PORSCHE 928 Resistor Pack - Need Help

[JayPoorJay]

Hello Good People...

I know this isn't the proper or correct sub category for this question BUT I looked through the categories and really couldn't find the exact proper section. I've gotten help from y'all here before and this category seems super well attended. If this is improper, I apologize and will understand if the thread is deleted...

Anyways. I have a Porsche 928 that has a very well documented problem - but no real well documented fixes. It has to do with a resistor pack that varies the speed of the HVAC fans by loading resistance on a 12v fan. I know so little about how to explain it I am a little ashamed. I believe a fix might be possible by inserting a correct bit of resistance wire - but I don't know how to BUY the right stuff.

Anyways, see photos, one of the coils of resistor wire has burned and needs replacing. I understand that it is resistor wire from reading posts in Rennlist - a global Porsche forum. A new part (what most people recommend doing) is 200$... There has to be a better way.

You all have guided me through some pretty cool stuff. I now have a working 70 LCD tv I found on the streets of NYC thanks to y'all and I'm hoping that y'all will come thru for me again. Deep bow to members here and many many many thanks. Also, I will post the fix y'all guide me thru on the Porsche 928 forum and others will benefit from the FIX.

I have levered up the HOLDERS for the coil and removed the old wire. On the sides of the unit it says something like .4 ohms and what I guess is the correct and needed resistance per coil so the fan will run properly. I will post photos of the markings on the pack.

What can I do to replace that wire/coil!

Thanks thanks thank ya!

[R_J]

Re: PORSCHE 928 Resistor Pack - Need Help

Where did the wire burn open, if it was near the end you could just clean the wire and re attach it. 1/2 a turn missing would not effect that speed very much. The resistive wire would need to be able to handle the same current so at .4Ω would need to be the same gauge

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

Dang You're fast...!

I messed up then, or I could use the wire left over, I have it here. I guess I would have to rewind it... The overall length is roughly 3inches. I think about 3/4 of an inch is the shorter section - see photo. I guess I could rewind what I have BUT I am willing to buy a spool IF I knew the proper guage and type.

I've been seeing "NiChrome"...? Is that the right-ish stuff? And if so, what guage and approximate length would I need to reproduce what the fan and gizmo are looking for?

[R_J]

Re: PORSCHE 928 Resistor Pack - Need Help

You need the same gauge wire and length that was there and for that length it would be .4Ω
I see a vaping site DPS metalworks https://jeancloudvape.com/boutique/o...ps-metalworks/ that sells 5" lengths that are 0.35Ω but they don't mention the gauge

Also NiChrome wire here https://www.amazon.ca/AWG24-0-5mm-Ni.../dp/B00TGQF6XK
0.5mm dia, 5.551Ω per Meter

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

You need the same gauge wire and length that was there and for that length it would be .4Ω
I see a vaping site DPS metalworks https://jeancloudvape.com/boutique/o...ps-metalworks/ that sells 5" lengths that are 0.35Ω but they don't mention the gauge

Also NiChrome wire here https://www.amazon.ca/AWG24-0-5mm-Ni.../dp/B00TGQF6XK
0.5mm dia, 5.551Ω per Meter

Damn R_J... Ok.

I think I'll go with the Amazon stuff and just trim some sections til I get to the .4 ohm range, coil it, and clip it in...? Is it really as simple as that?

In your experience will and is this type of wire be ridged enough to be clipped in and hold it's form? This is all new to me. I hate asking stupid questions, seeming all helpless and stuff, lol

[R_J]

Re: PORSCHE 928 Resistor Pack - Need Help

I have not repaired one of these before, If the resistive wire is of the same gauge it should work, here is another source of resistive wire: https://botland.store/1152-resistor-wires I'm sure there are many others
And I'm sure others here will have suggestions as well

This seems to be the schematic

[dskall]

Re: PORSCHE 928 Resistor Pack - Need Help

Nicrome is probably what was used originally. Just measure diameter of wire and get same. When you coil it make sure no coils are touching each other.

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

Y'all are pretty amazing. Jus say'n...

Ok. I'm looking at this. If you would, please help me with the math. I haven't grabbed a pencil and paper yet but could and should. I guess the goal is keeping the number of coils and the size of the coil within reason, so, if with the product in the link (and I could get it pretty quick - the first link RJ provided would be delivered sometime in June) is 5.5xx ohms per M or about 39 inches... If I needed to get that down to .4 ohms,,, what is the final length to produce the .4 ohm value. I can't do the math lol...unless I really sat with it a spell,,,

See if this link works - working on my phone.


https://<a href="https://www.amazon....ASN9QTB879</a>

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

https://www.amazon.com/dp/B00TGQG9RM...TJDRASN9QTB879https://www.amazon.com/dp/B00TGQG9RM...TJDRASN9QTB879

[R_J]

Re: PORSCHE 928 Resistor Pack - Need Help

5.551Ω per 39 inches, is about 0.142Ω per inch, so about 3 inches should be close to 0.42Ω

[redwire]

Re: PORSCHE 928 Resistor Pack - Need Help

The resistors are coils of nichrome wire, which cannot be soldered to especially because it runs too hot, you have to crimp the connection.
You could also use blower motor resistors from say an old Chevy (pic), at a wrecking yard, cut out the wire you need.

There seems to be a thermostat there, the switch contacts are open and control something. I can see 0.25Ω, 0.4Ω, 1Ω, 2.5Ω

Note that older blower fan motors (in any car) are notorious for simply drawing too much current as they age. So they can overstress the resistors, like yours- showing they've been running glowing hot.

[R_J]

Re: PORSCHE 928 Resistor Pack - Need Help

The thermostat: If the air flow across the resistors is too low and they get too hot, the thermostat (bi-metal strip) switch closes, bipassing the resistors so the fan goes to full speed cooling the resistor coils.

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

Understood, got ya, understood -

I have the wire coming... The minds eye plan (for now) is to cut a section of 3 to 3 1/2 inches and test the resistance - looking for .4ohms...? Then, wrap it around something of right width, and see if I can rightly crimp it into the pack housing. Fingers crossed.

Thanks for the info about the bi-metal failsafe strip. I was wondering what part that played in the pack. There is a thing with these cars called the "magic blower" situation where the fan suddenly goes full blast - I guess because it is over heating. Happens a lot in these cars.

As it was, with this pack, 1st and 2nd fan speed did not work. 3rd and 4th did. Hoping this will solve the problem ALONG with maybe getting into the fan and seeing if there is a way that I can get it to run more efficiently...? Could just be age. The car is a 1986 Porsche 928. Could just be that age killed the part...?

I am SUPER appreciative of all y'all's support!
Thank you!

ps. The remaining resistance wire in this gizmo is much much finer, and I would imagine much more difficult to replicate. I hope I don't have to get into the OTHER coils,,,,, EVER, lol.

[redwire]

Re: PORSCHE 928 Resistor Pack - Need Help

What happens is with age the blower fan draws more current and it either blows the fuse or cooks these resistors. You did give that '86 fan some oil?
The exact resistance is not critical, you could bench test the two or just ballpark the resistance value.
If the fan is drawing say 40% more current, from 6A to 8.5A that is 2x more power dissipated in the resistors, which can be too much. Porsche might have not so great airflow over them or the resistors are a bit thin.
It's OK to use thicker wire (longer) to get the same resistance, just make a bit bigger coils.

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

What happens is with age the blower fan draws more current and it either blows the fuse or cooks these resistors. You did give that '86 fan some oil?
The exact resistance is not critical, you could bench test the two or just ballpark the resistance value.
If the fan is drawing say 40% more current, from 6A to 8.5A that is 2x more power dissipated in the resistors, which can be too much. Porsche might have not so great airflow over them or the resistors are a bit thin.
It's OK to use thicker wire (longer) to get the same resistance, just make a bit bigger coils.

Thank you, Red. And thank you all.
This forum is a gift!!!

[Unspun01]

Re: PORSCHE 928 Resistor Pack - Need Help

Honestly, While the others have provided some good helpful advice, don't waste your time looking for exact nichrome wires or trying to order spools of expensive stuff.

Go to an autowrecker and look for any 1980's or early 1990's GM vehicle. They usually have a nichrome wire resistor pack in the heater core fan box located under the hood on the firewall. only need to remove 2 screws.

These usually never fail and you will likely find a very close match of resistance, coil size, and resistance to make a reasonable substitution to fix your car.

An alternate option is to cut back the damaged wire and unwinde the next viable wire loop on your existing pack and straighten with pliers and re-attach to the connector. This wire cannot be soldered so you will need to re-crimp the wire, or provide an alternate means to reconnect (such as under a threaded screw, etc)

Best of luck.

[JayPoorJay]

Re: PORSCHE 928 Resistor Pack - Need Help

Honestly, While the others have provided some good helpful advice, don't waste your time looking for exact nichrome wires or trying to order spools of expensive stuff.

Go to an autowrecker and look for any 1980's or early 1990's GM vehicle. They usually have a nichrome wire resistor pack in the heater core fan box located under the hood on the firewall. only need to remove 2 screws.

These usually never fail and you will likely find a very close match of resistance, coil size, and resistance to make a reasonable substitution to fix your car.

An alternate option is to cut back the damaged wire and unwinde the next viable wire loop on your existing pack and straighten with pliers and re-attach to the connector. This wire cannot be soldered so you will need to re-crimp the wire, or provide an alternate means to reconnect (such as under a threaded screw, etc)

Best of luck.

Thanks for the suggestion...
It's an old Porsche 928, and if you look, people are willing to pay a pretty penny for these cars, the values are actually pretty surprising. In that, I am willing to pay a bit to try and replace, repair, modify, save, stuff. At the same time,,, I'm not a rich man by any stretch of the imagination. Quite the opposite. So,,, with an old motorbike that I rescued (86 Goldwing Aspencade, a 1990 XJS, a 2004 XKR (that went for a swim in a Texas flood) and another old Jaguar XK8 with an exploded engine - (3 Jag convertibles) all rescues, all well and running - yes, every penny counts.

The resistor pack is quite expensive new. Couldn't do it.

I will definitely look online for one of the alternatives you mentioned that fits,,, or if it doesn't fit maybe rewire stuff? These have a unique position in the HVAC system (see Google) and although things can be made to work, I would like to save this part if I can... I gave it a shot.

The "repaired" part is below. Not pretty, but not too bad. I installed it today, left it running a while and it didn't default to MAX to try and cool the pack. So far so good. There is definitely a range of speeds. Very good.

Thank you guys for ALL your help. If not for this site,,, and the forums for the cars and bike - none of this, my hobbys would NOT be possible. Thanks for the help...really...

PS... Is it ok/cool to post stuff like this here? Even though it's off subject?

[Unspun01]

Re: PORSCHE 928 Resistor Pack - Need Help

Oh, sorry I didn't mean to confuse you. I didn't mean to imply that any old resistor pack from an 80s or 90s GM car would work in place of your Porsche resistor pack.

I meant that you could get various diameters and coils of nichrome resistive wires from one of those GM parts to repair your Porsche one. I definitely recommend repairing your existing one.

I don't know what the original one looked like but prominent loops are necessary for cooling of the coils as air passes over them. If your fix works, then thats good. Otherwise try to make the new loops as close to old design/shape as you can.

[stj]

Re: PORSCHE 928 Resistor Pack - Need Help

are these in series with the injectors?

i seem to remember a car that had this - or a link plug depending on the type of injectors fitted.

[redwire]

Re: PORSCHE 928 Resistor Pack - Need Help

OP's is the resistor bank for the cabin air blower fan, I think it's 3-speeds.

EFI started in the early '80's with ECU outputs not having (current) peak and hold output drivers for the fuel injectors. I think the '86 928 uses Bosch LH 2.2, which is group injection. So only one ECU output to drive all 4 injectors at once. There is usually a ballast resistor (on some Volvo using same ECU) used which is under the hood as a couple 25W resistors in a metal housing. It's just to limit current in the injectors during wide pulse-width (lots of fuel) driving.
But I don't see them on the car's wiring diagram. So the Bosch fuel injector output might be a peak and hold but I doubt it. There's 8 power transistors.