ABS and TRACS light under hard stop (Faulty MC sensor) Topic is solved

ryanmcgrim · Post by **ryanmcgrim** » 20 May 2016, 14:02

I've got a 96 Volvo 850 NA Auto that has been pretty good to me. I had the ABS module rebuilt last year in order to get Tracs and ABS light off, and it's been working great. I noticed this week that in the case of a sudden stop, the ABS and Tracs light will come one and stay on until starting the car again.

The car needed no brakes and a brake fluid flush, so I decided I would tackle that first. I replaced the rotors, pads, lines and fluid. All seemed perfect until today, when I had to stop suddenly due to someone pulling out without looking at all, and these lights popped on again, staying lit until restarting the car, where they would go off after a few seconds of starting the car.

In testing this, I found that these lights will come on under 100% of all hard stops, but will not come on during normal braking.

Any ideas on where I should start to track this down?

Thanks!

Post by **Matty Moo** » 20 May 2016, 17:21

Check your two wires that go from the abs module to the pump. Chances are they're bare and when the abs engages the module is picking up that they are touching.

Post by **dyn blin** » 20 May 2016, 19:34

I had this symptom when the vacuum line to the brake booster split. I replaced with silicone, 8mm. Some warn that the silicone will collapse, I've never had that occur, and it solved my issue.

ryanmcgrim · Post by **ryanmcgrim** » 23 May 2016, 09:40

Thanks for the info guys, I had a full weekend and wasn't able to check the car out, but will do so tonight. Hopefully it's one of these two simples sounding issues!

ryanmcgrim · Post by **ryanmcgrim** » 26 May 2016, 11:47

Well, I had completely re-insulated the two wires going from the abs pump to module when I put in the new module last year, so it was no surprise that when I checked it, they were still in good shape and not touching, so I moved onto the vacuum line to the brake booster, but it too looked to be in great shape. I'm not really sure where to go from here. I guess I'll just keep plugging away looking for things that don't look right in the rest of the system and pray there is something obvious going on.

Post by **abscate** » 26 May 2016, 12:03

I think its time to get/access a reader than can read the ABS codes to see if you can isolate this to a wheel or the ABS module

Maybe next time Im in Boston ??

Post by **scot850** » 26 May 2016, 13:43

I am 100% with abscate on this. Sometimes it pays to get the codes read (or buy a Dice unit for $100 US or so). It will save a lot of time and effort and mostly wasted money chucking parts at the car hoping to fix it.

Now if I can only get the suspension to give codes as to why it groans (it is non-electrical!)......

Neil.

ryanmcgrim · Post by **ryanmcgrim** » 12 Jul 2016, 07:37

I submitted an update to this post, but it appears that it never posted. The problem is now fixed. Thanks to everyone who chimed in with ideas, they were all good. The problem ended up being a faulty master cylinder brake pressure sensor. Somehow brake fluid was leaking though the pressure sensor, into the insulated wires behind the switch. The wires were wrapped with electrical tape and a casing, so there was no visible leak of brake fluid, just a very very slow leak that barely presented itself in the level of the MC. I guess under heavy braking, the leak at the pressure sensor was enough for it to pop a code. THey don't make this part anymore but i was able to get it at a junkyard. They didn't even charge me! (THe part is $200-300) otherwise.

ANyway, just wanted to update in case anyone else has this issue, it's a good place to check.

Ryan

Post by **scot850** » 12 Jul 2016, 16:24

Glad you found this unusual fault.

Neil.