2000 Jeep Grand Cherokee

Here is a 2000 Jeep Grand Cherokee 111,087 miles with a 4.7 liter engine. This is actually at a transmission shop that got it from another shop. The vehicle sets a P1768 code for transmission control relay always off and the transmission goes into failsafe. The previous shop has tried a Transmission Control Module, Transmission Control relay, and a transmission solenoid pack. These vehicles are very common in my area and they have various issues. Lets look at the code and code set criteria.

 Ok, so we have some code set and quite the number of possibilities. One item that is not listed is an issue with the solenoid pack or wiring. I have seen wiring down to the solenoid pack be frayed or shorted to power, ground, or another wire and set this code. Probably, what the other shop was thinking when they tried a solenoid pack. Lets look at the circuits involved for the Transmission Control relay.

  
 So we have a regular 4 wire relay setup. Fused battery power comes into terminal #30  and when the relay is energized it flows out on terminal #87 to the TCM (watchdog circuit) and to the transmission solenoids. Remember this is a 4.7 liter. The control side of the relay is a constant ground from G104 to terminal #85 and the TCM powers terminal #86 to activate the relay. I put that in bold because most relay control circuits are ground activated. This is just the opposite. When the TCM wants to failsafe the transmission it discontinues the power to the relay and power is cut to the transmission solenoids and the transmission defaults to whatever failsafe gear it is engineered for. So how should I proceed? Remember, I have to work fast and efficiently. I need to know is the TCM turning off the relay? Is the relay really turning off? Are there other factors involved? I decide I am going to scope the relay output, relay control, and the relay ground for starters.

 

 

  If you look on the diagram everything I want to see goes through two connectors C102 and C103. I am always very suspicious of Chrysler connectors and connectors in general. I don't like to just start disconnecting and inspecting connectors visually. The simple action of disconnecting and reconnecting can temporarily or permanently solve an issue. I backprobe whenever possible. Lets setup.

Here are the components labeled out. This is the right rear of engine compartment. Again, I am using Pomona backprobes available from Aeswave. They provide me with great confidence in testing with being minimally invasive. Let's go for a ride. I don't even get out of the driveway of the shop and the code sets and the transmission goes into failsafe. Lets look at the scope data.

 
Now I can get a picture of what is going on. The first thing I notice is my ground to the relay is flipping out. The next is the TCM is not commanding the relay off. The relay output is dropping out. Lets zoom in on that moment.

There it is the ground is being lost and then the relay output is being lost. Look at my scaling for channel 3 (blue). A good ground in this type of circuit maybe 200mv at the most. I clear the code and repeat this a couple of times always with the same result. We are losing the ground. Lets go find this bad ground.

 
Why does this always happen to me? It seems like every time I need to find a ground or a connector it is a N/S. A no show or a not shown. Ok, we know it is right rear of engine. I follow the harness and it disappears down the back of the motor. It is tight. My guess it is attached to the bellhousing or back of the cylinder head. These vehicles get transmissions, cylinder heads, motor swaps, etc. So what can I do to make sure this will fix this elusive problem?

I run a jumper wire from battery negative to the wire that is supposed to be grounded by G104. I clear codes and road test. I left my scope attached and recorded.

 Looks good to me. The transmission performed well. I figured I would also see how it is on the scanner as well.

Looking good. Switched battery and ignition feed voltages are right where they should be. No codes and like I said transmission performs well. I inform the transmission shop owner of my findings. I give him his options on repairing this vehicle. At this point the vehicle harness should be closely inspected and repaired/replaced.

Scantools.....

If there is one question I get asked frequently by shop owners, technicians, and emails alike is-What scantool should I get? Or, Is this scantool good? Or, I just spent $12k on this scantool and it doesn't do this procedure on this vehicle.

I typically will follow this up with questions of my own. Such as. What manufacturer is the bulk of your work? What functions do you want to do with the scantool? What is your budget?

Without knowing the answers to those questions it is awfully hard to give sound advice. I see many shops that use a $10k scantool like a $99.00 code reader. They would have been better served spending the $9K someplace else in my opinion. Like maybe some training. If that works for you and your shop so be it.

In this age of automotive diagnostics it is nearly impossible to be "loaded for bear" on every manufacturer. Unless, you have deep pockets. Even with that are you using all the capabilities of all your tooling? Lots of aftermarket companies make boastful claims when it comes to coverage and capabilities. Unfortunately, when the tool is in your hand and need to do a function they can fall short at the worst of times. I often say that a good aftermarket tool will have 85% capability on 85% of the vehicle lines. However, that 15% can kill you.





 

  Here is a small example of aftermarket tooling. Any one of these are a very capable scantool. Being able to read codes/erase, view scan data, graph, and perform bi directional functions on different modules. Some do it better than others. Some are strong on this manufacturer but weaker on others. Again the 85%/85% rule. Some, I have been impressed with and others well not so much. Sometimes it is just easier to break out an aftermarket handheld tool rather than hook up a OE laptop based tool to check data or codes. Time is money. Some aftermarket tools actually graph better/faster than the OE tool for certain manufacturers.

 

 

 

    Here are some OE scantools. These are manufacturer specific tools. Some are handheld and some are PC/Laptop based. The world of OE tooling is a convulated and confusing world. These tools walk the walk. With these tools you should be able to have 100% capability for that manufacturer. Notice, how I said should. They have their hiccups as well. It happens from time to time. Not often though. If you want to do a procedure from start to finish and want to be sure you can do it then OE tooling is for you.
One thing to point out in this photo is the GM Tech2. There is been much chatter about the demise of this tool. Well, it had a recent update and I used it to finish up an ABS control module setup on a 2013 Cadillac CTS just the other day. Granted, there is a PC based version of Tech2 called Tech2Win that could have done the same procedure.

 

 

 

 

Here is a screen shot from Tech2Win. I still prefer the handheld to the PC based. The point here is don't get rid of your Tech2 and if you do service a lot of GM vehicles a Tech2 is still a smart tooling investment.


You have to ask yourself those three question I posted earlier when choosing scantools. Some tool companies/vendors allow for a "test drive" of tooling. That is always a smart idea take advantage of  if possible. Nothing worse than investing money in a tool that disappoints. A test drive of a week should let you know if that tool is right for you and your shop. Beware, of any tool company that claims that you will never need another scantool.

Another point to remember is comfort. A tool that everyone is intimidated to use will be a tool that sits in your toolbox making you no money. There are plenty of techs throughout the country that are diagnosing issues with vehicles with tooling that many would scoff at. It works for them and they are using that tool to its potential. Many times it is not the tool but the tool wielder. The general publics perception that all you do is plug into the vehicle and the scantool "tells" you what part is bad is grossly exaggerated. The best scantool you have is your brain. Technicians fix vehicles not scantools. Remember that always.