If its a Bendix system the ecu should be on top of the rear relay valve. There will be LED lights on the side to indicate where the problem is. The fault codes can be reset by holding a magnet next to the lights for 20- 30 seconds.

If its a Rockwell/Meritor there may be a blink code switch somehwere, near the diagnostic connector? Hold the switch for 1 sec & release & it will blink out a 2 digit code. Hold it for more than 3 sec to clear the codes. Light will blink fast 8 times to indicate codes were cleared, The light will stay on till the truck is driven more than about 4 mph .

maybe try googling bendix abs, meritor abs

i think going to need more help on this than me.
 

ECU relay and where is this? and where does this stuff light up at? by pushing switch what does this do to light things up and hopefully the switch works. Totally confused.

Bendix has been "off and on" with those indicator lights; the ec-16 used them, the ec-17 didn't for example. ...so, I wouldn't look too hard for them. If they ARE used on the ECU, they are; green to indicate power, and a series of 3 red lights to identify the fault.

However, in an '01 9400, my fading memory tells me that the ECU will be found inside the cab, behind the driver's side kick panel (to the left of the clutch pedal), and it's not even easy to locate once that panel is removed; BTDT. There will be a small, round twist-type ?black? connector (maybe 12-wire?) that passes out through the firewall just in front of the driver's feet... I would first remove that connector (from the outside) and inspect for green death.

There is not much point in resistance testing the sensors unless you know which one to check; I would use allan's "sensor connector swap" idea to narrow it down if the firewall connector checks ok.

You didn't happen to get a phone installed recently did you? Just joking. Today I worked on a 2000 Mack that had an ABS light on. Driver said it started after he had a phone installed. Pulled the codes and it just showed a short to ground on right rear sensor. Harness from sensor to firewall connector tested good. Went inside and saw where they mounted some type of booster or something for the phone. It was screwed onto the metal panel under the right side of the dash. Pulled the panel to continue tracing wires from the firewall and found the Wabco ECU on the backside of the panel. You guessed it. The phone installer put a self tapping screw right through the panel and into the back side of the ABS ECU and shorted out the circuit board. New ECU is 985.00. Driver wasn't happy and I"m pretty sure who will get the next call on his new phone. The MACK VCU module was right next to it and it had a screw through the back of it also but it didn't go deep enough to hit the circuit board.

I think this is way over my head
 

I better see if my trucker neighbor can help me out on this.No didn't install a phone recently but if I had probably would have found the problem quicker. Me and that guy with the screw gun would have some serious talking to do! The nasty cold winter weather here is probably the cause of the problem,wash the ol" girl up and see what I can find. Sure alot I don't know about this truck wish I had a manual to explain and show me where this stuff is.