348 Dead HVAC

Hello, I have a colleague (non-FChatter) with a 1991 348. Called me today and said his HVAC went completely dead (no lights, no clock, nothing).

We checked all the fuses, and none are blown (I just read about the "hidden fuse", I think we may have missed it so I will give him a call and get him to check that one too).

I did a search on "348 HVAC", several threads popped up, but none with the same symptoms as this one. They are all either "lights blinking" or A/C blower not working, but none that are completely dead.

Any suggestions? We may throw his unit into my 348 just to see if the unit really is dead or if there is an electrical fault in his.

 

Definitely check the hidden fuse. If that does nothing, check the connector plug that plugs in the back of the climate control unit. Often just unplugging and plugging back in will make it happier.

 

+1 hidden fuse.

One mine, (1990) the multi pin connector on the back was missing its 'lock tab' or whatever you call it on the plastic harness. That allowed the whole plug to pull out of the controller on occasion, and it'd go dead.

As a bit of consolation, the failure mode on this is *rarely* a complete 'dead' unit with no lights. They go dead because of the fuse or harness. They screw up and go bonkers because of crappy electronics from Borletti (the manufacturer)

 

I had exactly the situation you describe and it was the "hidden" 30 amp fuse. Bet that will fix it. Good luck.

 

Thanks guys. It was the infamous "hidden" fuse. We missed that the first time we checked all the fuses, I suppose that's why it's called the "hidden" fuse lol.

One of pins was charred and part of the plastic on the same side was melted/charred.

New fuse, back in action. Thanks again.

 

Great news.

 

What you need to do now, to avoid the same situation in the future, is to replace that fuse and fuse holder with a higher quality unit, like a glass fuse.

 

I replaced mine with a 30amp circuit breaker (which kept popping - grrrr!!!!)

However, after recently replacing my fan motor power transistor it appears to have stopped doing that so although I don't have 100% concrete proof my guess is that the melted fuse may well be caused by the increased draw when a power pack is going bad...???

I did a lot of experimenting....

348 DIY "Restoration" - Page 157

 

Where is this infamous "hidden fuse" located BTW?

Thanks

 

Under the trunk lining, on the passengers side of the fresh air intake box. Dumb place to put it, but there you go.

 

Look for the 2 green relays.

The fuse melts and makes the connection unreliable
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I have noticed an obvious correlation between fuses that cause "problems" and the hi- draw circuits that these fuses are protecting. It appears a lot of these issues are related to old or worn fan motors. Remember the fuse is there to protect the circuit and if the engineer who designed the wiring loom determined that a 30 amp fuse is sufficient (example the hvac system) then a blown or melted fuse is indicative of another problem. As in cooling system fans, grime, salt water, dirt , age and generally the most common ie. a lack of lubrication on the spinny bits can cause a huge increase on the current draw through the fuse when rad fans "kick" on or hvac fan is turned on. I would expect some of the power window issues also have this common thread. I can tell you having spent a few years rebuilding a Jag XKE every electric motor in the car was disassembled and cleaned, bushings and bearings lubed, armatures polished and reassembled. Never had a problem with slow motors or blown fuses again! Even the windshield washer fluid motor was easily refurbished. Just a thought.........JGW

 

Interesting thought. To confirm it what we need is someone with a melted "hidden fuse" to check how the fan spins. I believe if the recirculation is set to off so the flap on top of the air box in the front trunk is open, the the fan is visible, and perhaps with a finger(ignition off) screwdriver, chopstick... whatever... spin the fan blades and see how rough it feels and how well it rotates.

 

I ended up replacing the fan motor resistor pack and so far it appears to have cured the problem.

I did check the rotation of my blower as part of my investigation and it rotated freely.

BTW I replaced the melted fuse with a circuit breaker which trips out in case of overload

Herses how I got the resistor out...

348 DIY "Restoration" - Page 154

 

Well, I think I found the beginning of my problem. Although the HVAC unit still appears to be working, there's no coolness at all. The system was working fine until recently.

Here's my "hidden fuse" and the relay connector nearby doesn't look so good either.
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Yikes. At least you found the problem relatively easy.

 

Yes, but only for starters. Next I need to find out what's causing the added resistance.

 

9 times out of 10 it's resistence in the wire due to corrosion. Check the terminals and repin if necessary.

 

I did all that when investigating mine and it was all very inconclusive. I even took measured the draw at each fan speed too - see link below

348 DIY "Restoration" - Page 157

I strongly recommend swapping the fuse for a breaker which "pop" rather than just melt then you can sort of learn what triggers it. I discovered in my case that as long as I didnt go above 4 bars on the fan speed it would be fine. Eventually I swapped the power transistor and this has 95% cured it. Just done a 400 mile trip and it popped once on a baking hot day with everything on full. other than that one occaison it worked perfectly at all speeds

 

Yep, the hidden fuse in my 348 was all melted when I first got it. It looked VERY similar to the melted photo above.

I eventually just cut it out, and replaced it with a dramatically larger glass fuse. MUCH more surface area for the electrical contact.

Here are a few pics I posted back in 2010/2011 when I first made this change:
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Thank you