S70 Tracs/ABS problem Topic is solved

shegarty · Post by **shegarty** » 27 Jul 2025, 04:34

Morning folks - I've been having some trouble with the ABS system on my 98 S70. Early last fall I started having intermittent issues with the system causing the ABS/TRACS OFF lights to illuminate. Read the codes which pointed to communication issues with the right rear sensor but since it was intermittent decided to let it go for a while and see what happened. I wasn't having issues with the speedo or odometer so it wasn't critical to address immediately. Over the winter it became permanent (still to speedo/odo issues) so I recently decided to start addressing the issue - also, because my oldest will be getting her licence later this summer and this will be her primary driver.

All of the research that I did said that it was unlikely to be a problem with the wheel sensor(s) and that the most likely culprit was the ABS module so I just replaced it yesterday with a rebuilt XEMODEX unit. However, I am still getting the same error messages saying that the right rear sensor is having communication issues. I can reset the codes but they return immediately. The strange thing though is that when I stream the live data each of the wheel sensors is reading the exact same speed and there doesn't seem to be any dropout on either of the rear sensors. I am going to inspect the wiring harness today but is there anything else I can test/inspect before biting the bullet for a new rear harness and two sensors? TIA

Vova585 · Post by **Vova585** » 27 Jul 2025, 04:58

"Returns immediately "? Meaning that without you driving the car, after erasing the code it is immediately back or after attempting to drive it?
If it returns immediately-electrical connector is likely an issue. If it returns shortly after starting to drive-try to inspect connector at rear sensor to harness joint point, can swap pins in the connector (RR to LR position so if the code migrates to LR your problem is in the abs sensor harness(I think it is the case where 2 sensors are sold as unit if my memory correct)). Another alternative is issue with the pin fitment at abs module(less likely).
More experienced members will hopefully correct me if I was wrong(it's been 15 years since I last had P1)

shegarty · Post by **shegarty** » 27 Jul 2025, 05:02

The fault will return immediately after clearing the code but it will still return clean data while driving. There is a common connector for both rear sensors but it’s a bit buried in the trunk under the beauty panels. When i pull it apart today to inspect it I’ll try swapping left to right if I can (if they’re not side specific) and see if the code migrates.

Post by **abscate** » 27 Jul 2025, 05:18

shegarty wrote: ↑27 Jul 2025, 05:02 The fault will return immediately after clearing the code but it will still return clean data while driving. There is a common connector for both rear sensors but it’s a bit buried in the trunk under the beauty panels. When i pull it apart today to inspect it I’ll try swapping left to right if I can (if they’re not side specific) and see if the code migrates.

Great diagnostic approach! You can swap the pins in the connector that lives near the fuel pump plate to see if it is a sensor or wiring issue. The one piece rear brake wiring loom is expensive in Volvo, but ATE is OE and don’t break the bank as much

Link to this replacement on a 1999 here

viewtopic.php?f=1&t=84246&p=461773#p461773

I went to the FCP site and the ATE IS NOW NLA. Genuine Volvo $133

Post by **abscate** » 27 Jul 2025, 05:38

Shegarty, just noticed this is for a 1998. Your rear brakes use a different wiring harness than the 1999 discussed above , sorry about that

shegarty · Post by **shegarty** » 27 Jul 2025, 08:12

Update - with the 98 the connector is one piece and idiot proof so you can’t install it reversed so I’m unable to swap connectors without splicing it. Testing for continuity one of the two leads to the wheel sensors comes back as a closed circuit and the other open. This would seem to indicate to me that I do indeed have an either a bad sensor or a break in the wiring somewhere.

Post by **scot850** » 27 Jul 2025, 08:24

Have you checked the reluctor rings? I'm still unclear if the code comes back immediately without driving the car, or only after reset and driving the car.

Neil.

shegarty · Post by **shegarty** » 27 Jul 2025, 08:26

Code comes back immediately after resetting before driving

shegarty · Post by **shegarty** » 27 Jul 2025, 08:50

Update - I was able to depin and repin the connector to swap inputs from the wheel speed sensors to the abs module and the fault follows and repin (ie swaps from RR to LR). This would further confirm that it is isolated to the loom or wheel assembly on the RR. Any pointers on how to examine the reluctor ring without disassembly?

Vova585 · Post by **Vova585** » 27 Jul 2025, 09:28

I doubt it is a mechanical problem with the ring itself. The problem would not return immediately if you are stationary and just cleared the code.
If you changed input from RR to LR inside the connector and issue moved to LR- your issue is actual sensor or a wire assembly. Remove wiring and inspect for rubs(break in wire) and try to restore if possible with soldering and marine grade heatshrinck.