Tachometer jumpy when using hvac

[KHull]

As the title states, just purchased this truck a month ago. Tach was on the fritz, at 70mph it was pegged in the redline, put a new tach sensor on it, seemed to clean it up a bit. Im starting to pay closer attention to it and notice when i use the lights/ hvac or anything electrical in the cab the tach jumps a bit. Also when i start the truck the highbeams are on for a while even with the switch off. I checked the grounds on the driver and passenger side kick panels and even ran them to negative on the batteries to eliminate a poor ground issue, still no change. At a loss here, any ideas would be great. Thanks

1993 F250 Dually Conversion.
7.3 IDIT 5Spd 4X4 Supercab Longbox

 

[Jesus Freak]

Hey, as far as the headlights go, you HAVE to add relays to the system..... Bronco Graveyard makes a plug and play wiring harness or you can make one yourself..... but its because of all the amps of 2 batteries are passing through your actually switch in the truck causing the internal breaker to trip on and off..... its very unnerving.....please fix that IMMEDIATELY!!!

As far as the tach.......who knows?..... im guessing its in the cluster itself..... maybe unplug it and replug it...... thats nominal at best, but fix them headlights, friend!!!

Edit..... rethinking your cluster stuff...... you might look into a 3G alternator conversion..... you probably got a serpentine belt set up, so i don't know if ferd put an adequate alternator in there by then, but the V belt driven alternators are woefully inadequate.....so.... theres my $3 on that...... could be a compromised voltage regulator, i guess..... anywoo.... keep us posted!!! Fix them lights!!! Don't drive at night until you do PLEASE!!!

 

[Greenie]

Years of running clearance lights on a travel trailer fried at least 1 - maybe two headlight switches before I switched out to LED on most lights (not on brake/turn bulbs though). It gets hot. The stock alternator on my '93 with the serpentine belt has performed adequately though the original started sending AC spikes that confused/damaged the TCM. Replaced both the alternator and the TCM. I also freshened up grounds - there are quite a few of them and none looked bad.

 

[DirtyWood]

Try tapping on the tach display to see if the problem is mechanical. I've seen gauges that were off but a good tap would bring 'em back--not a fix obviously but maybe a useful diagnostic.

 

[KHull]

So, I think ive got it figured out. Previous owner did the classic “farm fixes” and did some jank wiring, cleaned up the wiring a bit and pulled every fuse not relevant to me. I remember reading a while ago that someone had their horn pushing power and causing the same issue. So, pulled the horn fuse and lo and behold. Highbeams turned off, plugged it back in, highbeams came on again. Lights still all work like normal as well as my highbeams. This is just a temporary solution until i can get some time to work it all out.