4 wheel drive indicator light flashes on and off while driving

[TD97]

1997 Dodge, 12 valve, 4 wheel drive, standard transmission.

The 4 wheel drive indicator light started flashing on and off while driving just every once in a while after using 4wheel drive. Now it flashes and sometimes comes on steady while driving. I do not hear anything and when turning on a hard surface the truck does not lurch so I do not believe it is engaging. Any ideas where to start?

 

[petersonj]

Does your truck have a CAD (Center Axle Disconnect) on the front axle?

- John

 

[TD97]

It has a vacuum actuator on the front axle towards the passengers side. Not sure what is called.

 

[petersonj]

You have the CAD unit on the front axle. The 4WD indicator lamp is actuated by a switch inside the CAD unit. The CAD unit engages / disengages the right side axle shaft splines via a vacuum switch located inside the transfer case.

The mode of operation for a manual shifted transfer case is as follows:

* 2WD - Transfer case selector in 2WD - front driveshaft disengaged, CAD unit disengaged, 4WD indicator lamp off.

* 4WD - Transfer case selector in 4WD - front driveshaft brought up to speed and engaged, then the vacuum valve inside the transfer case is activated to engage right front axle shaft. The 4WD indicator lamp activates.

If 2WD is selected, then the front drive shaft is NOT engaged to the transfer case. The truck will not be in 4WD whether the CAD unit is engaged or not engaged.

You will need to verify that the CAD unit is working properly. It is fairly easy to disassemble and inspect for worn parts. It could be that you just have a faulty indicator lamp switch, or it could be more serious than that.

- John

 

[TD97]

You have the CAD unit on the front axle. The 4WD indicator lamp is actuated by a switch inside the CAD unit. The CAD unit engages / disengages the right side axle shaft splines via a vacuum switch located inside the transfer case.

The mode of operation for a manual shifted transfer case is as follows:

* 2WD - Transfer case selector in 2WD - front driveshaft disengaged, CAD unit disengaged, 4WD indicator lamp off.

* 4WD - Transfer case selector in 4WD - front driveshaft brought up to speed and engaged, then the vacuum valve inside the transfer case is activated to engage right front axle shaft. The 4WD indicator lamp activates.

If 2WD is selected, then the front drive shaft is NOT engaged to the transfer case. The truck will not be in 4WD whether the CAD unit is engaged or not engaged.

You will need to verify that the CAD unit is working properly. It is fairly easy to disassemble and inspect for worn parts. It could be that you just have a faulty indicator lamp switch, or it could be more serious than that.

- John

 

[TD97]

Thank You John
I’ve been laying underneath it.
I’ll let you know how it comes out.
Tim

 

[JMcCoy]

I would take the vacuum actuator off and make sure it didn't suck water into it. Water restricts the actuators movement. A little water will make the switch flicker, a lot will cause the CAD to fail. Ask me how I know.

 

[Chief USA]

Check the vacuum hose to the actuator to see if has cracked with age, broken or pulled off.

 

[TD97]

Thank Y'all for the advice. Truck has almost 600,000 miles and only had trouble once with the front wheel drive assembly which was in the CAD. Thank You John for the correct terminology, it helped me find some u-tubes on the system.
I found several hoses in the system which were spongy, sloppy, cracked or just not a good seal and replaced them and everything works as it should. The vacuum actuator was dry inside and everything looked good there, and the steel lines showed no signs of rust.
I changed the fluid in the transmission and transfer case and all the oil that was drained looked good. There was some silver metal in the transmission oil that was drained but not an alarming amount for the mileage.

Thanks again for the help,
Tim

 

[petersonj]

The CAD unit has served you well for almost 600,000 miles and it looks like it will continue to do so. Thanks for posting the results of your diagnosis.

Have you ever considered the 2WD low range modification? I find it very useful and smooth for backing heavy loads / trailers when steering sharp turns on any hard surface.

- John

 

[TD97]

I would be interested in that especially when we are picking up hay in the field. I looked into it one time but did not follow through.

 

[Cummins12V98]

I installed the 2 low kit on my 98 12V, it made maneuvering my RV on grade a breeze.

 

[JMcCoy]

Glad you got it figured. I have the lockout hubs for 2 Low. Absolutely awesome.