Tach and oil pressure not reading

[clbsinvaders]

Hey, my 86 is finally on the road, but there's no tach or oil pressure gauge working. New cluster. Same deal with the old one, no tach or oil pressure gauge working. This just happened all of a sudden. No reason why, either. Is there anything I can do? I have a track day Friday and I really want to run with a tach! Please help!

[mikec]

The instrument panel has a +12volt feed (via fuse #13 and the ignition switch ON position) and a ground... those are the only two wires shared by the whole instrument cluster. Those two power leads feed several guages, including the voltmeter. If the voltmeter works, then the basic power leads are getting to/from the instrument panel... your bug is outside the panel. My guess: somewhere along the car's history somebody pinched the wiring harness underneath the guage cluster or did something similar, crushing the tach input and oil guage input wire.

 

With the battery "-" post disconnected, remove the big connector on the instrument panel. Look for a yellow wire and a shielded (coax cable - like skinny cable TV/antenna wire). The yellow wire is the oil pressure guage sensor; the shielded wire is the tach input. Put an ohmmeter on the yellow wire and to ground; with the engine off you should see a couple hundred ohms - some continuity but not an open circuit. If you have an open circuit then the wire to the guage is shot... time to go hunting. Start by unplugging the wire at the oil pressure sendor (the round gold can near the oil filter) and seeing if you have continuity from there back to the yellow wire on the guage. If yes, then the sender unit is shot. No continuity = busted wire.

 

The tach input wire goes to the ECU connector and then to the ignition coil "-" post. If the tach input is dead, your ECU is probably throwing the "ignition pulse" error code too. Common problem: when air:fuel guages are installed this wire is tapped... and, because it's a shielded wire (a wire inside a braided "pipe") folks often short the wire to the braid - and the braid is grounded. That shorts out the O2 sensor, kills the tach input, and kills the ECU input leading to an ECU error code.

 

mike c.

[clbsinvaders]

Everything works but the tach and oil pressure. The voltmeter is at the center line. Normal? I really don't know, my first quest, and it hasn't budged at all.

 

When I first got it started, everything worked.

 

Now, is it possible that it is the incorrect ECU hooked up? I have 2, and my friends went a little giddy swapping them out to see if that was the problem.

 

One last question/statement: is there a thread in the FAQ to reading error codes?

[mikec]

ECU doesn't do much of anything with the instrument panel. It drives the boost gauge (87-later cars) and that's it. All of the other guages work just like a late -70s/early 80s carbureted car. Feed them +12volts, current flows through the guage, to a wire, to a sensor, the sensor varies its resistance to ground as it measures oil pressure, temps, whatever.

 

The voltmeter should sit at the bottom of its scale with the key out of the ignition, go to +12volts (centered) with the key in the ON position, and be somewhere between 13 to 15 volts when the engine is running. If it's stuck at +12volts regardless of the ignition key then somebody has hacked the wiring or you have an incompatible instrument panel and/or wiring harness - i.e. parts from some other year StarQuest - on your car. The instrument panels varied over the years and many are not interchangeable. The early model year StarQuests offered analog guages or digital displays - they don't swap either AND the sensors are different between analog and digital panels.

 

mike c.