prescott

Trying to hunt down a malfunctioning HVAC head (no A/C) and cooling fans come on upon cold start. Got a reconditioned AC head unit from Marc Anderson and after disconnecting the battery, I installed it. Radio installers had not connected the AC button (problem 1 fixed). Easy. Before buttoning it all up, I reconnected the battery. Turned the key and the AC fans did not come on, that's good. Fans didn't work, they did with the old head unit. Started the car, no AC, no fans.





Figured I've got a blown fuse. Turn everything off. Disconnect battery. Start going through all fuses related to HVAC. Got to 28 and 29 and noticed the discolored area above. Then noticed that 28 has a 30 amp fuse and not the 15 needed, and that 29 is 30 and not 25 AND they are fused together! A struggle to get them out and you see this. Can I replace the 5 fuse holder or do I need a whole new CE panel?

The Forgotten On

You can replace the 5 fuse holder relatively easily. Although I think it requires access to the back of the panel to remove the wires.

So it would be in your best interest to do a CE panel refresh and take it out of your 928 to see if any wires have melted behind the panel as those fuses got extremely hot and were possibly running over 30 amps through a 15 amp circuit.

prescott

Thanks Forgotten. I thought so. Putting in the order to Roger.

Is there a write up on how to do a CE refresh?

My brother in law lives in Newbury Park. He says there is somebody in TO with lots of 928s. Is that you?

Alan

This was NOT an over current issue despite how it looks. Damage like this never ever is with the correct fuses installed. Both fuses at 30A are in fact the correct size for this circuit - check again - you are mistaken about intended values.

This WAS a case of poor connections between the fuse and fuseholder. It is most likely the fuses are not even blown here - the current through the fuse will actually have been less than normal when this happened. Did you clean the fuse-holders, change the fuses or do any other maintenance recently? The good news is that the only damage on the wiring on the back of the panel will be right at the wires that connect to those 2 fuse holders (if at all) nothing else will be damaged due to overcurrent - only mechanical proximity to that heat.

This was completely unrelated to the HVAC changes - except perhaps that the fans would be on more of the time with AC in use - it would have happened in the summertime anyway.

You can replace the 5 fuse block - however you need a replacement that is the EXACTLY correct block - the correct numbers* and from the exact year (or you need to verify the internal connections - which can be very different).

Note if you don't do this you can severely damage your car!!!

Replacing the block is not as easy as implied - you need to understand the retention mechanism - source the replacement parts first - this will make it clearer to you how to remove the existing block without damaging the panel. remove the whole panel - it will make this much easier to do

Replacement parts may need to come from a donor panel - not sure if new is available still.

* Fuse-holders are all numbered from 1 - 45 - the labels are not easy to see with the panel in the car (under the bottom of the fuse). So there are 9 different block shells.
However internal to these fuse block the inputs at the top of the fuseholder may be connected together - this varied by year. Some have no inter-connections
some have all inputs bridged together and combinations & pairs in between. YOU MUST MATCH THIS CONFIGURATION.

Alan

prescott

Thanks for the good input Allen. The fuse box holder is toast as some of the metallic contacts/white plastic were melted onto the fuses. Once I get the CE box out, I will post photos of the wire connections. Thanks for the caution on the correct replacement fuse block. If Roger doesn't have one, I'll try 928 Intl.

I have not done any service on the fuse box since replacing a blown 53 relay October of 2014. However, I took the car in to get a new stereo installed about in December 2014, and just before that had my mechanic repair a leaking oil filler neck, intake refresh and oil change. The AC fan malfunction problem started AFTER the car had been to my mechanic (the AC didn't work anymore either), and they couldn't figure out the problem at first blush. Since it wasn't a serious problem during the Prescott winter, I told them I would bring it back when I had time. I have not noticed any electrical fire smell during my starts. Guess I found the problem....and a can of worms!

Imo000

I've seen crappy Chinese fuses melt like this before. It's also well documented on this forum.

Alan

I'd suspect that too - however these actually look like proper fuses. Hard to be sure - but they don't have the usual cheap fuse indication (slightly odd colors, strange rating markings, misshapen body molding and bright golden colored terminals).

Chinese fuse don't melt primarily because of problems with ratings (though they may also have them) - they melt because the dimensions of the blades and/or the plating quality is not up to standard and the electrical/mechanical connection to the fuse holder is poor, often the plastic compound used melts rather too easily also.

BTW if you have to get used parts you can't be sure of years - so buy several fuse blocks and test them to see which matches your configuration with a DMM in ohms mode. Its quite easy to do and you can mix and match terminals from spare holders to get the right configuration or cut the joined/bridged sections.

Alan

Alan

Read this for more context here:

https://rennlist.com/forums/showthre...eferrerid=6055

Alan

prescott

You are right about both fuses. They were correct 30A fuses. I got the wrong fuse info from the wrong year fuse chart. I really don't understand how they could have arced across like that! I've got emails into both Roger and Mark about CE panels/relays and fuses.

I hate it when I can't drive my car due to the wait on parts! Especially when we have great weather!

WallyP

The problem is not arcing.

When electrical power passes thru resistance, the result is heat.

The connections on a fuse should have virtually no resistance (yes, there is always some resistance, but it will be very, very low), so virtually no heat is generated.

If, because of corrosion, poor fit, etc., there is some resistance between the fuse legs and their panel connector, some heat will be generated. This heat will cause a little more corrosion, which causes a little more resistance, which causes a little more heat, which causes a little more corrosion, which causes a little more resistance, which causes a little more heat, which causes a little more corrosion, which causes a little more resistance, which causes a little more heat, which causes a little more corrosion... Eventually, the heat builds enough to cause the problem that the OP found.

When this heat damage is on the fuse holders, it is basically never the result of too much power flow, or the wrong size fuses, etc. Melted wiring is a different situation.

M. Requin

I do agree that the problem is not arcing, but I do think the pattern of overheating is odd, to say the least. I have spent some time trying, unsuccessfully, to come up with a hypothesis to explain this. Alan? Anyone? The best I can come up with ("best" may not be the correct word) is Chinese fuses. Some have been tested as basically being equivalent to buss bars.

Alan

As I already explained - Chinese fuses can be part of the problem - but for this kind of damage it is always due to contact problems. So it could be plating material, oxidation, terminal dimensional issues.

It isn't about over current so despite the fact that these fuse ratings may not be very accurate - that isn't the root cause of this. IT SIMPLY ISN'T EXCESSIVE CURRENT THAT CAUSES THIS KIND OF DAMAGE - many folks find this hard to believe - but as soon at the resistance starts building up in the fuse to fuse holder connection - the circuit current goes down - not up.

It will keep going down as the connection gets hotter and hotter and builds resistance. It is unlikely the fuse will ever blow electrically - though it may get ripped apart mechanically by the forces from the melting fuse body.

I suspect that these were actually acceptable quality fuses based on how they look (damage aside) - hard to tell for sure.

Now it is true that the fuse holders on the CE panel are of limited capacity and are not really ideal for 30A use (look at the pictures in the linked thread). Porsche should have come up with a larger fuse holder contact area IMO. You do not need to see much degradation before these high current fuses can start to see contact resistance build up. So using poor quality fuses in the past, excessive fuse plugging & unplugging or any attempt to mechanically clean the fuse holder with a contact file would likely start a downhill slide. Any water damage causing excessive corrosion or oxidation would likewise initiate long term demise.

Alan

Alan

They really didn't... they just got hot & melted due to poor connections to the fuseholder.

Arcing happens with high voltage - there wasn't any here, fuses blow due to excessive current - there wasn't of that here either.

Test the fuses - they are probably good - though it does look like the two blade contacts are almost touching now.

Alan

prescott

CE panel, new (non Chinese) fuses, and new relays are on order and in shipping currently....thanks Tom at 928Intl!

Cleaning the CE panel and prepping it for the new relays and fuses will be the next part of the project. I've always used deoxit D5on just about everything electrical on my car to good results. Alan has warned about stabilant 22a on relays that potentially could have inductive problems.

Is it ok to use deoxit on cleaning the relay contacts on the CE board? I know its ok on the fuse contacts. I was going to take the advise on related threads (I think Mr. Merlin) and clean the contacts with Deoxit, 91% isopropal alcohol, and a strip of satin cloth instead of metal files/emory boards and the like.

Thoughts?

Alan

Must have been a different Alan then (?) - I'd use stabilant on anything here - all relays have inductance - not really a major problem. Best thing you can do is to have good connectivity..

Alan