Hi all!

I bought a 1998 F250 5.4 XLT, 4x4, automatic gear shifting, electric 4x4 HI/LO switch.
I'm living up on a hill and when I drove back home last time there was snow and I had to use 4x4 all the way up back home. 4x4 worked fine. I parked the truck and switched of 4x4.
But when I started the truck next time and switched in a gear the 4x4 dash light started blinking. But different to many post I read so far the 4x4 dash light is not just blinking 6 times every 2 minutes, it is blinking constantly very fast (double time of flasher) as soon as I switch in a gear, even without moving the car. The dash light is blinking regardingless to the position of the 4x4 switch. I realized that interior lamp is flashing in same rythm as the 4x4 dash light.
When I shift from 2HI to 4HI 4WD does not engage but it sounds like shift motor on transfer case is doing something. When I switch to 4LO 4WD does not engage either but the gear reduction works.
I noticed that when I drive downhill and put on brakes the 4x4 dash light stops blinking for short.
What might be the problem?

What I tested so far:
- All fuses (under dash and under hood) are okay
- Relays under the hood (they switch when I connect them to 12V)
- Switched heathing mode selctor to panel. Air comes out of panel. Then switched on 4x4 and air comes still out of panel. Not switching to window. So vacuum pump system should be okay as well, right?
- disconnected car battery for a while. Reconnected, started engine, put in a gear and 4x4 dash light was NOT blinking any more. But as soon as I drove a few meters it started blinking again.
- When I drive the speed indicator works as well. So shouldn't be the rear speed sensor, right?

What else can I test?

Thanks for any advises!
Chris

 

It all boils down to a flashing 4WD dash indicator and airflow control.
.
First, the airflow is not using an air pump, other than engine vacuum.
Look for a vacuum hose issue.
Second the 4wd dash light is controlled from a module.
The module gets it's signal to light the dash indicator from the 4wd firewall solenoid, after it operates.
My feeling is you have a vacuum issue that may be affecting the 4wd but it could be a separate issue if you find the airflow problem and it does not fix the 4wd.
If not, the rapid flashing its the lamp circuit from the module.
Check for a leaky windshield that has dropped water on the module and caused corrosion. It is located on the back side the dash fuse panel.
I would check the harness plug for corrosion and being wet down at the transmission.
Your going to have to put some effort into checking all these areas.
Good luck.