We got back from a 8 day treck through the mountains of Colorado Sunday and encountered a couple of problems along the way, one that is solved and I thought I would pass some information along.

The problems: the speedometer quit working and the overhead temperature display will register correct outdoor temps as the temp drops but will never read higher as the outdoor temp rises.

I will get to the speedometer later but the temp indicator is still not working. I have been playing with the AAT (Ambient Air Temp) sensor all morning and have discovered that it is a thermistor (type of variable resistor that changes resistance with temp).

What I have tried:
pulled all the plugs that connect the AAT to the overhead, cleaned and re-greased them.
Tested the AAT with a ohm meter and it responds fine with a heat gun and ice water. I even went as far as replacing the AAT with a mechanical variable resistor and the overhead responds the same, goes down but will not increase in temp display.
If you unplug the AAT for more than 5 seconds and plug it back in, it displays the correct current temp of the AAT just fine.
Pulled the overhead console and re-soldered all of the infamous resistors that are known to break.

It still does not work right and I am beginning to believe that I have a microprocessor problem that has no easy fix.

Any thoughts or suggestions on this would be appreciated.

Now to the speedometer issue. I have been fighting this for a couple of months now. If I was traveling down the highway, the ABS light would flash a couple of times (quickly where you would have to be looking to notice it) and the speedometer would drop to zero or at least start to and then everything would work fine again. Very intermittently..... Once in a while I could pull a code that pointed to the VSS or as the code read, Speed Sensor A. I had a used VSS that I had replaced for a different problem that turned out not to be the VSS, so I swapped them again, no difference. The problem has been getting worse and would happen more often over the last two months.

After a couple of days of out trip, hitting some dirt roads, the ABS light came on more often and the speedometer would stop working for longer periods of time until it stopped working altogether and the ABS light was on solidly. We went through most of the week with it not working. Thankfully, I had an old GPS that I could put on the dash to use as a speedometer.

I began to notice that when we were on washboard dirt roads or hit a large bump on asphalt, the speedometer would jump up to speed and quickly die alone with the ABS light going out for as short of period. ELECTRICAL, it has to be an electrical problem figuring one of the multitude of harness plugs under the brake reservoir had got corroded and causing a bad connection.

Monday I began to start at the front of the truck and go after the plug on the ABS module located under the drivers side battery. Pulled the battery and battery tray and got lucky. There is a ground connection from the main harness that connects to the radiator bracket, just under the rubber adjustable hood stop that had backed itself about half way out. It is accessible without removing the battery but I did not notice it until everything was removed. I sprayed some electrical cleaner on the grounding lugs and tightened it back up and the speedometer works and no ABS light.

I may have loosened this ground wire when I installed my 6.0 intercooler but I doubt it since it is WAY out of the way to removing intercooler. Just something to check if you run into this problem.

 

As far as your AAT sensor goes, you may tap into the sensor output, and watch it with a DMM. See if its voltage actually comes back up, and your visual indicator infact does not. Just to be certain you do not have an underlying issue. If you can, start by connecting at the actual sensor plug. Glad you found your VSS issue, I would have guessed a possible corrosion in the wire to the VSS sensor, that would intermittenly lose a good enough connection to pass enough current for the pcm. I am not sure the impedance of the pcm's inputs, but I imagine they are faily high, resulting in low current sensors. The higher the impedance of the circuit, the more likely a chance of this happening is possible, corrosion is an interesting thing.

 

Bob,

Thanks for the heads-up on the loose ground causing speedo and ABS light problems. Nice find!!!

 

That's an idea that I didn't think of. I was playing with the AAT using a bunch of jumper wires with alligator clips so it would be easy to see what is happening with the voltage. Just have to pull the grill again

One side of the AAT is grounded, the other goes directly to the overhead. There is a conductor that goes to the info bus and one that goes to the PCM but I believe they are only used for calculation of MPG. I believe that the temp and compass heading processing is all contained in the overhead.

 

Well, the voltage drop across the AAT is consistent. At 75 deg the voltage drop is about 1.9v. As the AAT sensor heats up, the resistance decreases and the voltage drop decreases, as it cools, the voltage increases.

I am beginning to resolve that it's the board and not much else can be done besides getting a new one and that ain't gonna happen.

I have also found that the max temp display is 140 F and minimum is -40 F; but the manual probably says that.

Thanks Robin but I don't think it's a common point of failure because I don't remember ever hearing of someone having anything similar.

 

Thanks Bob for the tip on the speedo. Don't have the overhead so the only way I know what the temp is outside is to roll down the window.

 

My OH went awol w/in a month after I brought the truck home. Have seen the fix threads on here, but haven't gotten that far down my mod-list yet. Actually, I've been using Chris's OAT method and had completely forgotten about the OH display until I saw this thread!

Still, way down the list tho. Grill guard and front hitch are Mon, along with replacing my rear Class V w/ a 2.5"; next weekend if the wx is decent will be the WW/HPX.

 

If I have a similar problem I will be able to fix or eliminate that right off. I checked it right after I read the post because I was working in that area re-locating the DRL resistor. I also wondered what the ground was for and now I know. No such thing as TMI when it comes to our trucks.

 

Wow Jim, PMS got you big time.

 

Yeah, just keep spending more/more $. My credit card co LOVES me right now. Oh well, at least I'm getting miles for it! (Not to mention more enjoyment out of the truck!)

This site has cost me a lot more $ than the supporter fee, that's for sure! Otoh, it's also SAVED me a ton, too...

 

If I had to guess, the one that goes to the PCM is the 5V source (pwr in), and the one that goes to the info bus should be the output. But Im sure you already knew that.

Something to consider, and you may have already checked this, but when the temperature goes up, and resistance decreases (current flow goes up) What does the 5v input voltage go to, and what what is the sensor output voltage go to? I know you said the voltage drop changes, which would be consistant with the changing resistance, but what about the voltage value, of in and out? (in respect to ground of course) Secondly, have you found were the wire comes into the info bus (assuming that is the sensor output) and have you checked voltage there with a changing temperature?

 

Any updates?

 

Mike, I have not had much time the last week to mess with this very much. I traced the schematic back to where the AAT connects to the main harness through one of the 12 pin square connectors on the drivers fender. I don't remember exactly, but on our 02s, there is a string of 5 or 6 connectors all lined up in a row mounted on a bracket fastened to the plastic wheel well rock shield. It was the 2nd or 3rd from the left, a gray plug. I pulled it apart, cleaned with electrical contact cleaner, re-greased and plugged back together; no change.

Since the AAT goes directly to the overhead console processor, I am a bit confused as to your question of voltage in/out as one side of the AAT _IS_ chassis ground.

What has prompted me to somewhat give up on the problem is that the overhead processor only uses the one AAT for measurement of an increase or decrease in ambient temperatures. Since the decrease monitoring works and it responds quickly, it seems to me that it is a processing problem, not electrical. Also, once the truck sits long enough for the info bus or overhead to completely discharge, the temp registers the correct AAT upon startup.

I say I have given up 'somewhat' only because I have only driven the truck a couple of times this last week for very short trips in the morning. I thought that I noticed that the temp display went up 1 degree on both of these short trips so maybe something is working. It still does not respond with a heat gun in the shop. I will have to watch it closely on the next long trip I make when the outside temps begin to warm.

 

When I say voltage in, and out. In being the 5v source, which is probably the wire that goes to the pcm. Then the out wire, being the wire that goes to the console. The reason I ask for those, is to see that 1) the 5v souce remains constant, (not going into current limiting for any reason) and that the output voltage of the sensor changes accordingly. (all referenced to ground). You said the voltage drop accross the sensor changes. But that does not tell the whole story. That means the varistor is working properly, according to temperature. What that does not tell you though, is that the output voltage truely is going up. It just means the voltage drop accross the varistor is changing, due to a resistance change.

If I had an over head console, on any of our trucks, I would love to help. Heck I would even drop one, and ship it to you, to verify your theory, but non of the trucks around here have that.

If you decide that you cannot read/understand my babbling on (sometimes I cant even myself LOL) let me know, and I will pm you my phone number. Sometimes it just easier to explain what I mean. Good luck.