ABS Sensitivity?

It depends on the make and model of the car, but the short answer is yes.

The way some ABS systems work is by sensing a slight difference in rotation speed between wheels to detect slippage before a wheel skids and reducing brake pressure to that wheel to prevent a skid.

Sounds more like a warped rotor to me...or maybe someone just got a little oil on the surface of the rotor...I guess it could be hard spots created by heat on the rotor surface....

..."Hard spots" created by heat on a rotor surface are areas where the metal has been significantly altered due to excessive heat, usually caused by heavy braking, resulting in a localized hardening of the rotor material, often referred to as "hot spots" and can manifest as discolored patches on the rotor face, leading to vibrations and uneven braking when the brake pads engage with these hardened areas; this is typically caused by friction material deposits building up on the rotor and generating extreme heat during braking"....

https://www.reddit.com/r/MechanicAdvice/comments/1013gjn/spots_on_front_brake_rotors_symptom_or_cause_of/

https://community.cartalk.com/t/warped-rotors-after-just-10k-miles/153747?page=2

This just started after the new tires were installed.

I would hit the rotor and pads with some brake clean...

I know what Brake pad/disc pulsations feel like. This is not that for sure and the pulsation does not change with speed. it is the exact same feeling in the pedal as when you are sliding in the winter ice and the abs kicks in.

Just because it feels the same, does not mean there could not be another cause that produced a similar effect...It seems most likely that the brake rotor and pad interface has been contaminated or damaged in some way...I would at least try the brakcleen first...it's fast, easy and cheap...If that doesn't work then I would pull the ABS fuse and see if that fixes it...

https://mechanics.stackexchange.com/questions/32993/brake-pedal-is-pulsing-when-i-apply-brakes-is-it-caused-by-abs

You might just need to pump up the rear tires a bit....

GA: although this may not be a permanent solution, it should easily prove the ABS sensitivity theory.

the fuse is a good idea to isolate the issue!

Try switching traction control off?

As a test, you may want to switch / rotate the position of the two new ones with the old tires, just to verify your suspected cause of vibrations...

Since any vibrations can be caused by any misalignment, wheel balancing, difference in the rubber materials, the vintage and age, mixture of the raw rubber used, especially if the tires were of different age and brands?

It is also possible the source of vibrations may follow, may disappear or become completely unnoticeable after you switched tires mounting and positionings..

This is interesting but a little confusing. I haven't done the math, but it would seem like left side vs. right side differences would be greater on a curved road than than the rotational differences caused by differences in tire wear.

I'm guessing that the snow tires have a softer rubber composition and squirm a bit when you are breaking, triggering the ABS.

https://www.continental-tires.com/products/b2c/tire-knowledge/braking-with-new-tires/#:~:text=This movement comes from the,a large amount of squirm.

A softer rubber could be an issue, but those are usually on a high performance tire.

I have to agree with SE that it more than likely a warped rotor. On higher mileage cars/vehicles, you can feel that. Even on my Mercedes I can feel it.

Front pads and rotors were changed about a month ago. And prior to last friday no pulsing, got new tires on front and now the pulsing.

It could be a case of "brake shudder". I work for an automotive OE and we see a widespread problem of "brake shudder". Not sure if it is dust on the new pads that are not cleaned properly or a brake pad material issue. I would start by cleaning the rotors (disks) and pads and make sure there is no dust ingress while reinstalling. When you reinstall the tires, clean all dust off the rims to prevent dust ingress and install. If this does not solve the problem, the rotor/disk surface probably needs attention.

GM cars seem to be more fussy than other manufacturers about different size tires and the ABS.

Are you getting the ABS light on the panel?

These gearheads discussed this issue:

https://www.svtperformance.com/threads/different-tire-sizes-throwing-abs-codes.837612/

If the tires were not properly stored before purchase,they could have developed a "set" or slight warping in the tire.If so,it should disappear after a few miles of driving.To test the tires for out of round,or warping,jack the car up until the wheels just clear the ground by an inch or so. Then place a socket between the tire and the hard surface.Lower the tire slowly until it barely clears the socket.Now rotate the tire manually and watch for a change in the gap between the tire and the socket.A minor amount is normal.If you are precise in your lowering of the tire until you can just see daylight between the tire and the socket,it will give a very precise indication of warping.

Almost forgot:Be sure the air pressure is the same in all tires.

Good suggestion,… may not be the solution…

I store my car in the winter, instead of putting the car on blocks which was the procedure a generation ago for domestic cars, the recommendation for storage for the type of car I have is to over pressurize the tires to eliminate or reduce that warping as well as for the suspension.

If the front brake rotors are the "Hats" that slip over the wheel studs on the hub, rust and dirt on the mating surfaces may be the issue. Another possible source of the pulsation may be uneven torque on the wheel studs. Either of these conditions will warp the rotor surface and result in the pulsation you described.

But again if it had anything to do with the rotor(s) then this pulsing frequency would change with the cars speed would it not? In this case it does NOT.

It could be 40 Mph or 10 Mph and it's exactly the same.

Also it is NOT constant.

It is a pedal vibration for maybe a second, then wait a few, ten some more then wait, ba bla bla.

I agree that you should check tire pressure on all tires. I got home after getting 4 new tires one time and checked mine. One tire had 80 pounds in it! Let us know what you find.

If the problem occurred only after the new tires were put on, the problem is in the tires, but not necessarily the size of the tire. Put about 20 miles on the tires at highway speed and see what happens after they warm up and erase the cold set on them.