P 0502 Vehicle Speed Sensor A Circuit Low Input

[timcrytser]

I'm getting the error code P 0502 Vehicle Speed Sensor A Circuit Low Input. I replaced the two front speed sensors (after reading that that may be the problem on some forums). Alas this was not the problem as I still get a brake failure warning, ABS, and Anti Skid Lights still come on.

I found this on another site
"P0502 = Volvo code ECM 510F
Diagnostic trouble code (DTC) ECM-510F (no signal) is stored if there is no communication between the ABS/brake control module (BCM) and the engine control module (ECM) or if the communication is distorted.

This signal comes via the CAN network, is the ABS light on as well?"

Can anyone give some direction on how to fix this. So far I am stumped. I also found a search on another site that talks about the ABS Reluctor Ring.

[DeezlWeezl]

I know this question is almost 5 years old, but I did a search for this just yesterday because I had the same problem and it was unfortunate that it was never answered. Well, I worked my way through the issue, so I thought I would post what I learned...
Short version:
Use multimeter (ohm/resistance) to check if your sensors are even good.
Clean grime from sensors.
Make sure ABS sensor ring is cleaned of surface rust and aligned exactly in the center (inboard/outboard) of the sensor mounting hole (might be caused by pinched/crooked axle seal)


Long version:
P0502 deals with the ABS/Speed sensor (system) that's located inside the wheels and mounted to the spindles. I recently did a suspension overhaul (first time) and was dealing with the consequences of my own shoddy job. The end result was that I had both a P0502 and an ABS-012 (faulty sensor signal). Fortunately, they're both related...
Here's what I did:
First thing is use a multimeter to measure to see if it is, in fact, a bad sensor. The end connections to the sensors are at the top of the engine compartment, on the firewall in each corner. They are very easy to reach, but kinda tight to disconnect (the driver's side more so). Disconnect the wires, set your meter to ohms/resistance and touch the leads in the sensor wires. I saw a video that showed how to do it, but it gave a different "good" meter reading, so I did both driver and passenger side on mine to compare, since I knew the passenger side was good. I think I set my meter to 20k and got a steady .961 after holding it there for a second. I'm not sure what those numbers mean, but both sensors measured the same, so I had good sensors!
The sensor in my case (e.g shoddy job) was unmounted for a couple of months. This probably allowed water and junk down the mounting hold and onto the sensor ring that's mounted towards the end of the axle. With the sensor out, I looked directly down into the hold and saw the ABS sensor ring. Didn't realize it at first, but, the ring was offset. It should be aligned in-between the diameter of the hole (inboard/outboard). The alignment must be very sensitive, because I didn't have the axle retaining bolt in tight enough during one of my test runs, and it caused a fault...
When I took the axle out, I found out the axle seal was cracked, crooked and smashed, which prevented the axle shaft from being completely in the spindle (off by 1/16"-1/8").
While I had the axle out, just for good measure, I took a wire brush to the ABS Sensor ring and removed the surface rust. Since the sensor creates an electrical signal, rust will interfere with that.
As a final step, not that there is any metal contacts on the sensor, but I wire-brushed off the bottom of the sensor to remove any grime that might also have interfered with the signal.
Extra tip: If you have a multi-system scan tool that will do live data readings on the ABS system, the speed sensors are "left velocity and right velocity". I'm not sure if it's the same in all cases, but at a stop, it gave me a reading of .60 kph. But, both the left and right values should read approximately the same while driving, if once it gets about 10km/h apart, it throws the fault.

[Juliocgil]

Hi guys, I got this problem and also the abs module failure, and the way I fix all those things was just changing the pads and rotos of the front wheels and all those codes magically disappear, hope this help someone save some money. ( note I indeed need the new pads and rotors)

[dono]

TLDR - Try cleaning your ABS Reluctor Ring first, its easy to do and worked for me.

2004 Volvo XC70 2.5T AWD Wagon ~118,000mi.

I did an overhaul of my front end and replaced brake rotors, pads & flushed brake fluid. Of course without a lift there was lots of under the car grovel - so fun. After I buttoned it all up, on the way to get the alignment done I threw an ABS code 'Wheel sensor left front faulty signal/low voltage.' Wondered what I could have done as I was very careful with the abs sensor and safely tucked it away during axle removal and brake work. My first guess was the sensor had somehow been damaged (not any good at electrical but I did measure ohms & they seemed out of spec) so I installed an aftermarket sensor - after resetting the abs fault code I drove and no joy, the error message returned. The parts cannon was out now; I figured the after market part must be to blame so I ordered a genuine volvo sensor, a couple axle dust covers and a couple axle bolts (recommended to replace whenever removed). By the time I got the volvo sensor & the rest in the mail, I had had time to think and do some research and wondered if the reluctor ring (OEM part #30735955, 9466017) might have been damaged or the axle dust cap might have been crushed or misaligned during re-installation of the axle. I removed the wheel and examined the reluctor ring through the sensor socket.

There were two small dings on a couple of crenellation-like ridges on the ring, but otherwise it looked unremarkable and aligned. Before I re-installed the sensor/wheel I gave the reluctor ring (and sensor socket) a good cleaning with brake cleaner. I just sprayed the cleaner generously into the sensor socket while rotating the axle a couple times. I finished by swabbing the walls of the sensor socket, reinstalled the sensor/wheel, erased the abs code and went for a test drive. To my surprise the code has not reappeared. Luckily I had a return to FCP to send in anyway so the Volvo sensor, spare axle bolts & dust caps go back for a return. Again, Im really surprised that whatever grease, or brake dust came off (there was a fair amount that dripped out when I sprayed) was the cause of the abs fault but this appears to be the case.

[br0dy519]

Both times my ABS light came on it was the reluctor ring. I bought two off ebay, Febi Bilstein brand, for cheap. Both were already cracked and fell right off the axle. I always urge people to check these and have spares whenever they have planned work in that area.

Whenever you plan to replace a sensor, always ohm out the existing one before you throw a new one at it.