
(The 384 relay is the A/C clutch control relay on the main, 13-position, relay panel. It isn't a plain relay; it has some electronics inside it.)
These two coolant fan control relays are ordinary relays--just a coil that, when energized, closes a set of contacts.
Note that the V6 engine coolant fan functions identically, but the relays are integrated with the resistor and the afterrun coolant pump relay, out under the hood on the frame rail.

My experience with my 1.8T (owned since new) was that high speed never came on unless I was stopped in traffic on a 90°+ day with the A/C on. In that case, it would run on high for a short while, then go back to low speed. (That assumes your engine-driven fan is working, of course.)

The other possibility, quite unlikely, is that the fan temperature switch (lower radiator outlet) is faulty. You could unplug it to be sure, if there's any doubt.
 

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Here is a pic of the 1.8T dash wiring harness outside the car. The top harness (goes through the fender) has the fan control at the top right of the pic. Copy, save and enlarge to see it better.
100071
 

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Discussion Starter #6
By feel, I have identified 373 as the one that is opening and closing randomly. I can feel it clicking.

Any idea where I can get a replacement, I mean, other than eBay?

I also replaced the battery and am getting a good 14.1 volts now at the battery while the engine is running. However, relay 373 still sticks and that will keep the fan on unless I disconnect the battery. I also noticed that pulling the 40 amp fuse, pristine, below relay 373 will also kill the aux fan when the relay is stuck on. Also, removing 373, giving it a bump, and then reinstalling, must have opened it again, the fan stayed off. I'm really glad I've isolated 373 as the culprit that keeps the fan from shutting off.

However, I need my mechanic to help me if something else is causing 373 to misbehave.

I went into the ECU box. I was surprised to see that part of the ECU box is open at the bottom, so I can see down into the back of the dash and wiring looms there, but overall everything I could see looked factory pristine. There was no sign of water ever being in the ECU box or elsewhere below it. It all looks factory fresh in there.
 

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There are 3 coolant fan control relays in the 1.8.
1. Coolant fan relay.
2. First speed coolant fan relay.
3. Second speed coolant fan relay.

With ignition off, if you unplug "1. Coolant fan relay" does the fan stop ?
With "1. Coolant fan relay" unplugged, does the fan run when the A/C is on ?

NOTE:
1. Coolant fan relay, wiring colours:
30 (Brown), 86 (Brown), 85 (2 x Black/Red), 87 (2 x Brown/Black)
 

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Your VW dealer or online sources like ECS Tuning, of course... VW part number 4H0 951 253 A, but $34.20 msrp. ECS lists some less spendy alternatives.

You can also use Panasonic CB1A-R-12V from Mouser Electronics or Digikey, $5.30 plus shipping. This relay has the internal surge suppression resistor across the coil like the VW relay.

Your local auto parts store should have a suitable relay, as well, although typically without the resistor. The resistor is less critical in this application since this relay is not controlled by the ECM.
 

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Discussion Starter #9
I appreciate all the help! Thank-you!

In my last post, I incorrectly identified the malfunctioning relay as 373. It is 370. Yes, when I removed relay 370, the electric aux fan, which was stuck as long as the battery was connected, stopped. Relay 370 has prongs 30/86/85/87.

I think I did try to see if turning the AC on would turn the fan on with relay 370 removed, and I seem to recall the fan stayed off. However, that particular test I don't recall as well because when I first started removing the relay the fan was stuck on no matter what. It wasn't until after I removed 370 a few times that I physically bumped it such that the circuit opened up again. Only after bumping it and then re-inserting it did the fan stay off.

I did order a 4H0 951 253 inasmuch as it supersedes "8DO 951 253" stamped on the relay. Zygmunt had several choices. I picked the Bosch. The deciding factor was the same gray color.

I'll let you know if a new relay 370 solves the problem. I am greatly relieved if it is not controlled by the ECU, in which case, I guess the only other possible cause of the relay chattering would be worn wiring, probably close to the relay.

Meanwhile, I have discovered that nearly every point of attachment for the lower knee bolster has disintegrated. The two tabs that connect it to the dash frame by the left fuse cover have disintegrated, as have the two screw mounts underneath. They just crumbled. I can't find the part number anywhere on the bolster, but per patrickaccessories, 3B1-880-509-B-A94 appears to be the part number (black), along with 3B1-880-603 Knee bolster mount bracket, left; 3B1-880-604 Knee bolster mount bracket, right; 3B18800501A Knee bolster mount plate; 893-857-119-A Knee bolster clip; 3B0-858-035-A-2AQ Side cover, black, left; 3B0-858-036-A-2AQ Side Cover, black, right. A sprinkling of VW dealers around the country have reasonably good part listings with diagrams, but of course a lot of the parts are no longer available for purchase.

Aside from the crumbling dash, I will to be so stoked if the problem is confined to the relay itself with no other cause.
 

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I appreciate all the help! Thank-you!

In my last post, I incorrectly identified the malfunctioning relay as 373. It is 370. Yes, when I removed relay 370, the electric aux fan, which was stuck as long as the battery was connected, stopped. Relay 370 has prongs 30/86/85/87.

Meanwhile, I have discovered that nearly every point of attachment for the lower knee bolster has disintegrated.
Why not get a knee bolster from a junkyard ?

Is the relay that you are replacing in the top LH corner of the panel, as shown in this pic. ?
 

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Discussion Starter #11 (Edited)
A-ha! Now I know where that group of relays is,. The so-called "glove box frame support" metal bracket was blocking my view. No, the 370 I am working with is on a different relay panel lower down and just to the left of where the steering column comes through the firewall. Per your question, I just discovered that when I turn the AC on, the electric aux fan does run with this particular relay 370 removed. You can see the gray 373 relay behind the black and red wires. The chattering relay 370 I am working with goes in the empty butterscotch socket to the lower right of gray relay 373 and just above the red 50 amp fuse. Incidentally, IIRC from the other day, removing the orange 40 amp fuse there makes the fan inoperative.
100110
 

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Per your question, I just discovered that when I turn the AC on, the electric aux fan does run with this particular relay 370 removed.
The chattering relay 370 I am working with goes in the empty butterscotch socket to the lower right of gray relay 373 and just above the red 50 amp fuse.
You can see the gray 373 relay behind the black and red wires.
With all relays installed and....
With the A/C OFF, Ign ON, and the relay chattering or the fan running.
Does it stop if you disconnect the Red/Green wire from the temp switch in the lower radiator hose ?
Does it stop if you disconnect the Red/Green wire from the A/C pressure switch ?
 

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I suspect the relay in the butterscotch colored socket is the high speed relay, but Bentley is giving confusing info. If it is, I would expect the fan to still run (on low speed) when you turn the A/C on, even with the relay removed.

If it is the high speed relay, there are two things that can turn it on:
1) Coolant thermal switch at the lower radiator outlet, high temperature contact.
2) A/C refrigerant high pressure switch.

Both of these switches connect the relay coil to ground (red/green wire, ground is brown).
+12 volts is provided to the high and low speed relay coils by the 5A fuse on relay panel (switched with ignition).
Since the coil power to these relays is switched with ignition, the only (reasonable) way the fan would run with the ignition off would be if either the high or low speed relay contacts were sticking.

The 40A fuse provides the power to run the fan (as you've seen), always hot.
The 50A fuse provides power for the ABS pump.
 

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Discussion Starter #14
"With all relays installed and....
With the A/C OFF, Ign ON, and the relay chattering or the fan running.
Does it stop if you disconnect the Red/Green wire from the temp switch in the lower radiator hose ?
Does it stop if you disconnect the Red/Green wire from the A/C pressure switch ?"

I will try to replicate. When the fan stuck on initially manifested last week, AC of throughoutf, the fan would come on about 20-30 minutes into a drive, and from that point would not shut off even with car off and cooling to under 190F. So coolant temp appears to be implicated. I think you are definitely asking the right question(s) pointing me toward the radiator temp switch as a possible related problem or cause of the relay 370 misbehavior. Relay 370 would sporadically chatter for years before last week, when the fan began staying stuck on. So it seems like relay 370 is degrading progressively. Still, the ultimate cause of that degradation might be faulty input to the relay (possibly bad ground) causing it to chatter, rather than confined to a fault internal to the relay.
 
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Discussion Starter #15
"Does it stop if you disconnect the Red/Green wire from the temp switch in the lower radiator hose ?"

I found this connector near the main fan cowling. It's on there pretty snug. I tried to work it loose before trying to work with it while the main fan and pulleys are spinning. For now, I am going to just hope that a new relay fixes all the problems, and if not, pursue the temp switch and wiring input to the relay. I appreciate all the help.
100113
 

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"I will try to replicate. When the fan stuck on initially manifested last week, AC of throughoutf, the fan would come on about 20-30 minutes into a drive, and from that point would not shut off even with car off and cooling to under 190F."

This means the relay is faulty and needs to be replaced,
but the chattering would be caused by the faulty temp switch or its wiring. This would have damaged the relay.
 

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Discussion Starter #18
Thanks, Tom, I think you are right about the switch and/or wiring. Zygmunt got me the new Bosch relay right away. It's in the car. Everything is behaving presently, but so was the old relay during the last drive just before I put in the new relay. I think I'll wait to hear the chattering again to replace the switch. If it still chatters after that, search for wiring fault. I have a multi-meter and can do some basic things, but am learning the rest now. I was reading last night to start exploring ground at the relay terminal 85 slot or whatever. Obviously I'll check that. I need to get some jumpers with alligator clips. Epic exploratory electrical troubleshooting in prospect, huzzah! I'll let you know how it turns out, but it could be some time before I can get a multi-meter on it during a chattering fit. Lower dash apart I guess until then.
 

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Discussion Starter #20
Yep, I get it. The factory relay had probably hundreds of sporadic chattering episodes since early 2014 (I mentioned it, but my mechanic probably could not reproduce it, you know how that goes, as soon as you drop it off, the symptom stops). The replacement hasn't chattered even once yet. I just drove it on the highway for an hour at 88F ambient, the same temperature as the other day. Fan did not come on at all this time, only with the AC. It is possible I lost track of what I was doing diagnostically the other day. On the ride I thought was exclusively AC off, I seem to recall I turned the AC on briefly. From now on I will keep a notebook.

The very next time it chatters, yes, I will order for the radiator fan switch. ECS was the only place I found selling one. They have two Meyles, ES#: 3125739 (75-82C) or ES#: 2717988 (87-93C). I have asked them to help me determine which temperature range is correct for my car. They're distancing, so it may take a while to answer. I had a Bentley and a VAG-COM at one time, but they were both stolen out of the car.
 
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