F50 A/C: Fuse #2

I will try to explain this clearly:
USA model F50, which I have owned for many years.

Two dealers have failed to solve the issue, one said "just put a bigger fuse in."

The 10 amp fuse listed as #2 in the fuse block, controls the "air temperature control system and windscreen wiper control" per the workshop manual

The fuse has frequently failed, and to better determine if it was the fuse connectors (which are fine and have good clamping force), I have removed them from the fuse block and with a fuse in place, tracked the amperage draw through the circuit. When the car is started, the circuit draws 5.5 amps and will run that way for quite some time (10 minutes) and then the current draw will begin to climb, reaching approximately 14.5 amps and the fuse fails. The fuse fails not as a dead short, but as current draw/heat melting the fuse.

Fuse 2 has power coming form the Key on circuit (Blue black wire "LN") and the other side is a white ("B") wire leading to the multiplexing cable connection. The white wire eventually leads to pin B6 on the Number 94 "Control unit for for air temperature control system speed adjustment"


The following are controlled or connected to the control Unit (#94):
Pin A1 Speed/temp control switches (on the center tunnel aft of the gear lever)
Pin A2 As above
Pin A3 as above
Pin A4 to #95 Air Temperature control system control unit, then to #77 "Instrument Panel system connection with air temperature control system"
Pin A5 to fuse relay block "Evaporative Fan Control" Relay #23 (there is a large amperage wire (2.5mm) from #77 as part of this connection at the relay)
Pin A6 to fuse relay block relay #23 (this wire powers the coil of relay #23)
Pin A7 to #77 "Instrument Panel system connection with air temperature control system"
Pin B1 to #48 Cable Multiplexing connection
Pin B2 to relay block and relay #13 Water Electromagnetic valve
Pin B3 to #33 "Air temperature control system compressor control switch"
Pin B4 to #54 Cable Multiplexing connection
Pin B5 to #95 Air Temperature control system control unit, then to #88 "Dehydrator Filter with MIN and MAX pressure switches"
Pin B6 is the white ("B") from the power received from the fuse block/ignition circuit

Any ideas as to what is causing this amperage draw through the control unit #94, which seems to have all low amperage sized wires?? My guess is a relay is failed, however the A/C system works well until the fuse pops.

I noticed a current rise under acceleration to 8.7 amps, not enough to pop the fuse. a few minutes later the current started to climb over a 60 second period to 14.5 Amps or so, then fuse failed.
Thanks,

Groupe B

 

sounds like your only half way there now you need to current ramp the rest of the pieces on your list untill you can figure out what the excessive draw is

wouldnt it be sweet if you could go in with an SD2 and it was cool enough to throw a code or give enough data to help diag like "code 4532 excessive draw on watervalve" makebelive
code

probobly not, but it might,
never hooked my SD2 up to an F50 climate yet

 

Thanks, my thinking too. I was planning to test each circuit, but thought that I would toss the question out there to see if someone already knew that component XX had a history of failure. I will probably buy/borrow a couple more testers to shorten the process.

I love electricals up to the point where the small electronic box becomes the culprit. They do not give you the satisfying puff of smoke old school electrics do at point of failure.

RE the SD2: It was not a help; the two dealers both have/had that equipment and it turned up nothing. Therefore the "Put bigger fuse in it" notation on the service sheet.

GB

 

Good day GB,

Putting a larger fuse could result in wires melting and/or a fire.... For a dealer (or any technician for that matter) to say just install a bigger fuse without any knowledge as to what is happening is really reckless and irresponsible and does not reflect well on their skills. From the Italian wiring I have seen on the 308s and my Boxer, the original designer's were somewhat "optimistic" with regards to the various wire's current capability. My suggestion would be to keep doing what your are doing and locate what is causing the excessive load... Further, it seems that you are more capable and have the right approach as opposed to the "professionals" you sought.

Cheers,

Sam

 

Hi There,

recently i have same on my F355. I havent measured current yet, but 20A, 25A, even 30A fuses all melt with typical plastic meltdown for the fuseholder and heat wiring.
This specific fuse connects to a 4amp brown cable which is suposed to feed the fan, but then again fan itself works fine. I have downloaded all wiring diagrams for the complete A/C system and will check further whats causing this. It looks similar to what is described above so if i found something, i will post it here. Meanwhile any suggestions welcome here

J