ABS Activating a low speeds - but only when sensors are not reporting speeds

[Konito]

2001 525iT
New sensors on all corners
Speedo working

I hook up my analyzer (Foxwell NT630 Plus) to the OBD port. The car operates in one of two states, switching sometimes without any rhyme or reason:
1. ABS BRAKE lights on, all wheel sensors reporting speeds on the analyzer under live data screen. But the warning lights are on
2. No dash lights, no ABS/BRAKE lights. Car operates normally, except the analyzer does not get speed data from the sensors. Under these conditions, the ABS pump activates at very low speeds (<10MPH) nearly all the time, even during light braking.

Most of the time is spent on #2, and occasionally in #1.
And the data from the speed sensors is either available for all sensors or for none of them. Again, Speedo working ok all the time.

Any idea as to what could be going on and how I can diagnose or fix it?

[Chedley]

Bad sensors (are they BMW oem?) or bad sensor connections/connectors.
Scenario 2 above is normal: ABS activates as it should when it does not detect speed. So, it is definitely NOT the ABS. Do not mess with it.

[ross1]

Were the new sensors installed attempting to fix this issue or did one fail and you changed all?

I used cheap sensors a while back, ordered two to have a spare and BOTH were DOA.

[Konito]

Here is the challenge with putting the sensors at fault:
1. The speedometer is working fine. So is the cruise control. Even when the analyzer does not read inputs from any of the 4 sensors, the speedometer is working fine. Are these systems getting their data independently (Speedo and ABS)?
2. The analyzer reads the proper speeds from all 4 corners for a limited time, then ALL the readings go to zero exactly at the same time. I have a difficult time reconciling that ALL sensors stop reading at the exact same time becuase the sensors are bad. Plus there is a limited time when they work.

The ABS pump only activates at low speeds (under ~20 mph). I removed the ABS controller connector, cleaned the terminals and put dielectric grease on them. Seems to have tamed the problem so it now only happens under 10mph.

Sensors were installed becuase the speedo was not working and I had the light trifecta. Rear sensor wires were cut (I just bought this car). I ordered cheap sensors online. Now speedo works and lights are gone.

I do appreciate the advise about not messing with the ABS unit for now, but still at a loss as to why this is happening. I dont like the idea of changing things to test out theories but I may just have to do that.

[ross1]

Unless your current symptom was present before the ABS doesn't like something the new sensors are doing.
I'd double check the splices and if okay bin the bogus sensors in favor of others. I've had othe aftermarket sensors that are fine out of the box but the infant mortality seems very high.
Changing more than is broken at one time is the kiss of death as far as diagnosing goes. Fix what's broken then move on the proactive repairs if you feel you must.
Still have the original sensors?

[crdiscoverer]

I've been repeating this a lot around here, but check your ABS module, specifically the valve body. We have annual inspections here and my car was activating the ABS when testing the front axle brakes, something it didn't do before. Recently the issue devolved into the car braking less with one of the front calipers. It turned out to be a partially blocked valve, the solenoid for the front left inside the ABS module was getting stuck.

If the car "thinks" one of the wheels is not braking with the same force as the others, it will assume it's a slippery condition and activate the ABS. It just gets confused. That could be your issue.

If you have exhausted all other options, get a used module from a junkyard and test it. No need to reprogram it unless you verify that was the issue.

[theWalkinator]

Check the brake light switch up on the brake pedal.