Michael Benno

Some of you may have seen my posts on finding a resource to repair the fan final stage controller in 87+ cars. For those who didn't review the thread, the final stage controller recently died and I swapped it with a good controller that lasted about 10-days before dying in the same fashion. I sent them both in for testing and refurbishment, they both tested bad and are currently being repaired. Before I move forward with installation, I'd like to figure out why my car is destroying these units. I am hoping the deep knowledge here can help me find the cause.

Based on my research there seems to be two theories:
1. The fans are pulling too much Amps with overloads the components within the controller
2. Increased resistance in the connections in the fan circuit causing some sort of issue with the controller. (I am not sure how this would cause the controller to fail thouh)

Disclaimer: I do not understand electronics very well beyond reading the wiring diagram and testing voltage, amps, and resistance. So I'd appreciate your time to explain things.

Theory 1 fan amp draw
I started by testing the fans. The factory specification is 200watts each, when I convert that to amps @12volts I get 16.6amps.


I tested the fans in the shroud using a 12v motorcycle battery. The left fan shows ~13amp draw. The right fan was a little more strange. It would initially show ~13amps, but then it would slow in speed and the amps would decrease as well down to about 3amps and then it would stop and would not start (no amp draw). I'd wait for a few min and it would run a full speed at 13amp draw. I was able to replicate this behavior sometimes and sometimes it would just run at full speed. Clearly there is a problem with this fan but not in the way I would have expected. Interestingly, this fan motor was rebuild as evidenced by spot welds being replaced with rivets on the bearing retainer. The bearings on both fans seemed to spin freely. Since i have five other spare fans in a box, I tested all of them and they all similarly drew ~13amps. The fans ranged in age from 30k - 100k miles. I am thinking of swapping out the odd behaving fan with the lowest milage fan.

Rebuilt fan motor was the one behaving oddly

Any thoughts on this fan being the culprit?

Onto testing theory #2 poor connections...
The fan circuit is surprisingly simple excluding the control module and final stage amp,


When I was conducting the WSM described tests for the fan system. I noted the voltage (11.9v) test to the controller and to the final stage to be slightly lower than battery (12.6v). I did't think too much of it at the time since I am used to see a voltage drop at the CE panel in other scenarios (FOE harness tests). But I am wondering if that was a clue. Starting at the battery I cleaned all the connections in the circuit. I noted the fuse holders for the fans were melted fairly significantly. I removed the CE panel and inspected the connectors per the instructions in this post Sure enough, the connections were pretty heavily carbonized.



I removed the four bunt blades and gave them a good scrubbing on my wire wheel. Do you think this was the cause of the issue? How would the resistance in this circuit cause the final stage amp to fail?



So that is where I am at for now. Are there additional checks I should be looking at?

GregBBRD

Your answer is in the details....
Did the fan controllers both fail for the same fan?
Did they fail on low speed,, high speed,, or both?

The most common cause of failure is that the rivets which holds the power transistors onto their heat sinks break the heads off...and the transistors cook in a very short time, unable to disapate the heat.
Have you gotten any feedback from the rebuilder, yet?
It's entirely possible that your failures are coincidense...the rebuilder can tell you that.

BTW #1: I'd certainly "toss" any fan which was inconsistent in testing....there's nothing good occurring there....
BTW #2: And if one is bad....
BTW #3: Two new fans are chump change in the overall picture of your GTS....

Mrmerlin

I would replace both fans with new parts as well as both fan connectors make sure to solder the new connectors

Use Deoxit 100 on every connector and fuse

also inspect both fan fuses if either fuse is not tight then replace the fuse holders

At the battery Remove and clean all connections replace the ground strap if it’s original.
Consider replacing the positive wire from the battery

FredR

Sounds to me as though you may have more than one problem going on.

Irrespective of what is going on in the system the fuse holder should be able to handle the ampage whether it is the expected 15 amps or the draw has increased. If the fuse did not blow then you know it was drawing less than 30 amps.

To get some kind of realistic idea of what is going on with the motor you need to run them outside the car's electrical infrastructure such that it is running full bore on battery voltage or preferably alternator voltage- then you can measure the amps drawn directly. A 4mm2 cable connected temporarily to the hot post should suffice- just ensure you have a good connection to the motor. If you are using an inductive clamp you need one that can measure DC current. If the fans are drawing more than 15 amps time to do something about them.

You can also measure the resistance from the battery terminal through to the PWM control unit and calculate the voltage drop based on the motor ampage as tested. Same from the control unit through to the motor.

Mrmerlin

NOTE most fan issues occur from poor connections at the fuse panel,once the fuse connections get corroded then the resistance goes up and makes heat this will over time weaken the fuse holder pins.
NOTE The connectors to fans, get dirty and create heat thus weakening them and reducing power transfer, Hint see if your fan connector looks brown if so its getting hot.
HOT = poor connection
NOTE poor connections at the battery, many times the battery connections are overlooked , the 2 red wires on the POS terminal power the fans, make sure everything is clean,
NOTE if the car lives in a damp environment then it would also be prudent to replace the POSITIVE battery wire.

Michael Benno

Hi Greg, thanks for the feedback. I am waiting on feedback on which component inside the final stage amp failed. The failure mode was the same for both final stage amps: The left fan would run at high speed. The right fan would not run at all. This was the fan that behaved oddly when tested. Also, when I swapped the controller with a good one, both fans operated normally with variable speeds as needed. That lasted about 5-drives. I'll look for some new fans.

Hi Stan, thanks for the check list. As noted above, I think I'll start shopping for some new fans. Regarding the CE panel, yes, throughly soaked the whole panel in Deoxit D100 and then spray with Shield to protect against future oxidization. I also mechanically cleaned all the 30amp fuse holders and re-crimped the ends to ensure a positive grip. The battery ground strap is new and all posts and terminals on the battery were cleaned. I also cleaned the connections at the fans and the ground point for the fans (inside front fender). Can you elaborate on the solder connections comment? which connections? Also, can you elaborate on your comment of replacing the main battery cable for damp environments? What that only apply to the main cable or the two red supply lines as well?




Hi Fred, thanks for the insight, I bench tested the fans using the setup you descried.I yes DC amps. All the fans tested under the specified 16.6amps. I tested start up amps: 20-30amps for less that .5 second, and full running with blades: ~13amps, without blades : ~6amps, and stall amps >30amps and blown fuse on my test kit. I am assuming the PWM is the final stage controller? Can you go into a little more detail on how I might do this, as it's above my current skills. Thanks in advance.

GregBBRD

One of the discount Porsche dealers (Sunset or Marin) would be your most economical sources.
If the wait is too long (generally only in Germany) we (Mary) start freaking out when we have less than 4 new fans, ready to install or ship.

FredR

Michael,

The control unit sends a signal to the final element which is a PWM [pulse width modulator]. This thing works by high speed switching to vary the speed the fan runs at and does so in accordance with the temperature by the sender located on the front lower of the driver side end tank of the radiator . The WSM advises the programme in terms of temperature versus speed but realistically there is not much one can do with it- a scope will indicate what it is doing. Whoever is re-building these items for you should be aware of the input/output relationship and test accordingly.

Given the test results you obtained if the fan bearings are OK then no need to replace the motor units but remember these things do not last forever. The bearings are the likely failure point and then metal to metal contact etc does for them. Some folks have changed out the bearings themselves or so I understand.

GregBBRD

Fred:

Read Michael's first post above. His right fan seems to have a problem other than bearings...and someone has already had it apart.
DC motors (even when made by Bosch and installed by Porsche), with brushes and a commutator, have a given life span.
If a car comes into my shop, with over 100,000 miles on it, and it has one bad fan motor on it, 100% of the time I'm going to recommend that both fan motors be replaced.
This ensures that in six months, I won't have to explain to the same client why his car is overheating (again), from the second fan motor failing.
And why he needs to pay me labor to do this job again.

Of course, some people decline and only want one fan motor to be replaced.
Heck, if they want, I'll even install one good used motor.....I've got an entire shelf filled with used ones that I've taken off or salvaged for the past 35 years. Dozens of them!
And that's OK with me. At least I know that I wasn't the negligent one by not mentioning that both fans have the same exact hours on them and have run at the same exact speeds, during those hours.

If the car is here and having one of my strokers or even a stock rebuilt engine done....it's going to get two new fans.
Just like it is going to get a new water thermostat and oil thermostat.
There's virtually no choice, for the client.
And these parts are more like a "rounding error" in the overall cost of us building a new engine.

FredR

Greg,

I believe we are saying the same thing coming at the problem from a different angle as it were.

Michael says he has a bag of motors some of which are seemingly running OK so if they test out OK all well and good. The only difference is you make a sound recommendtion for your ciients that is bullet proof and quite rightly so. In Michael's case he is a dab hand DIY'er so if the spare motor conks out in a few months time or whenever he replaces it with whatever suits his mojo.

My concern in this case is "cause and effect". I have difficulty reasoning how one thing caused all the problem points Michael has listed. Thus the need to check the entire system out in case there is a fault we have not foreseen.

if fan motors have done 100k miles and have served the owner for 15 years I work on the principal they owe you nothing further. I thought my twin 12 inch SPAL fans were getting a bit noisy, pulled them apart locally, changed out the bearings so they felt like new and then in my friendly ac shop saw the last two 13 inch SPAL fans he had in his shop with identical motors to my unit at a super price to get rid of them- two new motors went in and I have two spare units just in case. One of the fans I am thinking of mounting as an auxiiary pusher fan for [hot] summer use.

I am not a big fan [pun] of the stock system in that I feel it is somewhat overly complex but it is what it is. If my system takes a dump I think I would consider wiring the fans directly with a couple of temperature switches staggered to being them in separately. I also have a programmable SPAL twin fan controlller that I could also use that I originally purchased for a prospective oil cooler project I prepared for should my under radiator cooler crap out given it has a bit of a dink it that I inherited upon acquisition.

GregBBRD

Having lived through several generations of water cooled Porsches, I can tell you this:
I'm a huge fan of the stock fan system, on the later model cars.
It is extremely rare for these 928's to have a blown head gasket, from overheating.
On the "simplier" fan systems (924 especially), there was a period of time where we could hardly keep up with the blown head gaskets.

FredR

There is no technical problem with the stock fan control system- the problem is age and unavailability of the control unit and final PWM element as and when they take a dump as gets ever more likely as they age.

Any "repair of PCB units inevitably costs in the region of $500 a pop plus time to ship and receive back. Thus I like to be prepared for eventualities and if I experience the problem Michael has I reckon that within an hour I can take the fans out of the final element loop and have them running directly as a stop gap. I also have the SPAL programmable controller I can install in an afternoon that would give me most of the controllability of the stock system.

Over here my ac system is running 9 months of the year if not more so having the fans run most if not all of the time is something of a norm for me.

Crappy systems on earlier models are what they are but I fail to see the relevance compared to fitting modern state of the art kit. I dare say there will be some after market kit that does what the existing system does only better and for much less dollari and that is what i have been keeping an eye open for. The Aussie outfit Davies Craig who make electric water pumps do a switchable piece of kit very reasonably priced [about $40] and a fully programmable controller that I intend to take a closer look at for about US$150.

When it comes to instrumentation simpler is invariably cheaper and more reliable. As you rightly say fan motors are loose change by comparison.