ammonman

Ever since I purchased my '90 S4 I have had a low voltage issue during summer. After much troubleshooting and monitoring of symptoms I think I've narrowed it down to a slowly failing Output Stage for the cooling fans. The Output Stage currently functions correctly along with the flaps. However, once the Output Stage has to drive the fans at full speed for a while (running both fans on high speed for A/C) and ambient temps are high (+90F and above) the alternator struggles to maintain voltage at idle and maintains lower but acceptable voltage above idle. When heavily loaded (full front A/C, partial fan on rear A/C, cooling fans on high, headlights on) it died on me a couple of times early in my ownership before I started managing electrical loading when it's hot out. If I shut off some load the car fires right up on re-start. With the Alt under heavy load as above, high ambient temp, and the car fully warmed/heat soaked then voltage at the jump post is 11.5-11.9 VDC at idle. During cooler months when no A/C is needed or ambient temp is low enough to allow the Output Stage to shed heat rapidly the voltage never drops that low. Heavily loaded at low ambient (or before the Output Stage heats up from use) the Alt will maintain 13-14 VDC. I have installed a rebuilt Alt from 928 Intl, replaced the aged wiring and connectors from the harness near the jump post down to the cooling fans, cleaned/checked/lubed the cooling fans, and checked the wiring and pin condition at the 14 pin connector (all fine). I haven't been able to figure a way to get my current probe down around one of the fan leads to check actual current draw during these episodes (only current probe I have is from my industrial maint. days so it is sizable.) My hypothesis is that the MOSFET's are aged and running hotter than they should along with the thermal paste being dried up or gone thus preventing good thermal conductance to the heat sink. As the Output Stage FET's heat up from aged components and poor heat conduction resistance goes up requiring more current to maintain output voltage. This results in a total current load above the original design load and the Alt can't keep up, resulting in voltage drop as it tries to maintain above rated current. Before I drop the $$$ on one of 928 Intl's recondition Output Stages I thought I'd tap the electrical wisdom here for opinions.

All feedback appreciated so don't hesitate to tell me I'm all wet.

Mike

Big Easy Sharkster

Mike,

Just to check, when you say 'output stage', your referring to #16 in the below pic. I renewed this unit about two years ago, got a rebuilt one. I am not an A/C user, I would rather just put the window down, there4, I just turn on my A/C a couple of times a year to run it. The last two times I've used it, which was for only 5-10 minutes, I noticed the dash voltmeter dropping from the 13-14 volt range to 11-12, which seems to parallel what yr problem is. Conclusion? Maybe not the output stage since mine is relatively new.

dr bob

Shortcut to end point:

Replace the ground strap.

The only partial failure of that fan controller that I've seen is one channel failing, leaving the other fan at full speed.


History:

My otherwise perfectly-mantained S4 had similar symptoms, especially in hot idle situations in traffic. With AC on and fans at full, and especially if the lights were on, dash gauge voltage would dip to 12 or less. A lot of sleuthing later, including replacing other stuff like the alternator and the positive battery cable, the ground strap did the trick for my car. (I should have listened to those who had passed this way before me, but I was too smart for that...)

Seems that the alternator was making enough current to charge the battery, but the weak ground strap was effectively limiting the charge the battery would get. At hot idle especially, the battery makes up for what the alternator doesn't produce. Once engine speed picks up, so does alternator capacity. The restores the battery charge and all is well. Since the battery wasn't seeing full current, it got weaker and weaker as the drive progressed. The battery is intended to make up the difference between alternator capacity and system load, so a good battery and good connections are requirements.

Bottom line is that, if you haven't yet done so already, replace the ground strap, clean and seal the battery connections, and see if the problem goes away. I bought 2ga ground straps for the local group online, and added my own shrink sleeve with a charging cable embedded. Roger bought some too, and sells them with the shrink sleeve already installed for little more than it would cost you to buy the pieces and build one yourself. Mark & Tom probably have a similar offering. That one little wear item instantly solved more than a couple mysterious symptoms. The car cranks much faster, etc, with the new strap.

soontobered84

Mike,
John Leidel has designed but not produced new circuitry to replace these aging secondary outputs. You might contact him via PM and see if he can shed some light. Also, if you would like to test your hypothesis in real life, I have several "spares" that are ostensibly "good" that I could send you to swap into your car for test. Just let me know.

Big Easy Sharkster

Bob,

Now that you mention the ground straps. I changed out my battery ground strap and the heavy gauge strap on lower passenger side of engine prior to seeing this problem with low voltage. Maybe I need to recheck tightness on these straps

ammonman

Big Easy,

Yes item #`16 in your pic.

Dr. Bob,

Ground strap at battery is new. All grounds cleaned annually as part of spring maintenance. Could be I need a new ground strap from the engine to the chassis. I'll get something generic and test this out. The thing that made me think of thermal breakdown in the output stage is while driving yesterday I saw a distinct jump on the dash gauge (I know they aren't very accurate but mine seems to be a pretty good indicator of ok vs. not ok voltage) when it started raining and the ambient temp dropped about 15 or 20 degrees. As soon as I got out of the rain and ambient temp climbed back up the volt gauge dropped back down.

John,

I was hoping John Leidel's project would come to fruition some day. I'll PM you about your kind offer of a test spare.

Thanks for the ideas guys. Keep 'em coming.

MIke

928mac

I have just noticed that only the passenger side fan is running.
Is there a fuse for each fan?
It did not make a difference weather the AC was on or not.

dr bob

Brad--

Fuses 28 and 29 in our cars. There is a dedicated feeder from the battery positive for each fan, two of the three minor connections at battery positive.

drwhosc

Well I have a similar problem. Only the passenger side fan is spinning. BTW I just replaced the end stage module, and no difference. So I am thinking a bad wire.. removing the wire at the fender well appears difficult, and I am stumped. anyone got advice on how to check the the wire to the fans. I know where one end is, I am assuming the other end is at the end stage voltage module.

Better yet. Don't care about the flaps, but I would like both fans to turn. Any suggestions on a direct wire with with a thermo switch.. Unfortunately I am in FLa now, so I need both fans.

ammonman

drwhosc, check both fan fuses as Dr. Bob suggested. You can also swap the connectors at the fans and the DS fan should run. That will verify both fans are functional.

Mike

Alan

Mike you are all wet...

The final stage provides a very significant load so it causes droop when the alternator is struggling anyway.

That the voltage would get low enough to cause the car to stall means you likely have some other electrical issues as noted by others.

Clean everything in the circuit , have the battery tested, consider a new regulator for the alternator and replace the ground straps (battery and engine/frame).

I suspect is has nothing at all to do with the condition of the fan final stage - sounds like it works just fine.

Alan

ammonman

Thanks Alan. I've been chasing this for a few years now. I'll have the battery checked even though it is only 2 years old. The Alt is a 2 year old rebuild from 928 Intl so I'll check to see if their rebuilds include a new regulator. I'll get a generic cable and parallel the engine to frame ground and see if that helps as the main ground strap was replaced last fall. My next thought is to parallel the power feed from the Alt to the jump post and replace the positive cable from the starter back to the battery and see if that solves the voltage drop problem. If the parallel from the Alt to the jump post makes an improvement I may spring for a new engine front harness from Carl.

Thanks for the advice.

Mike

Alan

Mike focus on the things you haven't already done already (with no major effect).

Just replace the ground straps - a good idea anyway over time. Check at least the ends of the main battery + cable for corrosion. Evaluate/clean all the main connections: starter, alternator, jump post , CE panel top.

As you say the additional connections betweeen alternator, jump post and starter could be suspect too...

Alan

928mac

Thanks Bob

GregBBRD

Actually, the Output Stage of the fan amplifier doesn't use but a tiny bit of current...the fans are what are using the current. Older fans do require more amperage to run....that is a given, as the motor and bearings age, but relatively easy to check. I replace quite a few fans that have "excessive" current draw (which is actually what causes most fan amplifier problems.)

If you have both fans running at high speed and the temperature gauge isn't buried into the "red".....you do have a problem. However, that problem might not be the fan amplifier. The "high speed" function is very difficult to get the car hot enough to turn on and normally should never come on.

I'd check the temperature switch, in the radiator, first. If the switch is good and the radiator is really hot enough to allow both fans to run at high speed, the next step would be to check the radiator for flow and efficiency. If the radiator is good and the fan switch is good, you would next have to think about water flow....is the flow inadequate due to a water pump or other cooling system problem?