1.
Am I correct in that there is only 1 relay on this whole thing???
Nah, you must have more relays than that. I can think of a few just off the top of my head;
You have a horn relay - it pulls in when you push the button on the steering wheel.
It''s true that the A/C compressor isn't controlled by a relay, but the blower motor on an A/C equipped truck is.
If you have power door locks, they are relay operated
There is a relay in the convenience center that operates a buzzer (or a dash warning light). It can be operated by two different switches. The door jamb switch pulls it in (if the headlights are left on (or the key is left in the ignition) and the drivers door is opened.
Also there is a switch in the seat belt retractor that will pull it in - if the key is in RUN and the driver's belt is not buckled. In that case it also illuminates the seat belt warning light.
See dude, life ain't that bad. Turns out you've got plenty of relays - and you didn't even know it.
2. As far as this:
Also, related: were does hot wire for in-cab electrical come from? I see one coming off alternator output to junction block on firewall. I also believe there is something off the starter motor too?
There are actually two battery feeds into the cab (through the firewall transit block).
One of them comes directly from the starter solenoid to the transit where it passes through the firewall and goes into the cab. Don't quote me on this, but I am fairly certain that on all square body trucks, it is always located in the top inboard slot. Like this:
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On the cab side of the transit it splits off into two separate leads. Those leads then head in two different directions.
One of them runs to the far left bank of the fuse block (the BATT group). That bank is not ignition switched and so it is always hot. Components that are required have power available at all times (the horn, the dome & courtesy lights, parking lights, clock, etc) are fed from that bank. The other one is the dedicated power supply to the headlight switch.
Now about the other power feed into the cab. This also goes from the starter solenoid and into the cab via the firewall transit. But this leg is the one that is visible at the firewall junction block.
Think of the JB as a hub. It is primarily placed on the firewall to provide a convenient location to network the truck's 3 primary electrical circuits (i.e. the battery, the alternator and all the ignition switched components). It's location also reduces resistance - due to line losses - in the wiring to any major components that are located within the engine compartment (e.g. the heavy duty blower used with an A/C equipped truck, an auxiliary electric radiator fan, under hood service light, or any other high draw optional accessories)
It is important to remember that, although the alternator is the truck's primary source of power nearly all the time - back feeding the battery (to keep it fully charged) and supplying all the ignition switched components, there are certain conditions under which the current flow in the battery positive cable is reversed. For example, when cranking the engine, the battery is providing power to the starter motor and all the ignition switched components. Or like if you are listening to the radio with the engine off, or you're at the drive-in movies and it's kind of rainy - you need to run the wipers once in a while.
Most alternators won't "boot strap" (i.e. begin to develop voltage without external field excitation) so at least a minimal amount of voltage must be available from the battery to enable the alternator to begin generating power.
3.
And finally, that ground strap on passenger side from firewall to block. Is it even possible to access the block's bolt/nut without dropping the engine? I don't see clearance to get a socket down there... Seems like this would be an important ground.
That strap is primarily there to provide an additional ground path for any electrical components that are mounted directly on the cab or have ground wires that are connected to the common bus block (located under the dash - by the e-brake - which must still pass through the cab body). If your cab components are working okay, they must have found another way to the battery's negative terminal - like through the cab mount bolts. But you're right, if that strap isn't there or not connected properly - it should be addressed.
I know exactly what you mean about the restricted clearance between the back of the head and the firewall - it is wicked tight. Here is what I do:
Forget about the socket/ratchet plan, it won't work. It's tough to just get a box-end wrench down in there - never mind trying to move it forward and latch onto the bolt head....****! I used to drop my wrench so many times! I would be standing in the engine compartment trying to get the bolt backed out - the wrench would slip and fall. After dropping the wrench, I climb out, crawl under the truck get the tool, climb back in and start trying again.
I take like 3-4 wrenches with me when I climb up on the engine - so if I drop one I have a few back-ups. Using the open end (that allows the wrench to engage the bolt head vertically), I break the bolt free. When it is released, I spray the hell out of it with PBlaster and find something else to do for a while (drink beer and watch TV).
Then, when the bolt has had time to soak, I switch to a flex head/ratcheting box end wrench (12 -point):
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I bought a set of Gearbox years ago. They are only for this type of job. If you use
any ratchet head wrench to release stubborn fasteners on a frequent basis, they
will fail - sooner than later.
I also bring a long screwdriver - or a small pry bar - with me to help push the box onto the bolt. Use the ratchet box end and the pry bar to back it out. That's it - that is my entire super genius idea. It does help a little bit - but not much.
I wrote the above procedure in the present tense. But honestly, it should have been written in the past tense. That was me when I was young, ambitious and principled - things have changed. Nowadays, if that strap is broken and everything is working - it just stays that way until I have to pull the motor. And, if I think there is a grounding issue with cab mounted electrics (there never is), I just make up a nice neat grounding strap and run it from the firewall to the engine block. The distributor clamp bolt is a great place to connect to the engine.
