1980 foxbody - pressing switch for high beams causing engine to cut out

[Based_Dookie]

I've been chasing an issue ive been having with my 1980 mercury capri (four eye fox) with an 87 302 swapped in . As the title states most times when I press pull the stock back for the high beams the vehicles voltage drops and causes the engine to either almost or completely stall. This happens regardless of if the headlamp/lowbeam switch is on or off. Currently I've tried replacing/upgrading the alternator with a 3g alternator which charges strong at 14.5 volts and I've cleaned up the chassis grounds for better contact, neither of these resolved my issue. I currently suspect the turn signal switch is at fault (it houses the stalk for the high beams) but would like to seek advice before I throw more parts at the car.

 

[Ryuk]

The first thing I'd do is check the sockets and relays. Something is happening when that circuit is closed so check every piece of that process.

 

[General karthief]

Also put a volt/olm meter on the battery and note working voltages and how far it drops when you pull/push the switch.

 

[7991LXnSHO]

Do you know if this problem is new, or has it possibly been present since you bought the car/ the swap was done?