Just bought an 85 720 from my dad for $250. Pretty solid truck overall, biggest issue was that he severely neglected the maintenance. In the 4 years that he owned it, he did NOTHING, and yes, that includes oil changes. But it also never gave him any troubles at all. Never broke down and was incredibly reliable. I drove it the half hour home, problem free. The thing drives and shifts like new, no issues.
Well, then I did the maintenance that it has needed for so long. Changed the oil, new valve cover gasket (old one was leaking) plugs, wires, cap and rotor. Fuel filter and air filter (had been running for who knows how long without one). Rear diff fluid, serviced the transmission, changed out brake fluid and clutch fluid. Flushed the radiator (and it reeeally needed it) and filled up with antifreeze. I think that's all.
So bear with me, kind of a long story but I feel everything should be included to help me troubleshoot. After all the work I go to fire it up, thinking that it is going to run even better from all the much needed love. It fires right up and sounds better than ever. About 45 seconds in, it starts stumbling and eventually dies. Wouldn't start back up, verified it had spark so I go to check for fuel and of course, it's not getting fuel anymore. After some research I find that the fuel pump relay is a common problem and seemed to fit my symptoms so I buy a new one, and it works.. Sort of. So I go through all the ground connections from battery to fuel pump and clean them up good, that works and the truck runs again.
It's running, and it seems to be running great. Only problem is now my battery light is on, fuel and temp gauges stopped working, and my turn signals don't work. WTF?! Another day of trying to figure this out, I unhook the negative on the battery while it is running just to see if the alternator was putting out enough charge to keep the system running. It is, with no issues, so I hook it back up. Voila! The gauges started working again! Everything seems normal, until I try the turn signals. Everything died again. Disconnect the negative and reconnect then all is well again. I even took it down the road and was able to use the signals. Get back home and park it for a while then go back to try again, and they're not working. Disconnect and reconnect, didn't do the trick this time...
I am so confused by this! I've gone through every connection and they all look pretty free of corrosion. I am missing the fuse box cover so I don't know what fuse is for what. I have tried researching this issue but haven't found anything similar to what I'm experiencing. Sorry for the long post, I hope somebody has time to skim through it and help me out. Thanks in advance.