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Randall C
Randall C, ASE Master Tech
Category: Cadillac
Satisfied Customers: 12890
Experience:  ASE Master Tech, 32 years exp,Cadillac 6 yrs
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Have a 1994 Cadillac Deville 4.9. Alternator will not charge battery.

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Have a 1994 Cadillac Deville 4.9. Alternator will not charge battery.Had it tested twice off car and is OK. Battery is about a year old when charged starts car OK. Charged it up overnight and showed on digital meter 12.35v. Start car and volts across terminals was 11.98v and increasing RPM, volts went down to 11.78.Idiot light does not light with engine key to on or when car is running. Alternator connection has an S wire(larger red wire)and L wire (smaller red wire)These wires had been spliced when new alternator was put on 2 years ago so checked voltage at S wire engine off showed same as Battery volts. L wire showed .025v. Engine running, S wire at splice was 11.48v and L wire was 10.78v.Could the lack of idiot light showing up means alternator is not sensing that it is time to keep battery charged and take care of other voltage needs on engine?

Hello, I can not say if you have a wiring problem between alternator and light but will confirm 100% that the light has to work for th alternator to kick in and charge. the light is part of the "power" styetem to the internal regulatr to activate "volt sense". I pu the diagram below and you will see the bulb in the circuit and the volt sense at alternator. A burned out bulb can allow the alternator not to work.




Randall C and 2 other Cadillac Specialists are ready to help you
Customer: replied 7 years ago.
Thanks for your reply. Is the idiot light something that is easy to replace or is it hard to get to and involves a lot of expertise to get at it? I live in near Atlanta and there is a car electric repair place I could contact about repacing the bulb but is 32 miles from me so if this is quite a project for one to do, would I be able to make it there on a fully charged battery ot woud I need to take along another battery as a back up?
Customer: replied 7 years ago.
Would have appreaciated to know if I could change the bulb myself/ Guess that was another question and the part about doing everything to get you help on your problem is not exactly true

Sorry, If a question is a 2-3 part qestion about same problem, we all usually help the customer. in your case, it got late and i logged off last night and did not see your reply till this AM. If i rememebr correctly, you can get to the bulb after the top of dash is off. Pulling the top off is one step to pull the cluster out to get to bulb. Directions are below. You may get top off only and reach to bulb from behind cluster, if not, you will have to pull the cluster out which after top is off, there are 4 screws. There just is not more info to send as far as pics etc due to gae of car. this is the best i have onhand



  1. Disconnect battery ground cable.
  2. Pull fuses A-5-IPC and B5-IPC from rear compartment fuse panel, pull fuse A3-IGN1 from engine compartment fuse panel.
  3. Using a small flat bladed screwdriver, pry defroster grill upward off of instrument panel upper trim panel, Fig. 17.
  4. Remove sunload and headlamp auto control sensors from defroster grill.
  5. Remove three upper trim panel attaching screws through defroster grill opening.
  6. Remove heat and A/C vents from front of instrument panel.
  7. Remove four upper trim panel attaching screws through vent openings.
  8. Remove upper trim panel from instrument panel.
  9. Disconnect electrical connectors from top of instrument cluster.
  10. Remove four instrument cluster retaining screws.
  11. Raise instrument cluster, then remove two PRNDL mechanism attaching screws.
  12. Reverse procedure to install.
Randall C and 2 other Cadillac Specialists are ready to help you
Customer: replied 7 years ago.

Thanks for getting back to me and sending the schematics on changing the idiot light. Getting at the light is quite involved so I have had the car towed to a Auto electric repair shop to have the kuight fixed so the alternator will charge again, I have an issue with too fast of an idle and will get back to you another time to see what suggestions you have for that problem.

Thanks for your help