It seems to me the more I read we are winning the information battle, the harder it is to get good diagnostic repair information, especially when it comes to antilock brake system (ABS) problems.

I'm seeing more and more ABS problems lately that are taking longer to diagnose because of inaccurate or missing information. And trouble code charts for ABS ... talk about taking the scenic route. Some of the ones I've seen lately look like they were crafted by a politician as an artful dodge to a question he didn't want to answer. An answer is inferred ... kind of, but not to the question that was asked.

I know it's not just me because I get a lot of these jobs from other shops. I've also read about similar circumstances to mine in the International Automotive Technicians' Network (iATN) knowledge base fix database (members only, www.iatn.net).

A couple of weeks ago I had a 1998 Cadillac Sedan Deville that was turning on the ABS light and the traction control system light. It had a code 1233, which my data information source tells me is a left front wheel speed sensor open or shorted. (I'm glad it's on the left because we replaced the right one about a month ago for a bad bearing.) Simple enough, I just get my scope and put an IV into the two wires for the sensor to check the signal. Figure 1 is my result.

This signal was taken rolling about 5 mph. Apparently this is an intermittent problem, so I clear the code and try the test drive again with the scope still connected. The light comes on and the code comes back while I'm backing onto the side street. I pull back in and clear the code and back up again while continuously watching the signal. "Hey dude, why don't you just back out in front of me next time!" I hear from a local as I keep my eyes on the scope. "Sorry," I say, without taking my eyes off the screen. The neighbors have gotten used to my driving. The ABS light comes back on and sets the same code and the signal never skipped a beat. I thought the signal looked decent but just for comparison I grab another lead and a pair of wire piercers and start an IV in the right side. This time when I back up I get a nearly flat line. (Sorry, no picture, I've stopped saving flat lines; use your imagination.)

Now I'm really confused. I went to my library of factory Cadillac service manuals and pulled the '98 SDV ABS section to see if I could shed some light on this problem. The manual says Code 1233 is the right front wheel speed sensor. That makes sense because it's the one with the bad signal. It also makes it a warranty job and the time is short enough that the customer is not going to want to pay the labor. I dig up the parts receipt and order a new hub. I go for another road test and everything looks good ... for about a block, then the light comes back on and it sets the same code. I check the signal and I notice it's shrinking the more I drive. Frequency looks good but it's getting noisy and losing strength.

I'm beginning to suspect a problem with the circuit, possibly a partial short to ground or high resistance in the connector. Now I'm second-guessing myself. I pull the car back on the lift, disconnect the speed sensor connector and recheck the signal right at the hub (Figure 3).

Definitely another bad hub. I call the parts house to see if they have had trouble with these hubs. After being assured mine was the only one, I get another new one. This time I bench tested the speed sensor signal. I'm not happy with it still. I decided to order a new Delco hub.

Figure 4 shows the bench test result.

I'm not privy to what the threshold signal voltage is or at what frequency it's tested by the Electronic Brake and Traction Control Module (EBTCM) to set a wheel speed sensor code, but with the Delco hub putting out nearly twice the voltage as the other one, I opted to go the extra cost. Problem solved. Recovering my time and the additional cost is another matter I'm taking up with the parts house. Hold your breath for me.

Not long after this episode I get a call from a local body shop that just finished a collision repair on a '99 Dodge Dakota. All that's left is the ABS light on the dash that won't go out. Fred, my friend who owns the body shop, figures there must be a wire pinched or shorted somewhere that was related to the accident. He's in a big hurry to get this one back to the customer. I like Fred, so I drop everything and jump right on it.

I hook up one of my scan tools and tap right in to the ABS system through the OBD connector. I get Codes 71 and 72. My data source leads me to a trouble code description chart where I learn:

Code 71 is "RAM Read/Write" and 72 is "ROM Checksum."

Set conditions for Code 71

Turn off ignition, turn ignition on, check for DTCs.

Is 71,72 or 73 present? Since a DRB is still only on my wish list, I cleared mine with an OTC Monitor 4000 enhanced with flame chromatization and an OBD generic cartridge. It must have worked because only the 72 code came back. According to the chart, the next step if one of the codes returns is to refer to symptom "replacing the controller antilock brakes," which it says you have to wish upon a star ... no, it says call STAR to get a new one. Then you can just reprogram it with your DRB and you're all set. I think my crock is about at capacity.

Call me stubborn if you want, but my towel doesn't get thrown in until I'm horizontal and there's been a 10 count. I still won't be the one to throw it.

After reading some good posts on iATN about similar problems with this code combination, I decide to try scoping the rear speed sensor.

While idling in drive I get the signal shown in Figure 5, which tells me to get a speed sensor. I call my local Chrysler dealer parts department and ask for a speed sensor. The parts guy asks me, "Which one?" I want to say, "the one that makes the sum of the CAB's ROM equal to the original sum stored in the ROM," but I just say the rear one.

I put it in and retest with the scope. Figure 6 shows the new sensor signal as being recognized by the CAB's RAM and ROM; I know because the light's out and I haven't even cleared the codes yet.

The right information fixed this one, it just wasn't where I expected to find it. Maybe if two or three more of these go bad, a bulletin will pop up in the subscription information source. If not, you can always just remember where you put this magazine.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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