83 Hurst Tach Issue

[LuckyHurstGuy]

Hey all, my tach has a strange fault I've been trying to troubleshoot that's been happening since I got the car about 10 months ago. When I start the car the needle barely moves at all, we're talking maybe a millimeter or so. Strangely, If I turn on the parking lights or headlights the needle will move a tiny bit more. Then, if I rotate the interior light dimmer knob the tach needle will finally show some real movement. It maxes out at about 1000 RPM even when I'm revving around 2k or more. The V8 jumper in the back of the gauge cluster has been hard-soldered.

I'm guessing it's gotta either be a bad tach or a ground fault. My other gauges are working (though I think my fuel sending unit is bad), and I show good voltage on that gauge. I read somewhere about strange tach faults that can occur from bad headlight switches and the one I have now was swapped from a pick-n-pull so unless that one was bad as well that's not it.

It appears pin 2 on plug 2 is the ground so I'm going to try and see if it branches to everything else. There may be a fault just before the tach I'm guessing. I'm no engineer but I took electronics in the Navy and I know my way around a schematic and a multimeter so I'm hoping I can figure this out. I'd like to try hard wiring a ground wire directly to the tach to see if that helps, otherwise I'm afraid the tach itself is probably toast.

Incidentally, I also need to track down why my turn signals and high beam indicators aren't working on the dash. Wonder if the two are connected?

Any opinions would be welcome. Much thanks!

 

[pontiacgp]

I believe that tach can work on an 8 or 6 cylinder. There is a jumper to set up the tach and sometimes that jumper comes loose which messes up the operation of the tach. .

 

[LuckyHurstGuy]

The jumper has been hard soldered in the V8 slot so that's not it... I pulled the gauge cluster last night and went on a cleaning spree and found several corroded bulbs and sockets and fixed those, my lights are all working now. Tach is still not working correctly, though. I'm somewhat at a loss everything seems to be correct so I'm forced to believe the tach itself is bad. Any other options?

 

[pontiacgp]

Bonnewagon (Mark) might have some ideas about the tach

 

[Judge Mills]

If you can get to it while the engine is running, I'd try to put your multimeter on the positive, then the negative, and record what reading you are getting. Go through the different scenarios you listed above.

Off the top of my head, it's a coin toss if it's the positive or the negative that is causing problems. But, since the dimmer knob is affecting the tach, I'm betting on the positive being the problem.