hey i have a 89 gtus... I've been having a issue. first off i bought a brand new battery today and also replaced the alternator, checked the coils and plugs.. everything is good but for some reason my cars voltage still drops. the car had a s4 alternator so the wires were cut. i reconnected them using butt crimps to make sure they were good but now i have no idea whats going on..

also my car has been idling rough and backfires alot during idle thus causing a thumping sound as if someones kicking the trans. its not consistan it happens on and off. i have been told it could be the TPS causing the rough idle and the backfiring is causing the motoro and trans to shake alot because of my bad mounts.

my logis is that the cars backfiring because of the the low voltage causing poor fuel delivery wven with the new pump which causes the car to shake and make that noise because of my bad motor and trans mounts... anyways idk some one help me out pleae

 

Never heard of an S4 alternator in an S5, but are you sure it is wired correctly as an S4 will be wired differently than an S5? There is a diagram of how this alternator is to be wired up in the FAQ section/thread (shows the difference between S5 and S4). If you were to take the voltage reading of the alternator's output wire what is it while idling?

The TPS is to be set w/the car fully warmed up (not partially or close to, but completely warmed up). There is a Green/Red wire at the TPS plug and it should read 1 volt w/key to on and again, the engine as warm as it can get.

 

the s4 alternator was done right and everything appears to be wired right.. since i put in the battery the car hasnt been idleng to crazy. ill probably check the alternator reading like you said and if its good i'll have to check for a bad connection or short somewhere.

 

update: i was right about the thumping sound it was the mounts so im gonna be ordering banzai poly motor and trans mounts still cant figure ot why the voltage is dropping the way it is so im going to run a wire from the battery to the alternator for now till i can figure it out.

 

Have you measured the voltage output of the alternator? It's the Black wire/cable bolted to the side of the alternator. It should read 14 volts approximately, car running.

 

i will try that as soon as i can find a volt meter. thanks for the help

 

i checked the voltage and its good so im gonna check for a bad ground and see if that helps

 

Did you check the voltage at the alternator's output wire? When you stated that the voltage was dropping were you relying on the voltage gauge? If yes, then next time you see it dropping at the gauge measure it at the alternator and see if it matches or not as the gauge might not be accurate.

 

I read the voltage after letting the car run for a while. It was at about 13.6 when the volt gauge dropped so I'm sure its accurate but ill try it again when it drops to about 12.? to make sure. I know for a fact I'm losing power though because, all the lights dim as soon as I start turning **** on. I've disconnected my radio and fog lights to make sure it wasn't one of those causing the issue. It drops the most when I put on the heat and turn on the headlights.

 

It should run from 14 to 14.4 volts. If it drops then the alternator has seen better days or the alternator belt is slipping or there is a weak link in the grounding system for the charging system. Alternator is the most likely scenario though.

 

I was planning on taking the alternator out and taking it to autozone to get tested if not then the bad ground was next on my list. ill update as soon as i check both of those possible issues. where would i look for the ground for the charging system?

 

Follow the negative ground cable down to the driver fender and it bolts to that area. And testing the alternator is for the birds as there have been numerous board members who had the alternator checked and it passed but never passed the mustard when actually in use. And many members upgrade to an FD alternator as it puts out much more amps.

 

I forgot to mention it is a FD alternator. I just picked it up..ill check the ground when i get home if its not raining and ill post back when i'm done.

 

Then why were you talking as if the alternator was an S4? Confusing. Chances are the FD alternator either has an overdrive pulley or an underdrive pulley. The underdrive will cause the alternator to produce less voltage while idling but produces enough output while driving and the overdrive one would tend to produce enough while idling but tapers off some while driving and from the way it sound you have the underdrive pulley.

 

But I've been having this issue since I had the s4 alternator.

 

Did you clean the grounding point yet?