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My '95 STS idles pretty rough in any gear, it ranges from 650 rpm to 1000. The interior lights and headlights both dim when the rpms drop or raise, what's up with my car?
 

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Ever changed the plugs & wires? Couldn't hurt to clean the throttle body either. I'm sure others will chime in with other good suggestions if these fail, but that's where I'd start first.
 

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I would guess that you are running on a low battery. To eliminate this possiblity, charge your battery with a charger til its good and full and see if the problem goes away. If it does, it will be either the battery, alternator or alternator belt. Pleas be sure to post your findings.

Also, trottle body cleaning and basic tune-up (plugs and wires) are always a good thing.
 

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Not sure if the '95 uses an ISC valve or motor but if it's the latter, that may be the problem if the previous items don't help. One way to tell if it is going bad is note if the ISC motor (if yours has one) ratchets after engine shutdown.
 

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My '99 SLS had the same problem with the lights. I it ended up that I need a new battery. After that was done, it was as good as new.
 

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Hello,
Forgive my ignorance, but what is an ISC motor? Never heard of that, and don't know what ISC stands for or controls. Just curious to learn more.

Could it also be a vacuum issue that's causing unstable revs?

-Jon
 

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Azriel said:
Hello,
Forgive my ignorance, but what is an ISC motor? Never heard of that, and don't know what ISC stands for or controls. Just curious to learn more.

Could it also be a vacuum issue that's causing unstable revs?

-Jon
ISC = Idle Speed Control. Earlier models used an ISC motor that extended a plunger to contact and push on the throttle lever to set idle speed. Later models use an ISC valve that meters the amount of air that bypasses the throttle plate to control idle speed.

You'll find pictures of the ISC motor, both assembled and disassembled in this thread.
http://caddyinfo.ipbhost.com/index.php?showtopic=7605
 

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CadillacETC1997 said:
. . . . Id bet my life its the battery
Please don't be so quick to wager your life on what might only be a loose battery cable. :)

Lights should not dim/brighten with rpm changes unless your battery/generator is incapable of supplying enough current or there is a high resistance path to the lights from the battery.

The PCM takes battery fluctuations very unkindly. VERY UNKINDLY! If that's your problem, it must be addressed first. Afterwoods, if the idle continues to "search," the ISC will be your prime suspect. As mentioned in the other thread, you can check the ISC switch from the heated comfort of your 8-way driver's seat. :)

Regards,
Warren
 

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So, did you get all that? Let's see what has been tossed your way:

Plugs/wires.....if your plugs were bad enough to make the rpms fluctuate that much, you would be getting a misfire code. However, if a coil pack was starting to go out, you wouldn't get a code with OBD1.

throttle body.....get used to cleaning your throttle body anytime you do maintenace under the hood. It is so easy and beneficial.

battery.....a weak battery would cause problems during start up or during high electrical load.

alternator.....If your lights are dimming when the rpms dip, you alternator is getting weak. Get it load tested for free at a parts store.

alternator belt.....a good possibility, if there isn't good tension on the belt, the alternator isn't putting out to its potential.

ISC.....a very good possibilty for causing the fluctuating rpms.

Even if the ISC is the cause of the fluctuating rpms, you still need to get that alternator checked (and check the belt). Your lights shouldn't be dimming at any rpm.





Ranger said:
Not sure if the '95 uses an ISC valve or motor but if it's the latter, that may be the problem if the previous items don't help. One way to tell if it is going bad is note if the ISC motor (if yours has one) ratchets after engine shutdown.

Ranger, my ISC has occassionally been making noise at shutdown for the last 30000 miles (3 years). It isn't dead yet. I believe all mine is doing is resetting itself for the next startup. I maintain a clean throttle body. Is there a difference is sound of the ratcheting you talking about?
 

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I am not really sure. I recall my '92 making that ratcheting noise and I never replaced til I traded it at 125K. If I recall correctly, it should not make any noise at all at shutdown if it is operating correctly but I could be wrong. Probably couldn't hurt to disassemble it, clean and greas everything if you are ambitious.
 

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Eldyfig said:
So, did you get all that? Let's see what has been tossed your way:
Plugs/wires.....if your plugs were bad enough to make the rpms fluctuate that much, you would be getting a misfire code. However, if a coil pack was starting to go out, you wouldn't get a code with OBD1.
Only reason I mentioned plugs/wires is I put in new plugs at 107k and my idle noticibly smoothed out. I did not get any misfire codes, they won't set unless you have a pretty bad misfire. This was just a slight miss. My plugs were between .065 and .070, with one around .075 when I pulled them.
 
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