Backup sensor theory

:banghead: I'm looking for some really technical info that I'm not even sure would be covered in a service manual.
I'm wondering if anyone has ever gotten into the "meat" of the backup sensors on a 1999 (or other similar) Windstar SEL, 3.8L engine. We got it two weeks ago and I've already fixed several little electrical problems and the right side door actuator, brake switch, etc. and I've got everything working almost like new except for the parking assist system. Like the other posters elsewhere in the forum, I get the automatic turn-off of the system as soon as I put in reverse. This indicates a problem in the system and I STRONGLY suspect one (or more) of the sensors is bad. Budget constraints keep me away from having the dealer scan the system for $100.00, so I'm hoping for some tech info as to how those sensors actually work. (There's only three wires going to each one) One is ground, one says power, one is the input to the PAM. However, that doesn't necesarilly mean "constant" 12v. to the power, it could very well be a short burst of voltage so as to send a short burst of ultrasonic sound from the transducers in the sensor. Now you see why I can't be quite sure as to their actual theory of operation since there's too many guesses as to how they might really work. I'm guessing that there may be electronics in the sensor assemblies themselves since there are no shielded wires connecting them. I'm guessing that internal electronics in them could preamplify the received echo from an obstruction, then turn that into a short, DC pulse to be sent to the PAM. A very small signal is most likely generated even without an obstruction so as to "tell" the system that the sensors are functioning and ready to go.
Please excuse the long-winded post but being technically knowledgible at the same time as being technically in the dark has really got me going on this one.

Thanx,
Phil

Sorry i can not be of more help but did see 2 factory ford replacement sensors on ebay. for 24 to 36 bucks.. It said windstar and other ford trucks. The other not sure unless you have the parts number..

Thanx for the info, I may just need that if I can't fix the existing sensor. The trick still is to find the bad one out of four, if there is just one bad one. I'm wondering if the two center mounted units can be removed without removing the bumper cover. There is a crack in the cover over the leftmost sensor which is why I think it possible that water may have gotten into it and damaged it.
It just seems most likely that one is all that is bad.

Phil

I removed the sensor from the cracked portion, left end, of the bumper cover. However, I broke it apart trying to remove it from the adapter. I sure hope it was (the) bad sensor. Anyway, I ordered the one from E-Bay with a winning bid of $30.00+$10.00 shpg. I hope I don't damage it getting it back into the adapter, which itself was damaged at the time since I thought the piece from the bumper was one whole piece. I may have to get the adapter from Ford and I'm betting it won't be cheap, even though it's just basically a plastic cylinder, with the end shaped to fit the curve of the bumper cover. I'm not sure I can re-use the old one anymore but I may give it a try. The end fitting into the bumper cover is not damaged but the end with locking tabs is.

Any update to this thread? How did you remove the sensors? Is it possible to remove them with out removing the bumper cover? Did you ever figure out a way to determine if they are good or bad? Mine started out giving me problems after the car wash, now it constantly bad. Like you, I do not know which one is bad. It would be great to determine a test to perform with a mulit-meter like most sensors. But then again, most sensors are only 2 wires...

Unfortunately, I still haven't solved the problem, I'll most likely wind up going to the dealer for a scan with their NGS scanner. (A $2500.00 proprietary scanner) The scan will cost me $100.00, but if I wish, that money goes toward the labor cost of the repair that will be needed. At least I know I have one good sensor now.
It was quite easy to remove the bumper cover, but you will need an offset screwdriver to remove one screw on each end that is mostly blocked by the rear tires. Of course you could remove the tires but that would be more timely than using the offset driver. All other screws come out easily with a long shank Phillips driver. There are 5 on top, visible when the liftgate is open, then 3 on each end, facing forward, behind the rear tires. There are 3 push pin type fasteners underneath which come out easily after you pull out the locking pin. You need to lift up and out to get the cover over the brackets that hold it snug to the van. There's a red locking slide on the wiring harness connector for the chassis to bumper. You may need, as I did, to use a sharp pointed tool, such as a strong, wire piercing test lamp point, to assist in disconnecting the sensor harness from the chassis wiring under the van. After removal, all four sensors wil be easy to remove. You will have the choice of cutting the cable tie at the sensor holder, unclipping it and removing the sensor and holder as a unit, or else carefully pushing the sensor out of the holder by pushing from the rear towards the front. In either case, you will need to remove the sensor from the holder in order to install a new one. The holders are made to fit the specific bumper for the vans. It could be hard due to all the years of dirt and water, etc. I pulled on mine, which resulted in breaking it apart, so make sure you only push it out rather than pull. The connector on each sensor is removed by pressing down on a rocker type lock tab on the connector.
In any case, I was never able to figure out how they "work". I did pick all the sealant off the broken part to see what all was inside, and there is a 16 pin circuit chip, a toroidal transformer, big capacitor, and many other tiny components on a small circuit board all tucked inside. Some yellow powder and a small foam cylinder came out and at the very back was the piezo electric disc that is what actually produces the sounds and acts as a mike for receiving the "echoes". This whole assembly was then all sealed in with silicone. No wonder the sensors are $106.00 at the dealer.

You can't pull ohms measurements off of all of your sensors, including the new one to make a comparison to determine the odd one out? Check for a sharp, pungent burnt smell. That will be your bad one for sure. Look for color irregularities. Check for ohms, continuity on the transformer, there should be low, no ohms and continuity. Can't remember on the capacitor.

I did try comparing resistance and they were all somewhat different, but none were far enough different to be any clue. As for the internal transformer, the windings got beat up pretty bad when pulling the circuit board out of the unit. Of course the piezo disc was broken too, as well as the wires from it being torn loose from the board. I'm sure the electrolytic capacitor was a filter cap for the B+ coming into the unit, so as to isolate the electrical "noise" from the sensors and vice-versa. The transformer possibly stepped up the voltage output from the piezo disc when a reflected sound was picked up. Thus, a lower gain, less noisy preamp for the mike could be used. When the units are powered up, the voltage on the center terminal, the input to the PAM, were all about two volts lower than the B+ voltage. So, as far as DC voltage measurements go, they were all the same. The PAM must send out a short, ground-going pulse, probably no more than about a half a millisecond, in order to "trigger" the sensor to put out a sound burst, then wait for an echo before sending another pulse. If no echo, then no "beep" in the van. Again, this is all guesswork. I'm sure there are at least a few of us out here that would like to know how that system really works.