[JoeyStrosnider]

A friend of mine has a 93 Daytona ES na 2.5L Auto. He has an aftermaket voltage guage (wired to the output side of alternator) and this past week the needle on the guage started "bouncing" rapidly from 10-14 Volts (The factory guage bounces back and forth as well). We thought the alternator was going bad so he went ahead and replaced it with a brand new one. However even with the new alternator its doing the same thing. He brought it over and I checked the voltage with my meter and with my probes directly on the alternator outputs it only fluctuates from 13.40-13.80 Volts with engine running/headlights off and with "everything turned on" 13.60-14.10 Volts. Then we figured since we had a spare ECM we would swap them out to see if maybe the regulator was going bad but only found the same problem is occuring.

I had this similar problem with my race car a year ago just before the alternator quit working and then a similar situation earlier this year when I had to replace my ECM. Could this possibly be another bad alternator or possible wiring issue?

 

[Bob Lincoln]

Put the voltmeter across the battery instead. That will show the regulated and decoupled voltage (smoothed out).

 

[JoeyStrosnider]

Did that as well and got the same results. Sorry I should've put that in the first post.

Would a bad ground possibly cause this? Possibly something else back feeding through the guages ground wire?

 

[Bob Lincoln]

A bad field wire or terminal might do this. Since you have a new alternator, maybe the wiring in the harness, especially field wires, has an intermittent connection.

 

[JoeyStrosnider]

I was thinking in that direction since both gauges are wired up separately from one another but both are acting the same. The wire could possibly be corroded somewhere and the resistance going through it has changed causing this. Maybe?

 

[dana44]

Last time I had that happen to me I ended up with an enginebay fire. There is some kind of short somewhere and it needs to be found quickly for safety reasons if nothing else.

 

[JoeyStrosnider]

Well now he apparently has more going on with the car now than before. He's telling me that Intermittenly when he's driving and if he hits the brakes that the radio will die out and when he releases the pedal it will come back on. He was driving home the other night and pulling into his driveway (headlights on of course) when he hit his brakes to slow down his headlights got really dim, lights on dash and stereo got really dim and the car almost shut off. Just so you know i did load test the battery and its good.

 

[Bob Lincoln]

So, he has a short in the brake light wiring. It might be split or chafed insulation on old wires. Or if the car was ever welded to fix ar leaf spring supports, it's possible that the welder melted the wire insulation. Happened to my brother's Duster years ago. Inspect the wiring in the trunk and work forward.