ABS help please.

[SS2115]

We inherited a low klm 850 sedan from a relation. It had been standing for long periods unused and is in excellent looking condition.
Once it started to be used after we got it, some problems have cropped up and mostly dealt with except for the ABS system.

All four calipers were completely overhauled as they were stuck causing the car to dive left and right. Fluid all completely drained through and all flex lines replaced for safety sake.
Car brakes really well but always has a tiny initial dive left and then straightens up. After investigating for a while, the shop concluded the ABS pump was probably also jammed or frozen somehow as the fluid to the right front caliper was slower moving than to the left caliper.

The ABS hasn't been working which we knew because you can lock the front wheels up quite easily without the jabber jabber of the ABS operating. However, no fault light and no fault codes. The globe is okay as its seen on the ignition on self test.

So eventually we found a good condition ABS pump which we swapped in this weekend. Bled the system out again. Car now has a slightly more travel pedal but brakes dead straight - no pull at all.
BUT - still no jabber jabber from the ABS. Brakes lock the wheels with little effort and the car really brakes hard to a stop. Great. But no ABS, and no warning light and no error codes.

We're at a loss.
I know its common for the module to fail but everything I've read and know is that the ABS light will illuminate with any module fault. Its even supposed to light if the ABS fuse is pulled. And also often the speedo stops working. We have none of these problems.

I'm wondering about the long skinny black relay that the ABS motor is plugged into. Could it perhaps not be firing the motor but the control module thinks it is? ie: is there any feedback from the motor to tell the control module its working. There are two blue wires which "may" be feedback wires - I don't know.

I'd like to get the ABS working as my son drives the car and its safer in the wet for the ABS to be working. He'll have a better chance of steering out of trouble.

[Matty Moo]

If the pump motor were to be shorted out the abs module would know and trigger a code.

Make me think that maybe somebody did some creative engineering with the abs light.

Pull the abs fuse and see if the light stays on.

[SS2115]

Hi.
Cant imagine anyone playing with the electronics as the car was purchased new by my wife's godmother when she was 75 years old and given to us last year after she stopped driving in 2015. Serviced religiously at Volvo Dealer.

But I went and checked anyway - started engine, no ABS light, pulled fuse# 29 for ABS Control Unit and ABS light lit up immediately.
Also pulled 30 amp fuse for ABS Pump which didn't light the ABS lamp but then its only for the motor so doubt it would light up except perhaps if the motor was unplugged (ie: continuity check such like a globe checker module does for headlights and tailights etc).

[jimmy57]

If you removed fuse you'd have to drive through 40 km/hr to allow the motor op test that would discover the motor fuse was out.
For that matter after you did the work you'd have to drive faster than that speed for system to ever be enabled the first time. Did it get driven 24 mph/ 40kph after the repairs?

[theWIFES_S70]

Hi.
Cant imagine anyone playing with the electronics as the car was purchased new by my wife's godmother when she was 75 years old and given to us last year after she stopped driving in 2015. Serviced religiously at Volvo Dealer.

But I went and checked anyway - started engine, no ABS light, pulled fuse# 29 for ABS Control Unit and ABS light lit up immediately.
Also pulled 30 amp fuse for ABS Pump which didn't light the ABS lamp but then its only for the motor so doubt it would light up except perhaps if the motor was unplugged (ie: continuity check such like a globe checker module does for headlights and tailights etc).

Matty Moo is the resident ABS specialist around here.

I think you're overthinking it. If your calipers are free and you replaced all your lines and you aren't losing fluid, there's probably nothing wrong. I'd guess you need to bleed your ABS unit of air a little more. Mark a line with a sharpie on your master cylinder reservoir and check it to see if it's stable. (Maybe you're losing fluid, from a line?) More than likely though, I think it's air in your abs pump. There are a bunch of individual channels that need to be bled individually (i.e., headache unless you know what you're doing).

A shop that specializes in Euro cars could probably bleed it for you for a hundred maybe one-fifty. I bet you an oil cap gasket if you do that, your braking will be smooth as butter! (Or rather, a smooth as 20-yr-old butter!)

[SS2115]

[For that matter after you did the work you'd have to drive faster than that speed for system to ever be enabled the first time. Did it get driven 24 mph/ 40kph after the repairs?]

I didn't know that and cannot say for sure if I exceeded 40Kph on the crash stop tests but would have thought I did at least a couple of times. But back residential streets so maybe I was more cautious and seemed fast but not over 40kph.

[More than likely though, I think it's air in your abs pump. There are a bunch of individual channels that need to be bled individually (i.e., headache unless you know what you're doing).
A shop that specializes in Euro cars could probably bleed it for you for a hundred maybe one-fifty. I bet you an oil cap gasket if you do that, your braking will be smooth as butter! (Or rather, a smooth as 20-yr-old butter!) ]

Air in the ABS shouldn't prevent the motor being actuated and getting the judder judder through the brake pedal and car of an ABS system working
And the brakes are perfectly smooth - thats the entire problem. ie: very smooth and no judder judder of the ABS pump coming in. I can lock the front wheels with little effort.

[theWIFES_S70]

And the brakes are perfectly smooth - thats the entire problem. ie: very smooth and no judder judder of the ABS pump coming in. I can lock the front wheels with little effort.

Uhm...

Just to recap:

You have perfectly smooth brakes on a 22 yr-old-car.

You think your ABS isn't working, because you aren't throwing a code.

You pull the ABS fuse and a code triggers. (So the electrical is working.)

Yeah...

My only suggestion would be to wait until it rains hard one night and slam on the brakes while the car is doing 40 mph+. To be absolutely sure it isn't working. The ABS has only ever triggered on our car (98 S70) during a really bad rain storm when I was going a little too fast.

I'm willing to bet (another oil cap gasket) that the ABS turns on. If it doesn't you can readily get those pumps on eBay for relatively cheap. But pump and install... I don't know. Your already doing right by putting your son in a P80. These cars are safe. I'd rather spend that money on replacing an airbag or something... But that's just me.

[SS2115]

[If it doesn't you can readily get those pumps on eBay for relatively cheap. But pump and install...]

I've already done just this! Replaced the ABS pump with a still wet one from an 850 and supposedly working ABS pump unit from an eBay seller.
Hence this thread because it has fixed the slight pulling we had and the pedal feels more 'normal' now that both calipers are moving at same speed and distance, but the ABS doesn't seem to be working.

I'm hoping it may be the road speed as mentioned. Or maybe I need to have one wheel on tarmac and the other in the gravel edge perhaps to get the ABS to activate?
Right now the car stops on a dime and can lock both front wheels. But experience with all other ABS equipped cars I've driven (including my present 2002 BMW) is that the ABS should activate immediately any wheel locks regardless of road speed.