Simple Wire Problems

Chris Foster - Diagnostician

Diagnostician - Chris Foster Chris Foster is a diagnostician for Certified Transmission and has been in the transmission business for over 30 years. He is an ASE Certified technician, and has also been in the Army Reserves for 31 years.

A few weeks back, we had a customer come to the shop and say "Hi! I was sent here by so-and-so, and he said you would be able to fix my car, because he does not have time to figure it out". "Well I am always up for a challenge, what is going on with it?" We then got into a discussion of how long he has been working on the car, and how everything "works great". She then explained to me that the dealer looked at it and said it was going to be $XXX to fix it, and she just didn't think they were right because "everything works ok... It is just the ALL WHEEL DRIVE DISABLED light is on, and nobody can find out what is wrong with it". "Sure" I said, "leave it with me and I'll see what I can do."

Wow, was I in for a treat. I brought the Buick into the shop later that day to perform the preliminary checks and noticed the information center flashed AWD DISABLED, and the message remained on throughout the entire diagnostic and road test. I instantly agreed with the customer that this was "bothersome" to say the least, not really even caring if the all wheel drive worked. Just this annoying message on the instrument panel was enough to make a person not want to drive the car.

I hooked up my scanner only to reveal that there were no codes present in any of the modules. "This is crazy," I thought to myself. "How can there be a warning message displayed with absolutely no diagnostic trouble codes? And where am I going to go from here? It has to be a bad scanner!" So I went through the pain of borrowing a scanner from a friend of mine at the local dealer to be able to read all of the ABS data and find out why the AWD DISABLED message was displayed. We all know the sad outcome of that tale! Nothing. No code in the PCM, BCM, ABS, IPC or any other module. Boy was I stuck.

I did some quick research on the Internet, and all that led me to was a whole bunch of forums with people asking how to turn off the AWD DISABLED message in their 2000-whatever vehicle. I read through them as time permitted, and needless to say, I did not find much help. I called some buddies at the dealerships and I got all kinds of answers, from "you have to replace the rear differential" to "replace the pump check valve" (whatever that is) to "try and reprogram it". All good information, but, just not really giving me anything solid to go on. I completed my Internet research and came up with the ever popular "tire size variation" starting point.

Well, sure enough, just before this all happened, the owner purchased two new rear tires, and was saving up for the front ones. The place that the tires were purchased at told her the front tires were fine, and she only needed rear ones. The tread depth was close, but had a significant difference. I consulted the owners manual (of all things) and there was a description of her concern in the 'warning lamps' section. It read "the AWD DISABLED message will be displayed in the event a temporary spare is placed on the vehicle." It then went on about the criteria for this event and stated "the message will go away once the tire variation was corrected."

"Perfect," I said. "Now I have documented proof to show the customer." She said she would have to save up for the tires, and if that didn't fix it? she would be back to see me.

As luck would have it, it only took her one week to get the new tires and to put 150 miles on the SUV. I only know this because that was the only thing different about the vehicle when it came back with the same annoying message displayed on the dash. "Oh well, I needed new tires anyway" is how we began that conversation. "That had to be done anyway? leave the rest to me" was my reply.

I put all other advice and thoughts of an "induced" problem aside and pulled up a wire diagram. "WOW! This looks difficult," I thought to myself. A power wire, a signal wire, and a ground, one fuse, a check valve, and a PCM make the whole program work. Way more simplified than trying to find out why shift solenoid 'E' in a 5R110W loses voltage when you select D3? or is it?

A week later, I had a Chevrolet Venture van come in with the same problem. I used it to write this article and take photographs as the repairs were made. Although the Buick beat me up a little bit, this one was "quick and painless" (we all know what that means).

I checked the fuse to make sure there is power on both sides, eliminating the ignition switch as the source of the problem. The simplified wire diagram didn't really show any types of splices or connectors that concerned me, and so I decided to check the entire wire, from the source to the component. Since here was 11.4 volts at the check valve in the rear differential and 0.01 ohms resistance from the fuse panel, I knew it was not a voltage problem. I then unplugged the PCM and found the LT BLU signal wire in pin 74. There was 0.42 ohms resistance in the wire from the PCM to the check valve, which should be ok.

Article Figure 1


That left me with the ground wire. It had 5.71 ohms resistance, which of course we need to be as close to 0.00 ohms as possible. A quick check of the simplified wire diagram shows the wire grounded on the B pillar. Easy enough, must be a "pass through" connector, maybe under the seat or in the fire wall. There is actually three connectors on a "pass through" bulkhead connector right behind the drivers seat- not shown in any diagram, exposed to water, salt and the elements, including the ever popular drive-through car wash that sprays the undercarriage.

Not being able to find a connector end, I replaced three of the female terminals, soldered the connections and applied a little dielectric grease around each plug to help prevent the problem from re-occurring. I'm happy to report that the AWD DISABLED light is off, AWD works, and the customer is happy.

Article Figure 2

The all-wheel drive Chevrolet Venture van came into the shop with the AWD DISABLED message displayed on the information center

Article Figure 3

The first thing I did was check to make sure there is power on both sides of the fuse, in order to eliminate a problem with the voltage source

Article Figure 4

The connector at the check valve shows it is a simple three wire system: Power, Ground and Signal

Article Figure 5

Checking the wires from the source to destination eliminates the need to make test points throughout the wire harness

Article Figure 6

It was pretty easy to locate the light blue wire at the PCM connector

Article Figure 7

The power wire had 11.4 volts on it, and had the same resistance as the light blue signal wire

Article Figure 8

There was more than usual resistance for the ground wire, just checking it at the frame, but also at the negative battery post

Article Figure 9

There is a pass-through connector just behind the driver's seat, not illustrated in the simple wire diagram

Article Figure 10

Once unplugged, I could see where the corrosion begins

Article Figure 11

The ground wire at the check valve was not much better

Article Figure 12

After replacing the three female terminals, and cleaning the male terminal, I applied dielectric grease to the connectors to hopefully keep this from happening again

Article Figure 13

All is good now.