I have a 1987 firebird 5.0. I'm having electronic issues and could use some help. First, put new battery in it, runs good till I turn on heater to high then engine sputters and battery guage goes down? Anyone seen this before?
How about when you turn on headlights, wipers, etc? Does voltage drop when other load using accessories are turned on, or just when heater fan runs?
If voltage only drops when heater fan is on, then I would suspect a short somewhere in heater fan circuit. Fan motor would be first thing to check. Followed by wiring, connectors, resistor for fan motor, etc.
We have the 88 service manual located in 2nd gen tech section. Click on master index to find it. The manual has full wiring diagrams and troubleshooting.
If you insist on 87 wiring diagrams www.bbbind,com has down loadable diagrams. I think they go back as far as 87, but I'm not 100% sure.