Rpm quit working, not the fuses. Help! - 1985 Nissan 300ZX

fuse ???

charge/battery/alternator lamp on is top clue
4wd, ill ignore.
you meant Hvac blower dead.? right, dash blower? same blower for heat and a/c so......???????
that to means blown fuse, or battery went DOA.
battery volts is 12.6v charged. and rested
running its 13.3vdc to 15v if not the alternator is OFFLINE.

with a voltmeter this is 5min work, with out days.? guessing?
ill do why> notparts
battery dead. (or discharged. the charge lamps comes out for alternator dead, due to many reasons, engine stalled,? low engine rpm, 400 is too slow, fan belt fell off or slips.
fuse blown. check fuse?
any were, related to alternator, and battery are suspect if alternator rpm is good.

no guessing here, that be silly, i use the voltmeter 1st.
12vdc across all fuses in all boxes, on both sides of fuse key on
got that?

bad cluster board

Just check the fuses Bro.The fuse diagram is on the back of the panel cover.

Check the fuse panel first. Has anyone been under the dash recently, sounds like a harness plug is not quite home.

The gauges all run off the same fuse, so they would all quit. No you likely have a failed instrument cluster.

If after switch on of the a/c there is no difference in the engine rpm with the solenoid latching up the a/c compressor with the fan working , then you must look for the fuse of the a/c which can be found near the main box inside the bonnet. replace and test. However if the ac works but there is no cooling then you must check for the gas pressure in the visual eye at the gas inlet. If you notice that there is heating in the cooling pipes then your compressor needs to be checked, gas leak in system, gas filling. Hope you check this out. Good day

did you check the fuse?
that sounds more like the problem!!!

The transmission quit shifting, and the speedometer quit working.
When you say the transmission quit shifting, do you mean you backed out of your driveway, started driving down the street, no speedometer, and you just stayed in 1st as the engine RPM's got higher and higher ?
Please explain what has happened. You should also have a check engine light on in your dash. You will have to plug a OBD II reader in to pull any codes, so we can help you find the problem. Please let us know.

check your fuses and relays