So I'm new to the forum but have been trolling around for a little while, getting some advice for my various DIY repairs.
Anyway, my first real conundrum ... a total stumper:
I have a 1983 300CD TURBO, 167k, good compression, glow lights all working, very little blow by, 25-29 mpg depending on how I drive, etc. Within the last 1,000 miles, I had the following maintenance performed on the car:

Start of Injection Timing
New damper bolt on IP
Air Filter
Fuel Filters
Climate Control Regulator Unit Replaced
Front Brake Pads + Sensors
Oil Changed
I also installed an aftermarket radio and new rear speakers, bypassing the little fader switch, and wiring the constant for the radiohead to the fuse for the clock and dome light.

So here's the problem (it's actually several minor problems but I think they're all related). On cold mornings (below 35F) the glow lights turn on and I wait for them to go out and maybe cycle them over one complete time and then crank and it the starter cranks slow, almost sounding like it's not going to start at all and then it just fires right up after two or three cranks interspersed sometimes over as long as five seconds. This doesn't happen EVERY cold morning, but more often than not. The weird thing is, that the other day it was 25F outside, and I cycled the glows and cranked it and it cranked normally, three or four cranks really quick one right after another and started right up.
Hmm...
Okay, so then yesterday the strangest thing happened. I started the car (started fine strangely enough) and it was raining outside, so I turned on the headlights right after the engine started running and the radio turned off, all the lights and everything going out, stayed off for about five seconds then clicked back on. I turned on the windshield wipers, same thing –– radio off, then back on. I put the car in reverse and drive away and flick off the headlights to see if it would happen again and the radio stayed on, nothing. So I'm thinking, 'Oh, just some weird fluke.' Then I flip the turn signal to the right, radio off and back on. Okay. I drive. Light city driving, turn signals here and there and it stays on, never shuts off again.
Then I get to where I'm going after about a fifteen minute drive. Run an errand, fifteen minutes later come back out to the car, glow it up and start it but it cranks really slow after cranking just fine before (and now the engine is warm!).
I turn the headlights on and the wipers on and the radio decides to stay on this time, but then I tap on the brakes and NOW IT'S SHUTTING OFF AGAIN!! Really weird. So I'm driving along back from where I came, turn signals and everything and the radio is staying on except when I tap on the brakes and then it turns off and comes back on and then after about five minutes of light city driving just decides to stay on for good. I decided at this point to check the clock on the radio to see if it had reset itself, and lo-n-behold, the time was correct which led me to believe the short was somewhere in the fuse and the electrical line it shares with the instrument cluster (maybe a relay or something?). Okay. So I know this is long, but there is more ...
This morning it was forty degrees and sunny and the car was REALLY hard to start, like it wouldn't even hardly crank at all. But the radio stayed on the whole time and didn't even flinch. Then I parked the car for about an hour and went inside and did some stuff and started it again and it was fine. The radio too. When I got home later on, I shut off the car and noticed a faint whir coming from under the dash behind the glove box. When I opened the door, this whir stopped. I closed the door and it started again. Opened and stopped. So I figured it had something to do with the little dome light door switch and there is some relay back there behind the glovebox making a buzzing/humming (real faint) noise that could possibly be some sort of SHORT CAUSING ALL OF THESE STRANGE ELECTRICAL PROBLEMS!?!?!?
Basic rundown:
Rain, radio intermittently shuts off when other functions on its same fuse circuit are engaged
Dry, no radio problem.
Cranks hard sometimes but not always (temperature independent)
Whir behind glovebox at shut off, stops when door is opened, restarts when door is closed
I know this was a long story, and quite a mysterious one too ... but does anyone here happen to have any ideas?

Sounds like you have a grounding problem and your battery/alternator may be on its way out. If I turn my key too quickly to the glow position, my wipers come on. Happens every time. If I turn the key slowly, the wipers won't come on. If I press the passenger side mirror switch to the back position, the wipers come on, every time. My car is slow to crank as well but I know I need a new battery.

The only thing behind the glove box that would whir is the climate control fan.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

Possibly corroded fuses?

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1981 300SD 512k OM603

Definitely sounds like a bad ground.

Remove and clean both battery and clamp terminals.

Remove and clean battery negative to ground terminal and chassis to transmission ground lead.

Check positive to starter motor battery lead for tightness and good connection.

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Beagle

silly, but could your ignition key be going out? Watch this video of mine at 8 minutes in to see what I mean.

82 300DT First run after getting it home. - YouTube

Checked last night. Lifted the car, and examined the starter and alternator and battery and everything seemed fine and tight – very little rust and no corrosion. So I was just standing over the fender on the battery side, tracing all of the cables that go underneath the coolant reservoir just trying to see if I couldn't find a bad connection somewhere and I traced one cable to that very odd-looking, hollow-spring relay mounted to the passenger-side fender directly in front of the coolant reservoir. That connection was VERY loose, almost completely free from the relay housing, so I connected it.

Looking at an electrical wiring diagram to figure out what that relay was, it seems that it is something for the climate control/AC system?? My AC compressor hasn't actually worked since like September (not that I need now anyway), but the climate control is otherwise fine, and I don't think the compressor itself is blown because the system is still fully charged. Okay. So the car started fine after that.

Next ...

Accomplished as well. No shorts, no weird radio on/off, nothing today. This morning it was 20F outside and I glowed it one time through, cranked it and it turned over right away, just fine, cranks like it should. So I'm going to assume the problem fixed for now and see if my compressor won't turn on whenever it gets a bit warmer outside.

Does it seem right, though, that a loose connection at that relay could cause such a myriad of idiosyncratic problems? Anyone with any thoughts care to share?

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1983 300CDT -- 177K

You are describing the Blower/Fan Control Resistor. Not sure how it works but it has to do with changing the speeds on your Blower Fan.

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84 300D, 82 Volvo 244Gl Diesel

The OE radio gets power separately for the clock, and the lights, and the sound. Fuses 1, 2, and 4, I think. The clock is straight from the battery, the lights via the headlight switch and the dash rheostat. Since you've changed that, it'll be difficult to troubleshoot.

But yes, there is a relay behind the glovebox in the coupe. It controls vacuum for the seat back locks. It's connected to the door switches, which are connected to the dome light, that you've connected to the radio. The hard starting may be due to somehow affecting the vacuum since you've changed this up.

Do you, by any chance, crank the car with the door open when it's warm, but closed when it's cold? That may affect starting if you're dumping vacuum?

Long-term, I'd fix the relay so the seat backs lock properly - and I'd try to restore the OE radio wiring (minus the speakers) since in the '83 coupe it should have been okay for an aftermarket HU. This'll mitigate electricaly weirdness.

Short-term, I'd make sure the relay behind the glovebox wasn't being engaged all the time. That sucker gets hot.

So you're saying the seat back locks (for the front seat) are vacuum powered? They have never worked since I've owned the car in that the seat back can be pushed forward or back without pushing that little button on the side. But all the other vacuum components work perfectly.

This may be an issue, but I don't see how hard starting can be a result of poor vacuum. The main components required to start the vehicle (glow plugs, starter motor, battery, etc.) are all electrical, except of course, for the ignition but its vacuum supply is only engaged for shutoff purposes.

Since I've initially posted this, I've had much success then further problems. I tested all the glow plugs and connections again, reading 10.5V consistently during glow and between .03 and .06 OHMS. The lights also shut off properly after an extended glow period, so they are definitely not the problem. I also don't think hardwiring the radio to the clock and dome light circuit is a problem because I've read about a lot of w123 owners who have done it without issue.

After I replaced all the fuses and such, the vehicle ran and started right up just fine for a day. But today, it wouldn't start at all after I drove it last night. The battery was dead. I jumped it and then drove it around, started it again later and it was fine. I just now decided this evening I'd fiddle around with the multimeter and get some readings at different places, but it's dead again.

At this point the only thing I can think of is the brushes are worn down on the Voltage Regulator, making intermittent but not consistent contact and perhaps even expanding and contracting with fluctuation in ambient temperature to cause the random non-starts, non-charges. Any ideas? I'm going to replace the thing anyway just to see if that's what does it.

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1983 300CDT -- 177K

Yes. The seat back locks are vacuum powered and electrically triggered. There is relay/solenoid behind the glove box that "opens" to dump vacuum when either the doors are opened. The solenoid is wired to the battery.

You state you wired the radio to the dome light power. That circuit is also wired to the door switches (otherwise the dome lights wouldn't come on when the doors open). So now you've introduced some sort of connection between the solenoid and the radio. Maybe benign and a red herring, maybe not.

Do the seat back locks engage when the car is running with the doors closed and the button not pushed? If not, then they're not working at all.

Lots of W123's aren't coupes and don't have the additional solenoid. I've also recommended to 240D owners that didn't have the solid state radio that the nearest steady +12v in the console was the dome light switch, so yeah, I get it that it can be done, if necessary.

Your battery died and you've got buzzing behind the glovebox in a coupe after you've affected the door switch circuit in some way. Sounds like maybe the solenoid is being continuously engaged.

Pull the glove box, unplug the wire and see what happens. What's the worst that will happen? Maybe the seat backs will start working, but they just won't auto-dump vacuum when the door is open. And maybe the battery won't drain...

Maybe there's nothing wrong with how you wired it, maybe the door switch sometimes sticks. You've got a coupe, you've got to accept you're going to have some special issues that "lots" of other W123's don't have.

http://www.peachparts.com/shopforum/diesel-discussion/295616-definitive-83-fed-300cd-vacuum-thread.html

http://www.peachparts.com/shopforum/car-audio-multimedia/284681-240d-frying-radio-fuses-4.html

Yes, four pages of troubleshooting a radio install and popping fuses. At the end there's a radio schematic. And yes, the constant power for the OE install is on the same circuit as the clock.

Alright gentlemen, here goes ...

I replaced the voltage regulator, disconnected the relay/solenoid for the seat back electric vacuum dump mechanism, then jumped the dead battery and got the car started. That's when I decided to run some tests.

After letting the car run for about ten minutes I checked the battery at idle (750RPM) and got 13.4V. Revving the engine up to 3000RPM, the voltage at the battery terminals increased steadily all the way up to 14.5V and then with the throttle suddenly let off, dropped to 12.5 for a split second before settling back between 12.9 and 13.4 depending on what devices were engaged (heater, headlights, radio, etc.). I didn't check to see if any particular electric device sucked more voltage away than another, just turned a bunch on and then off to see the difference in Voltage at different loads. is this something that should be done?

The reason I ask is because it seems to me that the fluctuation in voltage at the battery terminals is more likely an alternator problem especially now that the regulator has been replaced. What do you guys think?

The battery held a charge for a few stops and starts today, but then after a drive this evening, heat running, headlights, radio, etc. I went to start it back up again later and ... nothing. Only 12.04V at battery terminals (not enough to crank). I still haven't seen a warning light mind you. Had the battery tested and checked out okay. Checked the grounds again, closely inspecting for corrosion and tightness, still okay! At this point, I'm really at a loss. Any advice would be of great help.

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1983 300CDT -- 177K

How old is this battery?

May need to look for a parasitic drain. Pull one fuse at a time while watching an ammeter inserted in series with the battery.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

Two months. Just bought the car. Guy that had it hadn't driven it in a year so we went out and he bought a new battery for it and we installed it together and fired it right up. Ive done a little bit of routine maintenance on it -- oil change, break pads, brake fluid, tranny fluid, new tires, fuel filters, timing adjustment etc. But this is the first 'issue' I've had after driving it daily for the last two months and even on a few long road trips. Really the car runs great with excellent acceleration and fuel economy between 25 and 29 just depending on conditions of course. Starts just fine too when the electrical system is functioning.
I'm taking it to have the alternator checked right now, but I'm thinking there's a short in the system somewhere. I'm going to get the car started and let it run to get some idle readings from the multimeter. Ill post them up here when I do.

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1983 300CDT -- 177K

Well, the alternator is fine according to O'Reilly and I'm inclined to agree with them. Voltage readings at idle (750RPM)

14.25V nothing on
14.25V radio on
13.05V heater full blast + radio
13.58 radio + headlights (no heater)
12.98 radio + headlights + fog lights
12.54 radio + headlights + heater (no fogs)
12.28 radio + headlights + heater + wipers

At that point, it seemed to me the biggest drop occurred when the heater was engaged. I turned the heater off with the radios headlights and wipers running, and the alternator began t steadily charge the battery.
So I just had the heater regulator replaced and the AC compressor does not engage, but the system is charged and I don't think the compressor itself is actually blown. Is the blower motor on its way out? Or is there another possible short in the HVAC system? Any ideas?

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1983 300CDT -- 177K