Weird Heater Blower Relay Burnt Issue
07-31-2011, 08:05 AM
#16
Burning Brakes
Thread Starter
Yes, that's a good idea with the fuse, sometimes if they are stressed, the fuse element goes a little heated and bendy and it will then blow later at a much lower current than what its rated for.
But sure, the plug I replaced above is the same part number as the one used for the a/c condensor blower and oil cooler blower relays in the front fusebox. So some ballast-resistor issues might possibly mean that someone else might have to do the same DIY replacement up front one day.
But sure, the plug I replaced above is the same part number as the one used for the a/c condensor blower and oil cooler blower relays in the front fusebox. So some ballast-resistor issues might possibly mean that someone else might have to do the same DIY replacement up front one day.
07-31-2011, 12:08 PM
#17
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OK, now that I'm over the envy of that clean-enough-to-eat-off engine bay, I don't see that you ever determined the cause of the burned relay. That would concern me.
I know that these heater blowers are historically (and I'm going back to my mid-70's cars) prone to freeze up, either from bearing wear in the motor and/or the squirrel cage fan binding on the fan housing. When I was totally broke in the late 80's, I replaced the bearings in my '75 fan with some brass bushings fashioned from bearings out of a pick-n-pull wiper motor off a Japanese car.
The fans are really quirky and if it's just the fan binding, they can freeze up but then be given a new lease on life by a little adjustment of the fan mounting to the housing. The one in my 964 did that, and after adjustment it's now been working normally for about two years.
I'll bet the fan froze at some point and burned that relay. You might want to keep an eye (or ear) on it.
Great first post. Good luck, and if you need help with the Pcar, you won't find better than these Rennlisters.
I know that these heater blowers are historically (and I'm going back to my mid-70's cars) prone to freeze up, either from bearing wear in the motor and/or the squirrel cage fan binding on the fan housing. When I was totally broke in the late 80's, I replaced the bearings in my '75 fan with some brass bushings fashioned from bearings out of a pick-n-pull wiper motor off a Japanese car.
The fans are really quirky and if it's just the fan binding, they can freeze up but then be given a new lease on life by a little adjustment of the fan mounting to the housing. The one in my 964 did that, and after adjustment it's now been working normally for about two years.
I'll bet the fan froze at some point and burned that relay. You might want to keep an eye (or ear) on it.
Great first post. Good luck, and if you need help with the Pcar, you won't find better than these Rennlisters.
07-31-2011, 08:32 PM
#18
Burning Brakes
Thread Starter
Yes, it does bug me too that the cause for the relay meltdown in the first place isn't really obvious.
After the repair was done, I'd pop off the fusebox cover after a drive, and have a feel around to see if anything is getting hot or if there are any weird smells. Did this for several days and still check it from time to time. Seems to be ok.
I think what might have happened was that the relay might not have been plugged in right. When I got the car, it was half-unplugged and sitting crooked. It makes sense that the subsequent poor connection from something like that would cause a lot of heat when the full 30A goes through it.
When I refitted the new relay, it did the same, it was hard to get it to plug in all the way. Eventually I bent one of the relay prongs slightly, and only then did it slot home nicely. I guess it's possible that at some point in the past, the relay died, and whoever refitted it did so roughly, and bent one of the female connectors inside the plug. And then the relay doesn't fit and later...voila...you get relay plug on toast
After the repair was done, I'd pop off the fusebox cover after a drive, and have a feel around to see if anything is getting hot or if there are any weird smells. Did this for several days and still check it from time to time. Seems to be ok.
I think what might have happened was that the relay might not have been plugged in right. When I got the car, it was half-unplugged and sitting crooked. It makes sense that the subsequent poor connection from something like that would cause a lot of heat when the full 30A goes through it.
When I refitted the new relay, it did the same, it was hard to get it to plug in all the way. Eventually I bent one of the relay prongs slightly, and only then did it slot home nicely. I guess it's possible that at some point in the past, the relay died, and whoever refitted it did so roughly, and bent one of the female connectors inside the plug. And then the relay doesn't fit and later...voila...you get relay plug on toast
08-01-2011, 08:30 AM
#19
Rennlist Member
The internals of that relay look very similar to a DME relay; two coils, etc.
It would have been nice to post the relay schematics...for example, this is for a DME relay...
A burned contact or connection is usually due to to high a current or high resistance contact. A high resistance contact can make it look like a connector is at fault.
The following relay from a fellow 993er shows a section of the board that overheated (center top)...
You'll also notice one solder joint that is totally unacceptable (upper left of the large solder pad).
On top of it, one relay contact measured varying resistance (when switched) between 50 and 200 milliohms. A relay contact in a relay of this capability (15A) should be in the 10 milliohms and under range. After cleaning the contact, it measured 2.2 milliohms.
Why is this important? To keep contact temperatures down. The same applies to a crimped on connector (which is why I always solder the tip where the wires exit the crimp. The key is not applying too much solder or it'll wick down the wire and make it inflexible which will cause it to break in time due to vibrations).
The following is another DME relay where the solder joints in the yellow rectangle were faulty; all solder joints on this board were re-soldered...
The above DME relay failed for the first time after nearly 16 years (it was the original that came with the car based on the date code). All it took was removal of the relay (they are in there tight and it took quite a bit of force to pull it out) for the cold solder joints to finally fail....and for the car not to start.
It would have been nice to post the relay schematics...for example, this is for a DME relay...
A burned contact or connection is usually due to to high a current or high resistance contact. A high resistance contact can make it look like a connector is at fault.
The following relay from a fellow 993er shows a section of the board that overheated (center top)...
You'll also notice one solder joint that is totally unacceptable (upper left of the large solder pad).
On top of it, one relay contact measured varying resistance (when switched) between 50 and 200 milliohms. A relay contact in a relay of this capability (15A) should be in the 10 milliohms and under range. After cleaning the contact, it measured 2.2 milliohms.
Why is this important? To keep contact temperatures down. The same applies to a crimped on connector (which is why I always solder the tip where the wires exit the crimp. The key is not applying too much solder or it'll wick down the wire and make it inflexible which will cause it to break in time due to vibrations).
The following is another DME relay where the solder joints in the yellow rectangle were faulty; all solder joints on this board were re-soldered...
The above DME relay failed for the first time after nearly 16 years (it was the original that came with the car based on the date code). All it took was removal of the relay (they are in there tight and it took quite a bit of force to pull it out) for the cold solder joints to finally fail....and for the car not to start.
Last edited by IXLR8; 11-20-2011 at 06:04 PM. Reason: Typo
08-01-2011, 10:04 PM
#20
Burning Brakes
Thread Starter
Thanks, that makes sense.
If the relay wasn't plugged in properly, it would have a high resistance/poor contact at the spade, and then the contacts inside the relay get burned, which causes more resistance and then the whole thing spirals into a melted doom
But I did the same as your advice with the solder, I melted some solder onto the tip of the wire where it poked out of the crimp, and then used a small blowtorch to get ti to wick down into the crimped area. The wire was still flexible after that, so I don't think there was enough solder to wick down that far (it's good advice not to use too much solder!)
I got a new DME relay, and the old one is residing in the tool pouch as a spare
If the relay wasn't plugged in properly, it would have a high resistance/poor contact at the spade, and then the contacts inside the relay get burned, which causes more resistance and then the whole thing spirals into a melted doom
But I did the same as your advice with the solder, I melted some solder onto the tip of the wire where it poked out of the crimp, and then used a small blowtorch to get ti to wick down into the crimped area. The wire was still flexible after that, so I don't think there was enough solder to wick down that far (it's good advice not to use too much solder!)
I got a new DME relay, and the old one is residing in the tool pouch as a spare
05-09-2013, 09:02 AM
#21
2nd Gear
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Burnt blower relay
Great post, I learned so much! Thanks! I've had this same problem on my 90 C2. I have 2 questions...what is that silver box in the relay/fuse box? I have the plug for that but not sure what I'm missing there....also, I need to order a new relay connector also (same as yours) which one of those numbers is the part number on your pic? Thanks!
I ordered this little guy from my friendly neighbourhood Porsche dealer:
The thing is, there really isn't much room to work. This is after the heater downpipe is removed to free up some space in front of the fusebox.
Unbolt the relay holders from the fuseboard...
My theory is that when I bought the car, the relay was half hanging off and the heater fan was working only intermittently. The resultant poor connection caused a bit of arcing, which melted the plastic connector. I then discover the loose relay, push the relay back in position and it all works again. But all that melted plastic made it a high-resistance connection, and the 30A draw of the heater fan then melted it some more.
The heat then melted some of the internal insulation in the relay, and caused a short, which made the heater fan run all the time, even with the engine off.
To free up even more room (you'll notice that the length of wire going to the relays are very short) I even removed the back of the fusebox too.
My plan was to remove the existing terminals from the melted plug, replace the ones that were poor, but recycle the good ones. So this meant dremeling the old connector to get the terminals out.
You can see that the terminals have barbs, which mean that once they're popped in place, they don't wanna come out You can also see that they're quite delicate.
Once the old plug is all hacked apart, the terminals are free (and you can see only one of them is burnt, but pretty bad)
...the remains of the old plug
Pop on a new terminal to replace the burnt one (it's just a normal crimp spade connector, with solder melted into it)
Carefully slide the terminals back in place..
And button it back together again.
I put the heater on full blast, and went for a bit of a drive, and it seems to work fine again. The blower fan doesn't stay on after the engine is turned off (like before) and the fan comes on only when you dial up some heat.
Took the fusebox cover off and the relay and wiring aren't getting warm, and there isn't any weird smell, so fingers crossed this has fixed it for good.
The thing is, there really isn't much room to work. This is after the heater downpipe is removed to free up some space in front of the fusebox.
Unbolt the relay holders from the fuseboard...
My theory is that when I bought the car, the relay was half hanging off and the heater fan was working only intermittently. The resultant poor connection caused a bit of arcing, which melted the plastic connector. I then discover the loose relay, push the relay back in position and it all works again. But all that melted plastic made it a high-resistance connection, and the 30A draw of the heater fan then melted it some more.
The heat then melted some of the internal insulation in the relay, and caused a short, which made the heater fan run all the time, even with the engine off.
To free up even more room (you'll notice that the length of wire going to the relays are very short) I even removed the back of the fusebox too.
My plan was to remove the existing terminals from the melted plug, replace the ones that were poor, but recycle the good ones. So this meant dremeling the old connector to get the terminals out.
You can see that the terminals have barbs, which mean that once they're popped in place, they don't wanna come out
You can also see that they're quite delicate.Once the old plug is all hacked apart, the terminals are free (and you can see only one of them is burnt, but pretty bad)
...the remains of the old plug
Pop on a new terminal to replace the burnt one (it's just a normal crimp spade connector, with solder melted into it)
Carefully slide the terminals back in place..
And button it back together again.
I put the heater on full blast, and went for a bit of a drive, and it seems to work fine again. The blower fan doesn't stay on after the engine is turned off (like before) and the fan comes on only when you dial up some heat.
Took the fusebox cover off and the relay and wiring aren't getting warm, and there isn't any weird smell, so fingers crossed this has fixed it for good.
05-09-2013, 11:45 AM
#23
"I know that these heater blowers are historically (and I'm going back to my mid-70's cars) prone to freeze up, either from bearing wear in the motor and/or the squirrel cage fan binding on the fan housing."
"The fans are really quirky and if it's just the fan binding, they can freeze up"
"I'll bet the fan froze at some point and burned that relay."
That's it!
"The fans are really quirky and if it's just the fan binding, they can freeze up"
"I'll bet the fan froze at some point and burned that relay."
That's it!
04-06-2014, 09:39 PM
#24
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I have the same question as post 21. The grey metallic box that is in the last pic on post 4, what is it? I don't have one but am curious.
I plan to resurrect this thread as I have discovered the exact same issue with my relay and relay holder (melted). I'm getting strange signals to the relay at odd times and wish to check with a few experts.
I plan to resurrect this thread as I have discovered the exact same issue with my relay and relay holder (melted). I'm getting strange signals to the relay at odd times and wish to check with a few experts.
04-06-2014, 10:45 PM
#25
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So after 12ish hours of troubleshooting, I have had two moments where the rear engine fan seemed to work as expected, except for not turning off. Of course, there was a lot of other weird activity of the rear engine fan during the day, but I'll cut to the chase.
Pulling the relay, I tested the high amp spades (low speed and high speed) for continuity - checked ok.
I tested the low amp signal inputs to the relay, they closed the relay as they should. so the relay is fine.
I then tested the voltage of the signals to the relay box (the thing that the relay clips into). I note an ever present 12v at the low voltage signal side, telling the fan to run at low speeds even with the car not started at 0C outside. So I thought that I have a bad NTC temperature sensor, so I pulled it and checked it for resistance as it hit it with heat. The ohms decrease to zero with heat applied, seems to be per design. I didn't check a 20C and 45C as I should, but I'll order a replacement regardless.
So I reassembled everything, and as predicted, the fan turns on to low speed without the key in. I think to pull the plug off the temp sensor, and there is no change. With a decreasing resistance with increasing heat, an open circuit should suggest the engine is cold, no? Does the CCU need some feedback from the NTC, rather than just a completely open circuit?
Struggling here... anyone?
Pulling the relay, I tested the high amp spades (low speed and high speed) for continuity - checked ok.
I tested the low amp signal inputs to the relay, they closed the relay as they should. so the relay is fine.
I then tested the voltage of the signals to the relay box (the thing that the relay clips into). I note an ever present 12v at the low voltage signal side, telling the fan to run at low speeds even with the car not started at 0C outside. So I thought that I have a bad NTC temperature sensor, so I pulled it and checked it for resistance as it hit it with heat. The ohms decrease to zero with heat applied, seems to be per design. I didn't check a 20C and 45C as I should, but I'll order a replacement regardless.
So I reassembled everything, and as predicted, the fan turns on to low speed without the key in. I think to pull the plug off the temp sensor, and there is no change. With a decreasing resistance with increasing heat, an open circuit should suggest the engine is cold, no? Does the CCU need some feedback from the NTC, rather than just a completely open circuit?
Struggling here... anyone?
