Captain_Slow

About three weeks ago I replaced the fan controller with a good used one (the one by the passenger seat) and my fans returned to normal - both fans working at less than full power (I think they are supposed to run at 50%). They worked perfectly for almost a month.

Yesterday, I drove the car up to Summit Point to watch the PCA event. On the way back decided to use the AC. First time this summer. I probably had the AC on for about 30-40 minutes of highway driving.

Close to home I turned off the AC and rolled down the windows. Driving through town I could hear the return of the single fan running on full power and cycling on and off. I pulled over and sure enough, the right fan wasn't working (same as before). My immediate conclusion was somehow the car is eating fan controller boxes (the PO had also replaced it).

This morning I checked the fuses. Both are fine.

Then I started the engine and let it warm up, waiting for the moment the fans start. I was expecting just the left fan, cycling on and off. Instead, both fans worked, at the normal 50% level. NOTE: The controller box installed by the PO would NOT run the right fan at all. Perhaps this problem starts off intermittent and then becomes permanent/fatal (for the controller box) over time.

Any thoughts or ideas?

I'm going to run it again (without A/C) and see what happens when the engine gets hot. I'm thinking (but have no confidence...just a wild guess) maybe the right fan is drawing too many amps at high power and being shut down by the fan controller box.

If the fans run fine when the engine is hot, I'll switch on the A/C and see if I can create a repeat of what happened yesterday. Not sure what insights this would provide, other than being associated with the A/C being on....but it wouldn't be the first time there was a correlation without causation.

Captain_Slow

Did the idling tests with hood closed. Let the engine idle for 20 minutes after the fans began to remain on continuously. Then 20 more minutes with A/C switched on. Fans worked normally for both tests. So it's looking like this will be an intermittent thing...for awhile anyway. Possibly due to old oxidized connections, wire, failing sensors - anything in the system that can deteriorate after 26 years in a 928 engine compartment.

When either the final stage or the controller box dies, I'll buy the DC Controls fan controller (and possibly put in one of the modern lower profile fan kits).

dr bob

Got an IR temp gun? Might shoot the fues holders. Then go after the wiring between finals stages box and the fans.

ASSuming the two fuses for the fans are identical, you can get a relative reading on motor current by measuing the voltage dro across each fuse. Needle probes fit through the little openings on each fuse. Meter set on millivolts, larger voltage drop equals higher current flowing. That might tell you whether the fan motor itself is the problem, eating the final stages boxes and all.

Captain_Slow

Thanks Dr. Bob. I do have an IR gun. The fuses are fine (and less than a year old). No signs of either heating up, but I will try the gun.

The wiring between the final stage and the fans is fine. Fraggle measured both for either voltage or continuity -- whichever it was they were both fine. I'd already performed the potentially dangerous jumping of the pins (sockets actually) in the final stage connector, powering up both fans at full power.

You are spot on with my concern about the motor. However, if it is my final stage going it is a strange example. We swapped Fraggle's final stage into my car - right fan still didn't work. We put my final stage in his car - both of his fans worked.

Now all evidence pointed to the controller box. I swapped in another Rennlister's box and both fans turned on, at half speed.

I bought the used controller box, put it in, and again my fans worked perfectly.

Perhaps after all these years there are variances in how sensitive a controller box (even from one box to the next) is to slowly failing final stages, or fans. I have thought that the right fan, which spins with just slightly more drag than the left, might be failing and drawing more amps, within a range of values that will trigger some controller boxes, but not others. In other words, I may be at a tipping point where fan and/or final stage are failing, but not consistently triggering the controller box to shut down the right fan.

However, the original box and the replacement put in by the PO were consistently seeing a fault for the right fan (final stage or fan itself). The used controller I put in 3 weeks ago may not be as sensitive....or, it's just a matter of time before it shuts down the right fan consistently.

I'm obviously grasping and using a lot of imagination....but one thing is very certain - the right fan stopped working yesterday and the left fan went back into full power mode and cycled on an off as before.

Is it impossible for the controller boxes to be fried by faults in the system? Theo and Fraggle confirm that is is very unlikely because only low level sensor inputs go into the controller box.

outbackgeorgia

I have had issues with the connectors AT THE FAN. Intermittent contact. Clean the fan side with deoxit, replace the wiring side connectors. Don't really need the fancy plastic covers, just be sure of rotation direction, as swapping the wires will reverse direction. I eventually replaced all fan wiring to resolve the issues.
Dave

Captain_Slow

Dave -
Thanks for this idea. It sounds as likely as any other culprits. The reason I haven't looked at the wiring or any other possible causes is because the replacement fan controller box completely fixed the problem.

Theo emailed me this morning and said he has had an 88 S4 that would go into the fail-safe fan mode after driving for about an hour. Shutting down and starting the car again would return the fans to normal....for awhile. I think he indicated this was the beginning of the end for the final stage (intermittent fail-safe mode when the fans are running on high). This makes sense as the current and heat at the FETs in the final stage would be most stressful on the aging components.

I've got some deoxit and will give that a try today.

MainePorsche

Jon,
I thought you'd gone through the continuity eval of the wiring, and the diagnostics of the controller and the final stage. ?

Alan

The aging of the semiconductor components would mostly lead to their death...

That it works again suggests something different. An intermittent continuity break in the wires to the fan is one possible cause - an intermittent continuity break in the controller final stage circuit board/solder joints is also possible. I'd suspect something more mechanical like this related to heat & expansion/contraction

Alan

Captain_Slow

Alan -
Thanks. This makes sense. Theo is offering similar ideas, particularly after the A/C has been on for over an hour. I'm going to check how much current the fans are drawing, apply deoxit to the connections, and check the grounds for the fans. Most likely this is mechanical expansion/contraction issue. I'll just drive it until it progresses from intermittent to all the time.

Captain_Slow

Craig -
Yes, I did check all the wiring and checked the continuity and voltage. Fraggle did also. But we did it all relatively cold. The problem now may be heat related. It is really strange that the replacement fan controller box immediately cured the problem, almost.

I would like to find the problem. It might be something straight forward. But it's really starting to look like these systems can have many potential age/environment related faults that will produce the same symptoms (single fan on, cycling on and off, while the other fan does not work). Theo is a purist (and a very skilled electrical engineer) and will always encourage me to keep it original (I get that...and if I had his knowledge and skills I might stick with it). But I'm not Theo or Alan, and therefore at some point I'd have to have an expert find the obscure faults and correct them, rebuild final stage at $600 a pop. Even the used controller from 928 Int'l was $200.

I know enough to know when it makes more sense (for a non-electrical engineer on a tight budget) to go with a modern fan controller replacement. The DC system doesn't have several sensors monitoring ATF temp, intake temp, A/C pressure, etc. However, after reading the technical materials provided by Brian (he's the one Craig stated has an "Alan level" of expertise) at DC Controls, I feel the DC system provides similar cooling protection, if not better, because it makes the fans truly continuously variable, and monitors coolant temperature and makes very quick fan speed adjustments to maintain a selected temperature. Like motor mounts, we may be looking at a practical compromise solution emerging as most suitable for owners who just want to do the best they can and still be able to afford to keep and maintain their car.

The high current capable DC controller (FK-95 I believe) costs less than $200. Th internals are not protected by a seal that can fail (e.g. final stage) - the entire guts are immersed in epoxy so that when used on off-road vehicles it can handle being submerged for river crossings. I like that. Tom Cruise would have liked that.

MainePorsche

Jon,
The dc unit was just delivered and is now in my hand.
I'm a 'purist' too, but my main concern was the amp rating for FET and other components. As Carl (FLYVMO) did his impressive rebuild with many components not OEM, and did in fact choose the dc unit for his fan controller, something must be said for his reasoning of this choice. From my inquiries into the dc unit the resultant temp controlling via fan mimics the original system in its final result - 'the cooling'. Porsche did do it through coolant, ATF, intake, AC pressure sensing but the dc unit cools at a temp controlled by us - one temp is not elevated at the exclusion of others - especially true if one chooses to have automatic fan activation when AC switched on.
As for circuitry the system, it is very simple - one sensor input - the rest is current in/current out, ground, and AC activation
I will soon do the install and post up the info regarding it. Like I've said in a PM to you, I have a plan for its placement, esthetics, and ventilation. I might just be using the original final stage cover as part of the esthetics - have to do the final measurements when I get it out of the box.

MainePorsche

Jon,
Won't be possible to use old final stage cover to keep 'original' esthetics. The new controller is about 1/3 the size (about size of a pack of cigarettes), and is very thin EXCEPT for the 4 cooling fins it has - this will not allow it to fit under the old cover.
Simple wiring - labeled as IN, OUT, & GND. The plug in connectors for optional settings are smartly done on one side - wires/connectors included.

Captain_Slow

Craig -
Can't wait to see how you do the installation, preserving the stock appearance.

Captain_Slow

Craig -
Take a look through the circular hood latch receiver in the front apron. See all that space under the apron above the cooling flaps? I was thinking you might be considering installing it under the apron, with mounting hardware hidden by the fined cover of the final stage (minus the guts).

MainePorsche

Did think about that locale for it is the highest velocity air flow area - it is also the highest moisture/road 'grime' area as well.
I am going to put it where the old final stage was. Some holes will be drilled to allow air from under - the fins will be up top.
Will keep you all posted.