Chris: This is great information. I will go back to confirm the readings I took but mostly they are in line with the expectations for a properly operating system so I am still at a loss as to why no squawking. I may just hook my scope to the output pins and see if there is a signal present.
One mystery: the caravan trailer sensing pin 8 shows 12V all the time whether car is in reverse or not (pin 9 does go high and low correctly when reverse is engaged). What makes that change state? Could that be disabling the entire module if it thinks a trailer is sitting there all the time?
I can say that all return pins from all sensors show ~9V when reverse engaged and ~10V when not engaged. The + pin to all sensors is 12v all the time regardless of whether reverse is engaged.
What we need is a truth table! Thank you for this. --Tom in Tucson

 

Tom, measure the - lead to chassis ground. Is that going between 12 VDC (when reverse is not selected) and 0 VDC (with reverse selected). That would explain those readings.

You may be on to something. If the tow module is having issues. I would try unplugging the "TT3" connector from the towing module and seeing if that may make your back up sensors work.

 

All readings I took were to chassis ground. To recap: the signal from trailer towing is 12V all the time no matter what. I cannot find a TT3 connector or module anywhere in my car. I might try interrupting that line? I do not have any of the later enhancements that are described in some of the wiring diagrams. Mine is an early 2001.5 car. The reverse light pin does go to 12V when white rear lights come on and to 0 when car is in any other gear. One wire to the sounder is 12V all the time; the other sounder wire goes to zero when reverse is engaged (with me standing at the open boot poking around.) If that zero is a path to ground, the sounder should sound but it does not. I will try putting power and ground to the sounder out of circuit to test it once and for all. Generic piezo sounders with built-in oscillators are generic $15 devices all over the web.
Are you and me the only people who have ever dug this deeply into this specific problem? It's fun! Thank you.

 

I haven't necessarily dug into the system that far. I am just looking at the diagrams and stepping through all the possibilities and trying to come up with a way to figure out the issue using the minimal number of steps. But then, I do this sort of thing for a living as I am an instrumentation tech at a nuclear power plant. So, I play with electronics all day long.

 

Hi thomash85715,
The following is an extract from Fig. 18.1 of the 2.5L & 3.0L – 2001.5 Model Year Electrical Guide-

CONTROL MODULE PIN-OUT INFORMATION
Control Module

Pin Description and Characteristic
B+ RB7-1 IGNITION SWITCHED POWER SUPPLY: B+
PG RB7-3 POWER GROUND: GROUND
D RB7-5 SERIAL DATA LINK
I RB7-8 TRAILER CONNECTED STATUS: GROUND = TRAILER CONNECTED
I RB7-9 REVERSE LAMPS STATUS: B+ = REVERSE LAMPS ON
D RB7-10 LH CENTER SENSOR SIGNAL DATA
D RB7-11 LH SENSOR SIGNAL DATA
O RB7-14 PARKING AID SOUNDER +
O RB7-15 SENSOR POWER SUPPLY: B+
O RB7-16 SENSOR GROUND: GROUND
O RB7-17 PARKING AID SOUNDER
D RB7-23 RH CENTER SENSOR SIGNAL DATA
D RB7-24 RH SENSOR SIGNAL DATA

So if your seeing 12V on Pin 8 it should mean the Trailer is NOT connected.

 

My reading comprehension skills used to be pretty good but I have a lot of trouble finding stuff in JTIS. Thank you for providing this. I will NOT have to cut the trailer wire now to pursue this repair. If only radio shack still existed, I would just buy a generic sounder from them today and get to the bottom of this.
And Chris, now it is clear why you call yourself Thermo! Congratulations on working at an important job that you must be proud to do. Have you ever visited the Conowingo Power Plant from a previous era in electric power generation? We took the tour once when we lived in Philadelphia.

 

Hello All: Short answer to puzzle thanks to Occam's Razor is: bad sounder. My local radio shack is morphing into a sprint store but until that metamorphosis is completed, they can still sell you a generic piezo sounder for less than one dollar. I wired the thing in to red-blue and yellow wires that go to the original dead sounder and stuck it in the baby seat anchor latch on the rear deck. Presence of giant subwoofer amp makes access to original sounder rather difficult.
Only way I could confirm sensors were clicking--and eliminate computer board as culprit--was with screwdriver on sensor and handle on my ear bone. So thank you SiHarvey and Thermo for guidance on this. --Tom in Tucson
PS: finishing other people's sentences is bad enough but replying to myself on here? even worse.

 

Tom, I actually got the name while in the service. I was the "poor sap" that was stationed on a submarine and got to play "Homer Simpson". Now, that was a very interesting job. Nothing like sitting 20 feet from a running nuclear reactor and sleeping with your feet tapping against nuclear tipped missiles (that will make you feel all warm and fuzzy inside). So, my civilian job now is somewhat "boring" as I only get to do the electronics vice playing the man with 20 hats (on a sub, you have to be the operator, trainer, QA guy, supervisor, worker bee, repair technician, and a few other things all at the same time). I got the name complements of a Lieutenant on the sub. He made the mistake of telling us "lowly" enlisted guys that we were nothing more than trained monkeys (see a light, perform a sequence of actions, go back to doing nothing, while this is true, that is a different story). Well, we took offense to it and decided to name all of us after a monkey. Since I control the reactor and its temperature (not its power ironically, that is a different person's job), I became known as Thermostatic Monkey. I then shortened it a little to Thermomonkey. People have since shortened it slightly more to simply Thermo.

As for finding a new piezo, e-bay is a wonderful place. That or a place like Grainger will have parts like that. radio Shack is learning that there are not enough people looking to make their own electronics any more with way too many big name stores providing the same parts for way, way cheaper.