98 TA electrical problem

When I hit the brakes the check gauges light comes on and all the lights including headlights go dim for a fraction of a second. It also does that randomly.

When I have the air conditioner on and am stopped the batter voltage drops down to 8-9 volts and the air conditioner slows, the rpms surge, and the speedometer shows me going ~15mph. When I take off it's okay.

I have a new battery and the alternator has been checked twice. Any ideas where to start checking for a short? The garage says it is just a 'coincidence' that it happens EVERY time I hit the brakes. They don't know about the battery issue.

Thanks for any ideas, I'm lost.

The root problem is not like a short, but a resistive connection. The main power stud, main cable, tap cable and bolt, and bussing connections at the underhood electrical center are suspect. Marginal battery connections will also produce those symptoms. The main BODY and ENGINE fuse connections are also suspect. The ignition switch itself could also be the cause.

Locating the problem will probably be easiest by running the engine and adding some electrical load, such as lights, AC, HVAC blower, and whatever else you can (not the flashers or turn signals since the load is so cyclical). Measure voltage drop across connectors or circuits. In other words, check between the alternator output stud and battery positive post for any voltage drop. Anything more than about 0.15V is a potential problem. You can test for drop at the ignition switch by accessing the connector under the dash and I/P. You can check the UEC bussing and MAXI fuse connections by measuring between the battery POS and test points right on the tops of the fuses.

Great, thanks!