Lock up torque converter

[Vacdaddyt]

Just got my 91 back on the road from a motor change out. I'm now moving on to the next thing i don't think is right with the way it drives. Just cruising down the road at say 60 mph the truck seems to not drop back on the rpm and lock in. Maybe I'm too used to the gmc and dodges dropping back a couple hundred rpms, it runs around 2300 to 2500 rpm at 55-60. How do I check this , the gas mileage is 17 on the best of days and I drive too much( a lot of hwy miles). My other explorer that I had could get up to 21 on easy trips so I'm trying to find where I'm losing it.
The motor has around 120k on it , new plugs, wires, air filter and gaskets. I changed the filter and oil in the auto trans, it was black. I overfilled it and had to suck some back out and the new was really black also. Should I get the trans flushed better or do you think it is some thing else? My scan tool does not have the correct adapter for it to scan it. Any comments are appreciated.

 

[Roadrunner777]

What gearing do you have in difs?

 

[Vacdaddyt]

The tag in the door says D-4, The tag on the axle says 3 L(i think L) 73 38 1F21 and on another line S 6298

 

[MrShorty]

Just cruising down the road at say 60 mph the truck seems to not drop back on the rpm and lock in. Maybe I'm too used to the gmc and dodges dropping back a couple hundred rpms, it runs around 2300 to 2500 rpm at 55-60. How do I check this

Standard road test for the torque converter clutch:

1) Get on the highway, light enough traffic that you can maintain a constant speed of ~55 mph.
2) While maintaining your speed with the right foot on the accelerator, "tap" the brake with the left foot. The engine speed should momentarily jump a couple hundred RPM, then drop back as the TCC unlocks then locks back up.
3) If the TCC shows that it is "locking," slowly increase throttle. Engine speed should only rise as vehicle speed rises, until the PCM decides to unlock the TCC, when the engine speed should jump a little.

 

[Vacdaddyt]

I'll try this the next time I leave town to the north, there the road is very level for ten miles or so. I have noticed the rpms are lower when I am not using the cruise control.

 

[Vacdaddyt]

MrShorty, I tried your test several times on the 16 mile trip into work. The needle never jumped like you described when I pumped the brakes, even when you could feel them hit and slow the vehicle. The rpms would go up to 3000 if you sped up and then fall back when the speed leveled out but no jump when tapping the brakes. The trans seems to shift O.K. through the gears though. Whats next?

 

[MrShorty]

Now we need to look deeper into the electronic controls, to see if the problem is electrical in the engine management system or something mechanical/hydraulic inside the transmission. In recommending these tests, I'm assuming you have some skill reading a wiring diagram and using a multimeter. If not, then these circuits are relatively simple DC circuits, so this could be a good opportunity to begin learning these skills.

Here's how I'd probably proceed:

1) Pull codes from the computer (see the stickies in the EEC-IV forum). See if there are any codes that might prevent the PCM from commanding TCC lockup.
2) Carefully check Brake On/Off (BOO) switch. Do your brake lights work (even when the brake pedal is just "tapped")? This is also checked during the KOER test. The road test described is using the fact that the PCM commands the TCC to disengage whenever it gets the signal from the BOO switch. We just want to make sure the BOO switch works so that doesn't confuse our testing.
3) The TCC solenoid circuit is a pretty straightforward DC circuit, with the PCM acting as a ground side switch. Put your voltmeter on the circuit so it is measuring the voltage drop across the solenoid. Enter the output state test (see my "notes on pulling EEC-IV codes" thread). You should be able to see the PCM turning the solenoid on and off. A test drive with the voltmeter hooked up will tell you whether the PCM is commanding the TCC to lockup or not.

At this point, if the PCM can command lockup, and is choosing to command lockup but the transmission is not responding, then the fault is likely internal. If the PCM cannot, or does not choose to, command lockup, then you would need to delve deeper into the engine management system to determine why.

 

[Vacdaddyt]

looks like I need to get the adapter for my scan tool and find a diagram that I can read without a magnifing glass. How easy is the wiring to get at where i need to put the voltmeter in the solenoid circuit for the TCC? Or is this up by the brake pedal. By the way there is a molex plug hanging by the drivers side of the transmission with two wires in that is not attached to anything that I can see. It comes with the harness down from the back of the engine to just above the shift mechanism. I'm going to research this further as I get more info. Thanks

 

[MrShorty]

How easy is the wiring to get at where i need to put the voltmeter in the solenoid circuit for the TCC?

I use connector at the PCM, since all of those things attach at some point there. You need to be real careful, though, so you don't damage any of the connectors. There's also a connector under the hood on the driver's wheel well that can be a convenient point to access the TCC circuit.

By the way there is a molex plug hanging by the drivers side of the transmission with two wires in that is not attached to anything that I can see.

I'm not sure which circuit is in a two wire connector. At the transmission, the TCC circuit is part of a three wire connector -- one hot wire, the wire for TCC control going to the PCM, and a third wire going to the PCM for 3-4 shift control.

looks like I need to get the adapter for my scan tool

"Need" might be a bit strong. You can perform all these tests without the scan tool using a paper clip or similar. On the other hand, if you are looking for an excuse to by the adapter, go for it.

and find a diagram that I can read without a magnifing glass.

I've had good success on these circuits using the wiring diagrams in Chiltons, which can be downloaded free (with registration) at autozone's website.

 

[Vacdaddyt]

update

I found it!!!!!!! The wire i was describing that was hanging loose did have something to do with the problem. Long story short this vehicle was a lab vehicle for the local automotive program. When i got it, various things were missed when students put it back together so finding a loose wire shouldn't always mean it was never hooked up . The socket lies just above the pan line in front of the shift lever. With better light and a bigger mirror I found it this morning, still do not know what it is attached to but I plugged it in and now at 60 mph it is running 18-1900 rpms. I also tried setting the bands like one of the other posters suggested but don't know if there is any change. Time will tell, I put around 200 to 300 miles a week so I'll find out.

Thanks MrShorty for the advice.
Yes buying a tool is maybe a little severe but I have a scan tool already and have been buying the accessories to mate to the different vehicles as I need them. I still ordered a used one for 20 bucks delivered.

Now to figure out what is wrong with the fuel gauge, I probably should look for a loose wire there also.