Speedometer not working on 1998 S70 T5

spoton22 · Post by **spoton22** » 24 Mar 2010, 15:20

Need help! The speedometer on my '98 S70 became intermittent for a couple of weeks and has now stopped working. I have read all the forums and as a result already completed the following. Note ABS and Tracs failure light has been on for many months.
First I used the proceedure prescribed to repair the ABS ECU, re-solder all joints etc. re-installed without any success (no speedo and warning lights still on). Purchased a reconditioned unit from FC Groton (not very impressive, while in a sealed bag and labeled as re-worked and tested the unit was very dirty and had remnants of spider webs - gives impression not a lot of care taken by the remanufacturer!) anyhow, re-installed new unit and still no success. Then pulled each speed sensor cleaned, checked resistance all seemed in order. The rear units were caked with dust and dirt but I did not remove rotors to clean out.
Checked wiring from rear to connector in trunk but could not trace the wiring from the front sensors. Does anyone know the which pins on the ECU connector are the speedsensors as would like to check wiring continuity as well?

Appreciate advice as to what to do next.

Post by **jblackburn** » 24 Mar 2010, 18:01

Do you have any other ignition-switch related problems? E.g hard to get out of park, intermittent headlights, blinkers, etc?

Are your ABS/TRACS lights still on after the reconditioned module?

I've got the pin-outs, though I believe the speedo signal comes directly from the ABS module...I'll double-check that here in a few minutes.

spoton22 · Post by **spoton22** » 24 Mar 2010, 18:19

Hi, have had no problems with other ignition switch related items. Yes, the ABS/Tracs light is still on after the reconditioned module. If you have the pin outs can you tell me which contacts trace to each speed sensor?

Post by **jblackburn** » 24 Mar 2010, 18:43

It looks like B18 is going to be the speedometer signal to the ECU box, but the speedo signal comes directly from the ABS module to the instrument cluster on 96+ models, after taking the input from both front wheel sensors.

The signal leaves the ABS module at pin 6, and then heads directly to the instrument cluster at port A3.
I will attach both of the PDFs for the schematics for you to take a look at.

My suspicion is that if one of the front wheel sensors themselves or the wiring to them is screwed up (I nearly snagged mine replacing a strut mount), the ABS module will refuse to function. Honestly, I have no idea how you'd check the continuity of these wires short of shoving a huge length of wire into one end of the voltmeter and stretching it down under the wheel arch, and putting the other prong on the ABS module side. Unfortunately, the OBDII system is not very helpful for ABS codes as the older cars which would tell you EXACTLY what were wrong with them, and you may have to get the code read by a dealer or someone with a bit more advanced OBDII reader.

spoton22 · Post by **spoton22** » 24 Mar 2010, 21:16

Thanks very much for providing this detail. I will study and trace wiring and let you know how I go

spoton22 · Post by **spoton22** » 29 Mar 2010, 19:07

Thanks to jablackburn providing the wiring diagram / pin out for the ABS unit I was finally able to resolve the issue. Turns out the fault was in the cable from the wheel sensor connectors - interestingly both front sensors had failed cabeling. In one case the cable break was at the connector the other some 5" back from the connector. The benefit by measuring the sensor resistance from the ABS connector is that you can isolate the problem to the wiring or the wheel sensors - i.e. if you measure the sensor once removed from the vehicle it is most probable that it reads correctly. When measured from the connector if the cable is broken the resistance reading with be erractic and very high. Additionally, if you have long enough leads on your meter (or make extensions) you can do a direct continuity test to verify the connection status. I found the correct resistance measurement should be between 1300 -1800 ohms.

The wire within the insulation had corroded badly and thus was intermittent, in the case of the wire broken 5" back I was unable to strip the cable back to a point where I could be confident of the wire so ended up running a new wire. The front sensor wiring is joined to the wire connected to the connector with a crimp connection hidden under the fuse box (this is a little difficult to access if you end up replacing one of the cables as I did). I suspect that both the cabeling and ABS ECU was faulty so by replacing / repairing the module was probably a necessary step as well.

fraigan · Post by **fraigan** » 29 Mar 2010, 20:09

i've got an issue with swapping out my instrument cluster (metric units) to a standard u.s. cluster in my '98 xc.

quick question: is the ABS unit you're referring to in your thread the black box located above the instrument cluster ? i'm not able to get a constant readout with the cluster i swapped out, and i'm wondering if it's related to the ABS module. that being, if i kept the original ABS unit and switched out only the instrument cluster, would i get proper signal.

anyways, thanks for your time - i'll also post a new topic about my swapping out my instrument cluster following your response.

cheers!

spoton22 · Post by **spoton22** » 31 Mar 2010, 21:27

The ABS module is not located in the instrument cluster, it is the black module mounted underneath the ABS pump which is located under the master brake fluid container see: https://www.matthewsvolvosite.com/forums ... 651#p46820

Regret I do not know the answer to your instrument cluster problems - except to say the speed and odometer is definately derived from the ABS module, but the black box you refer is not the ABS module.