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Dodge Ram 1994-2001: Why is My 4WD Light Flashing?

There's nothing more annoying than when you see a flashing light illuminating on your dash—it always spells trouble. But when that light in your Ram truck is flashing "Service 4WD," you might be able to remedy the source quickly with a little bit of troubleshooting.

By Keith MacDonald - July 20, 2015

This article applies to the Dodge Ram 1500 2nd Generation (1994-2001).

Whenever you witness a warning light flashing on your truck's dash panel, the illumination normally spells trouble. When that light reads "Service 4WD," the transfer case may be partially (or even fully) engaged in 4WD mode. However, sometimes it's not an issue with your shift linkage or gear box at all–it might be the vacuum-operated sending unit or "switch" that is simply stuck in-between modes due to age or clogging road grime. The cause may even be a leaky vacuum line leading to the unit. In any case, it's not too difficult to diagnose and remedy the flashing 4WD warning light.

So how do you stop an annoying "4WD" light from flashing on your Dodge Ram's instrument panel when you're certain the truck is in 2WD? Continue reading for the answer.

Materials Needed

  • 27mm deep well socket (or wrench)
  • Ratchet
  • Utility knife
  • Replacement vacuum light switch/sending unit (if needed)
  • Tape

Step 1 – Check the shifter

The gears may not have been shifted correctly.

Normally, your transfer case must shift into neutral before 2WD or 4WD can be engaged. If it fails to engage, shifting the transfer case around might stimulate and restore vacuum flow to the light switch. Dodge mechanics suggest trying these quick-fix methods if the "Service 4WD" indicator light comes on:

  1. Shift the truck into 4L.
  2. Then shift into 2H.
  3. Then shift back into 4H.

If the indicator light does not turn off, try the reverse gear method:

  1. Shift the truck into 4L.
  2. Drive straight in reverse for approximately 20 feet.
  3. Then shift back into 2H. The indicator light should turn off.
Figure 1. Transfer case shifter.

If the two shift reset methods do not turn the light off, move on to the following step.

Step 2 – Check your vacuum lines and the switch

Is it sticking?

  • Locate the vacuum switch connected to the transfer case of your truck. You will see four vacuum lines attached.
  • Number the vacuum lines with pieces of tape as you remove them, just to be safe. Putting them back the way they came out is always a good idea.
  • Visually inspect each rubber vacuum line for cuts, chaffing, or pin holes.
  • Use a 27mm deep well socket (or wrench) to remove the vacuum switch. Be extra careful of the four plastic nipples on top, or you'll have to buy a new switch.
  • Remove the switch and clean it thoroughly with electrical contact cleaner or alcohol until it moves freely.
  • If the switch is stuck, a new vacuum operated actuator switch will cost approximately $17 to $20 at the auto parts store.
  • Figure 2. Location of Dodge Ram vacuum switch.
  • Figure 3. Remove the switch using a 27mm deep well socket and ratchet.

To re-install the switch, attach the vacuum lines, and remove the "Service 4WD" light:

  • Screw and tighten the switch back onto the transfer case. Tighten it to a snug fit; you do not have to torque it down.
  • Reconnect the vacuum lines.
  • Shift the transfer case into 2WD and start the engine.
  • Drive the truck in reverse for 10 to 20 feet, and then shift it forward to drive off.
  • The "Service 4WD" light should stop flashing. If not, return to Step 1 and try the reverse gear method again. If cleaning the switch didn't remedy the problem, you might want to purchase a new one.

Pro Tip

Take extra care when removing and cleaning the switch. Chances are it just has some grime in it, so save yourself $20 by carefully removing and thoroughly cleaning it.

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