Repairing a heater resistor - HowTo

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janijoeli
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Repairing a heater resistor - HowTo

Post by janijoeli » Sun 11 Aug, 2013 19:06

Is only the fastest speed on your Z3's heater working? If so, it is very likely that the heater resistor pack is broken. A new resistor costs £30 or so, which is an awful lot considering that it is just three springs and a cutoff switch attached to a small piece of plastic. Manufacturing costs cannot be more than 30p. Instead of bending over for a car parts dealer, I chose to attempt to fix the switch... with great success! :D

Here is an excellent writeup about how to get to the resistor, but the author rather just replaces the resistor with very minimal attempt to fix it first. This is where this guide steps in.


DISCLAIMER: If you attempt this repair, you attempt it at your own risk. I take no responsibility if you end up burning your car, killing your cat, getting cancer from breathing solder fumes or something else, or if this fix doesn't work on your case. However, it worked fine for me.


THINGS NEEDED FOR REPAIR
Mandatory:
Soldering iron (35-50W or a basic temp adjustable works fine)
Solder
Multimeter
Small flat screwdriver
Electronics cleaning spray (a.k.a. IPA solvent a.k.a. IsoPropanol Alcohol)
Cotton swabs (a.k.a. Q-tips)
An hour
A bit of patience

Optional (but very useful):
Canned air
Solder paste
If needed, a bit of very fine sanding paper (400 to 800) for the switch contact
If needed, a de-soldering pump
Coffee, tea or other (may be adult) beverage


HOW DOES THE DAMN THING WORK?
So, let's start by analyzing what this magical little dongle does. In short, not very much.

When the heater motor is on speed 1-3, the electric current consumed by the heater motor goes through the resistor pack. On full speed the resistor pack is bypassed, thus speed 4 is not affected by a broken resistor pack.

Resistor pack consists of three small springs. The current going through the springs heats them up. Heating takes energy, which is off the energy going to the heater motor as electric current - in other words, some of the electric current "disappears" as heat, and a little bit less current comes out of the pack than went in. The less current, the slower the fan motor spins. By changing on how many of the three springs the current goes through, the resistance can be varied, thus getting three speeds out of the pack. Simple.

Below is the circuit diagram for the resistor pack. There are four connector pins. Current comes in from pins 1, 2 and 4, one at a time (adjusted by the heater switch), goes through 1, 2 or 3 springs, and comes out from pin 3, which is connected to the heater motor. If there is no connection between one or many incoming pins and outgoing pin, then one or many speeds are not working.

In addition to the springs, there is a safety switch just before the outgoing pin. I guess it is supposed to break the circuit if the pack gets too hot, to prevent it catching fire.



FINDING THE BROKEN PARTS OF THE CIRCUIT

In this picture, pin 1 is on the left and 4 on the right. I am measuring resistance between pins 3 and 4 (thus connection through all springs, and the safety switch). There is no connection. If there would be, then probably just a proper cleaning of the pins and the connector under fan motor would be sufficient.


Next I measured the connection pin1 -> pin3 and pin2 -> pin3. Neither was connected. This tells me that the fault is in a part of the circuitry shared by all three inputs - that is, the safety switch, and the end of R3 which is connected to pin 2 and R2.
Pins 1, 2 and 4 have a connection with eachother (with very little bit of resistance), which indicates that there are no faults anywhere else.

As a sidenote, all pins are nice and clean with a shiny layer of tin on each of them, so no problem there. There would be no point in sanding them as some have done. In fact, this would make them worse, as the tin layer would go from smooth to rough, leaving more surface area in contact with air, thus speeding up oxidation/corrosion. I just gave them a good rub with a cotton swab soaked in IPA solvent, just in case.


FINDING POTENTIAL WEAK SPOTS IN THE BROKEN PART OF THE CIRCUIT

So as the connection is broken somewhere between R3 and pin 4, let's find all the joints between those two spots, measure the connection through joints, and clean and solder broken ones. Or as I did, just solder them all.

From this photo can be seen that the safety switch itself has three potentially broken joints: the contact on the switch itself, and both ends of the switch.


From the other side it can be seen that the switch is just riveted in place. The copper coloured contacts (under second pin from the left and the one at the right front corner) belong to the switch.


I measured the part of the circuit that goes from pin2 through R3 to where the switch connects to, and it was OK. However, both ends of the switch didn't connect with the circuitry at all, and the switch itself had no connection, so I decided to solder these three spots.


SOLDERING WEAK SPOTS

First I soldered the riveted connections on top (connector) side of the resistor pack. As the ends of the switch and the metal strips connected to the springs are made of some other metal than the rivets, the surface under the rivets has oxidized over years and connection lost.

All you need to do is:
- Scrape off any built up sh*t and oxidation from both the metal strip and the rivet with a small flat screwdriver, until both surfaces are clean and shiny.
- Blow the dust off with some canned air if you have some.
- Wet a cotton swab with IPA solvent (use liberally) and rub the cleaned areas throroughly.
- Again, if you have some, use canned air to blow off the solvent and dry the contacts, or just wait until they are dry.
- If you have solder paste, it will make soldering a whole lot easier. Rub a thin layer of paste everywhere on the cleaned parts of the metal strip and the rivet with a cotton swab.
- Now solder the metal strip and the rivet together. If both surfaces are clean (and especially if you used solder paste), after a fair amount of heating the solder should just spread nicely and evenly on the cleaned areas. If not, you need to either clean the surfaces better, use solder paste, or your soldering iron doesn't have enough puff to heat up the metal strip. Having an adjustable soldering iron with enough wattage (35-50 should do) should help immensely. Just set it to fairly hot, and increase temp if solder doesn't stick to the metal strip. I have a cheap Velleman one and it worked fine.

Here is a photo of the cleaned contact, before solder paste and soldering. It looks like not properly cleaned, but that's just because of the direction of the scratches on the surface and direction of light, it really was shiny all around the rivet.


Both contacts soldered. I used too much heat on the connection under pin3, as the plastic started to melt a bit.



Then I switched over to the spring/switch side of the resistor pack. Here half of one of the switch end connections cleaned. You can see how much stuff had accumulated on top of the connections!


Throroughly cleaned and soldered:


Then cleaning and soldering the other end:





What comes to the switch contact itself, I would suggest cleaning it first to see if just cleaning would help. Tear off most of the cotton off a swab, like this:


Wet the swab with IPA solvent. Then hold the switch open by prying it upwards from just behind the contact with a screwdriver. Be careful not to overdo this, as otherwise you might bend or even break the switch. Rub both contact surfaces with the cotton swab.
If this doesn't help, run very fine sanding paper between the contacts, cleaning both sides, until it works again. Alternatively, scrape the surfaces with a small screwdriver. Note that it must really be a small one, otherwise you'll end up bending the switch.
If even this doesn't help (as it didn't in my case), cover both sides of the contact with a thin layer of solder paste and use very little bit of solder to re-surface the contacts. Use a screwdriver to pry the contacts apart until the solder has cooled off to prevent them from sticking together. You want just a very thin layer of solder! Otherwise the switch might not work as intended. If you have a de-soldering pump, use it to suck out any excess solder.

A re-surfaced switch contact:



After soldering all connections, re-measure resistance from pins 1, 2 and 4 to pin 3. If all look something like this... A winner is you! :cheers

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Brian H
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Re: Repairing a heater resistor - HowTo

Post by Brian H » Sun 11 Aug, 2013 22:56

Nice write up, I am sure it will be helpful to others :thumb: Just a note: The resistor pack you have and its location is for cars with AC.

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janijoeli
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Re: Repairing a heater resistor - HowTo

Post by janijoeli » Mon 12 Aug, 2013 00:06

Cheers Brian, I didn't even know the resistor on non-AC cars is different!
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Brian H
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Re: Repairing a heater resistor - HowTo

Post by Brian H » Mon 12 Aug, 2013 10:09

janijoeli wrote:Cheers Brian, I didn't even know the resistor on non-AC cars is different!
Yes, the resistor in the non AC cars is in the centre console in the passenger footwell, The size and shape of the resistor pack is different but I would hazard a guess that it is the same style circuit and the same principle as above would apply for the fix.

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Gazza
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Re: Repairing a heater resistor - HowTo

Post by Gazza » Mon 12 Aug, 2013 11:38

Moved to the Z3 Knowledge Base

Thanks :wink:
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Gazza
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Re: Repairing a heater resistor - HowTo

Post by Gazza » Tue 16 Feb, 2016 11:30

I changed my resistor today, the clips holding the plastic cover were a bit fiddly and didn't just 'pop out' as suggested.

All in, the whole changeover took about 45 minutes.
Gazza

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wonderloaf
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Heater Control Resistor - additional fix

Post by wonderloaf » Tue 16 Feb, 2016 19:47

janijoeli recently posted an excellent article on how to repair a heater resistor, but also after my recent experience of this problem just wanted to add another possible another 'fix'.

What I found was that although I went through all the actions of cleaning, soldering, buzzing out the circuit as detailed in the article I still couldn't get mine to work properly. It was only then that I realised that there shouldn't be a gap between the two leaf springs of the thermal switch, so I put a flat blade screwdriver under the lower U shaped lower contact and tweaked it up until it only just contacted the upper leaf spring, creating the required electrical contact. After that all is good .. in the end the fix was really simple!

Thought I'd post this in case other people have the same experience as me, I was about to get a new switch from BMW but this managed to save me a few quid! :)
2001 Titan Silver 2.2 Sport
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Gazza
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Re: Repairing a heater resistor - HowTo

Post by Gazza » Wed 17 Feb, 2016 17:49

If you do the Wonderloaf modification, you must test the bi-metallic cut-out switch by heating it to ensure that it breaks contact when the circuit is overloaded.
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