ABS resistance checks out, module repaired, still have codes

Brickbox8398 · Post by **Brickbox8398** » 04 Jan 2025, 07:28

I hate to be another poster of this topic since this has been beaten to death, but I think I have something different going on here. 98 V70XC had the light on. I had my S70 module repaired by mattymoo a long time ago when I was less experienced, about 12 years ago. However this time with all the writeups available I felt pretty confident I could repair it myself on this car. So I resoldered the connections, which cleared most of the codes and got power back to the unit and pump. But I have actually gone back and re-heated the rows of signal pins 2 more times since the light still wasn't going away for the RF wheel (fault in circuit). I did find a semi broken/corroded wire inside the sensor plug so i fixed that, no dice. Using a multimeter I found that I still had an intermittent continuity issue with that wire, so I cut and replaced it close to the firewall, chocking it up to internal damage due to previous front end work. The light still comes back for that wheel sensor immediately after clearing the code.

Note: My soldering skills are minimal, but I am pretty sure I am doing it right and putting enough heat into the pins. And since I have reheated the connections more than once you would think one of these days I'd get it to work out! I have seen many poor quality solder jobs on these boards throughout my research and I don't think mine is that bad.

Here are my resistnace readings at the controller plug (all within the expected 900-1380 range):
LF= 1.078kO
RF= 1.058kO
LR= 1.076kO
RR= 1.043kO

All checks out right? This tells me my sensors and wiring to each wheel is good

I haven't been able to find expected resistance AT the ABS controller pins (controller out of the car), but I took readings anyway because i thought it might tell me something and here is what I got:

LF= 3.78MO
RF= 7.27KO
LR= 7.75MO
RR= 7.76MO

I opened the controller again and targeted pins 18+3. Used my solder popper and resoldered those pins. Now for the RF pins I am getting 3.97MO. Not sure why front and rear sensor values are almost halved, however at least now both my front pin values are close? And all sensor pin pairs read within the MO range. So I plugged the controller back in and of course the code is still there and comes back immediately after clearing (VOL-FCR) which tells me there is still a problem. Anyone have any ideas? I have been trying to avoid the hassle of sending out my controller to midwest (although a great option I am thankful for), and I am pretty certain I don't need to replace a wheel sensor based on my findings. Again I am pretty sure my solder job isn't the worst, I heated each pin up until the solder was liquid and you could feel the pins wiggle slightly. But maybe i messed something else up. I am open to critque.

Post by **scot850** » 04 Jan 2025, 07:43

Have you checked the reluctor rings at the end of the axles for dirt or corrosion? I would suggest trying to remove the abs sensors from the wheel end (soak th e10mm bolt with penetrating oil for a while first to reduce the chance of the bolt snapping), remove the bolt and then try to remove the sensor. If you can get the senor out, see how dirty it is and clean it off. Look down the hole and look at the teeth on the reluctor ring and see is they are caked in dirt or rusted off or the ring has not swollen and cracked.

Neil.

Brickbox8398 · Post by **Brickbox8398** » 04 Jan 2025, 07:53

scot850 wrote: ↑04 Jan 2025, 07:43 Have you checked the reluctor rings at the end of the axles for dirt or corrosion? I would suggest trying to remove the abs sensors from the wheel end (soak th e10mm bolt with penetrating oil for a while first to reduce the chance of the bolt snapping), remove the bolt and then try to remove the sensor. If you can get the senor out, see how dirty it is and clean it off. Look down the hole and look at the teeth on the reluctor ring and see is they are caked in dirt or rusted off or the ring has not swollen and cracked.

Neil.

I figured the sensor and reluctor ring cleanliness wouldn't give me a "fault in circuit" code. But those are the two last things I haven't checkd yet so I will have to rule it out. I've been putting ATF+acetone on the 10mm bolt for a week so maybe i'll try removing the sensor today. Reluctor rings on all corners look crusty (car sat for 6 years) but I haven't given them a close look yet.

Post by **scot850** » 04 Jan 2025, 07:57

Be careful checking them as they can get a hairline crack in them with age as mentioned.

Neil

Post by **volvolugnut** » 04 Jan 2025, 09:23

Did you or anyone else use RTV (Room Temp Vulcanizing) to seal the box? RTF has acidic acid which can cause corrosion in the enclosure. Your circuit board looks a little crusty to me.
volvolugnut

Brickbox8398 · Post by **Brickbox8398** » 04 Jan 2025, 09:46

volvolugnut wrote: ↑04 Jan 2025, 09:23 Did you or anyone else use RTV (Room Temp Vulcanizing) to seal the box? RTF has acidic acid which can cause corrosion in the enclosure. Your circuit board looks a little crusty to me.
volvolugnut

I have read about that and no i didn't use RTV. Whatever i used didn't smell like vinegar which was the tell tale of what not to use. The inside might look a little crusty but a lot of that is plastic dust stuck on the remaining rubberized board coating after dremeling open the box. It also might look a little dirty from handling and opening/closing the box 3 or 4 times. I will be taping it up from now on until I get this figured out.

Brickbox8398 · Post by **Brickbox8398** » 04 Jan 2025, 12:11

I don't want to speak too soon but it seems to be fixed! All I did today was remove the suspect sensor and cleaned it along with the tone ring. As I said the tone ring was crusty, and i did find chunks of rust on the tip of the sensor. It IS a Volvo sensor by the way. Anyway, not sure how the above results in a "fault in circuit" code, to me that sounds like there is a break in the circuit entirely. You'd think it would only result in an "extrapolation" code. I think it is fixed though, and at least I learned something

Post by **scot850** » 04 Jan 2025, 13:47

When they get crusty and dirty, or the ring cracks is throws a fit as it cannot read the signal it expects.

Hopefully this is the issue and is a permanent fix.

Neil.

Post by **abscate** » 05 Jan 2025, 04:45

A good scanner can read wheel speed info and tip you off to a bad sensor or tone ring. I had one on SWMBO e83 that would shut the power down after 20 minutes in highway, that was unpopular to drive. No codes, I caught it but watching the wheel speed readout getting noisy above 30 mph

Post by **Matty Moo** » 12 Jan 2025, 19:27

If/when you originally scanned it, before repairing it, and got just an abs-312 (circuit fault) without an extrapolation or wrong wheel speed code with it then it is an actual fault in the circuit. With a module fault you'll always have one of those two preceding it.

If you want to rule it out send it in and I'll test it. Just pay postage.

I can recall maybe two times since 2010 where I've seen a circuit fault code come up and not clear that was an actual module fault. I'd guess 1/2000 on the 96-98 modules.

For as well engineered as the 96-98 cars were I never understood why the front abs wires were part of the actual harness without a plug-in to replace them. They did it from 99 on though.