Need some expert help, fuses blowing and cold start
 

Ive read a ton of forums and can't seem to find the answer or answers that might solve this problem so I thought id just ask, im truly hoping someone makes me feel stupid by solving this as im just a bit stumped right now. There's 2 concurrent issues and I have no clue if they are related. The car is a 1983 row 911sc, 3.0l. It has the classic retrofit air and upgraded alternator in it, as well as the classic retrofit cdi (all new a year and a half ago) otherwise pretty stock.

Problem 1. Im blowing fuses. sometimes 2 or 3 in an hour, sometimes one a day, the issue being, its not always the same fuse. This has happened when the car is starting, its also happened while running. It started with the rear defog fuse, then window, then 3 times I had the fuel pump fuse blow while I was driving, that seems to have passed and its gone back to blowing random fuses, like the front blower motor.

Ive unplugged the fuses in the rear, there isn't a rear defog even plugged in, in my car. There's no o2 sensor plugged in, no lambda. Ive checked the volts at each lead at the fusebox and its all 12v, although the interior voltmeter in the lighter socket is saying it hits 13 to 13.6 sometimes. What could be blowing random fuses? What's the best way to track down why? The car has been driven by me for 2 years now and this has never happened. Could it be the starter? Could it be blowing fuses upon startup? I've had some weird starts lately where it kind of starts before it fully turns over.

Problem 2. Could this be related. If I set the engine idle when the engine is warm to 950, the car runs beautifully but upon cold start it runs around 500-750 and it stutters like it wants to die. If I adjust idle at cold start to 950, when its warm it runs at 1200-2000 rpm. I started replacing the vacuum lines. I replaced spark plugs, dist cap and rotor. The dizzy has the vacuum with the single line so advance only, I've tested it with suction from my mouth and it appears to work fine but I can't necessarily feel any suction from the advance line. The wur is the correct row wur, all the other lines are tight and in the right place I went over that with help from the threads. I checked idle, I've never touched the air mixture but I've checked and set the timing. When she's warm she runs like a bat out of hell. Just those few minutes of warm up.

any thoughts 911 geniuses...

Could any of this be related.

Have you checked the alternator to see how well the voltage regulator is working?

Not sure if this will help but it's always a good idea to clean & tighten all your grounds including the ground cables on the engine/trans and the negative battery cable. Many electrical problems associated with these cars are the result of bad grounds. Also make sure the fuse holders are clean and hold the fuses tightly and the screws that hold the wires on the fuse panel are clean and tight. Good luck

Thanks, I've actually started on this, I've been sanding down all the contacts on the panel that have been blowing and bending the holders so they are tighter. Next up the battery contacts and ground contacts.

I haven't yet, is there a special way I do that?

The alternator is a bit hard to get to on the 911, but the way I did it recently on my Saab is just using a DVM on the + of the VR to ground and monitoring voltage. If it fluctuates a lot, the VR is probably not working well. Don't see why you couldn't do this on the battery terminals, but you'd want to check your grounds first.

ill look into that, the alternator is basically new, year and a half old, a high amp 175w, so not old tech either.

If your VR is out of spec, you'll see it at the battery with a simple volt meter or digital multimeter; no need to tap the VR directly (much too difficult on the SC since the VR is built into the alty). As Otter74 noted, clear the grounds first - engine-chassis strap, alty to case behind the fan, and be sure all the connections to the starter are clean and tight since pos (+) from alt to batt runs through the starter.

First I think you need to be more specific on exactly which fuse in what slot is blowing at what time. You stated you have had random fuses blowing. It might be possible that you have several wires from different circuits intermittently shorting either to ground or to other circuits. Look at the schematics for your year and model highlight each blown fuse to see if there is a correlation. Blown fuses are due to excessive current draw in a circuit that has suffered a change in circuit resistance such as two wires shorting to each other. Did the blown fuses start after the CR 175 amp alternator install? If you are seeing + 13.6 volts at idle then your voltage regulator in your alternator is working. installation of the CR Electrocooler system adds additional hot and switched circuits to the fuse panel. Are any of the blown fuses related to any of the added circuits? Leaving the AC system off do you still blow fuses? I will assume you are running the original fuse panel and have not upgraded to an ATO fuse panel. The original fuse panels after 40 years can cause corrosion (resistance issues) at the fuse connection points which create a heat issue that can lead to damaged connections or overheated wires.

Look for heat discoloration on the fuse block wire connections/ fuse holders/bases/buss bars.
Also check the back of the fuse box. I have found loose bus bar rivets.
If the fuel pump blows repeatedly and you’ve verified consistent good battery voltage then dig further into the pump.

thanks for the response, the alternator has been in for a year and a half with no issues, same as the ac, so I don't think it has anything to do with those pieces of the puzzle. The fuses that were blowing have been the rear defog( its not even hooked up and I've pulled the fuses from the back) then it moved to the fuel pump, that happened three times, then the blower motor, then the power window fuse, its jumping around, this is all within a week so Id agree that its probably its ground related. Ive ran the system both ac on and off and sometimes it blows and sometimes it doesn't, the ac system is plugged directly with a separate fuse to the battery, only the ac button light and trigger is run to one of the ignition fuses (25amp fuse 2, or 3 I can't remember off the top of my head). yes to the original fuse panel, I checked the screws and its tight, I've sanded/filed down the fuse leads on the ones that have been blowing. Its literally melting the fuses to liquid, leaving behind the metal sometimes. The stereo I've double checked its also on its on fused line. Its quite confusing as this wasn't happening before a month or so ago. I lost my window functions, replaced the switched and everything worked and then a couple weeks go by and this starts.any of that help?

the fuse pump fuse went 3 times, but it hasn't happened in a week its now other fuses blowing, it didn't start there either. The starter has been weird the past week, the cdi box does a tach sweep before it turns on at cold start and sometimes the car just starts in the middle of that, could that be popping them? maybe the starter has gone ? is that normal?

Intermittent shorts from multiple circuits. Seems that tea tray is on the right track with the recommendation to check the back of the fuse holder to make sure the wires aren’t grounding out. The fuse holder or the wires might be to close to the fender wall or other wires.

Also check the fuse holder in the engine bay to make sure you didn’t change the bridged fuse situation.

And check to see if the fuel relay rubber holder/ grommet is’nt melted and shorting.

Did you connect the three compressor defeat wires that come with the AC?

From the 81 schematics:
Rear defogger switch fuse 12 (25A)
Rear defogger fuse 24 (25A)
Fuel pump fuse 16 (25A)
Fresh air blower fuse 13 (25A)
Power windows fuse 21 (25A)

All of your blown fuses are 25A, I wonder why that is? No lower rated fuses are blowing?

Have you looked behind your fuse panel to see if there are any chafed cables fried wires or loose connections/ jumpers?

Be careful when accessing behind the fuse panel (it’s a real rats nest of wires).

If this was my car:
I would tackle one circuit at a time. I would connect an amp meter in series with the fuse that blows the most. I would not use the AC for these tests and I would replace the defogger fuses for the testing even if they are not used. If the current is close to 25 Amps I would then trace the circuit through the devices checking contacts/wires and also removing connected devices to see if the current draw drops considerably. Once you isolate a high current circuit you will need to do resistance test to check the continuity of the circuit for shorts.

It is my opinion there is no magic bullet to fixing you problem. You will need to take your time and check each circuit until you find the intermittent fault.

Sorry, I wished I had a better answer or suggestions.

Can’t help with the electric but the idle issue is totally normal; that’s why early Porsches have hand throttles and later ones have WUR or some IACV.

Cold air is thick and doesn’t move as well as hot air, so when the engine is cold it’s harder to get air in and keep the idle where you want it, once the engine gets hot the air moves easier and you get the correct idle.

Something in your CIS (assumption, as you didn’t clarify if you still have CIS or have carbs/EFI-ITBs) isn’t working correctly. Start with the WUR.

Check your earth straps, one from battery to body, and the one from the engine to body, even if they look clean, take them apart, give both surfaces a good clean, put them back together using some copper slip between the connections, the fuse issue may be because the engine or management system or other high current items may be using one or more ancillaries to get an earth return, worth checking.
Hope this helps.
A...

I have seen these symptoms where a mouse has gnawed on wires and several have bare spots in a bundle.

Indeed!
Forgot to add to my previous post; as well as the two main earth straps, there are multiple small earths located around the car too, these are also worth checking out.

Thanks everyone for your comments, I will try these things and report back, really appreciate it.

From what I've been told the compressor defeat wires are only used when not upgrading to the 175amp alternator. So no they are not connected and have Never been an issue before.