700r4 tranny and diesel info???

[libtec]

Thanks for looking. I know this is the 6.2 area but I have a tranny question and to put it simple I haven't found any resources that describe both a diesel and how the 700r4 tranny go together. I have the Haynes manuals for chevy trucks (24064), diesel engine repair (10330) and gm auto transmissions (10360). Each one describes a little info on my 1990 6.2 but none of them describe what on the diesel motor controls the tranny, none of them provide any type of wiring diagram for a diesel or gas and how they might interact with the tranny, basicaly they leave a big gray area. What I need to know is does anyone have a wiring diagram for a diesel suburban? Do you know where you can get one without having to buy a factory service manual? I am trying to figure out why my lock up torgue convertor is trying to lock anytime I run the truck at a constant speed, regardless of what range I am in (OD, D or 2). The transmission book lists the coolant temperature sensor, vehicle speed sensor, Throttle Position Sensor, and the brake off switch all as being inputs to the CPU that controls when the lock up is engaged on the convertor. All fine and dandy but none of the three books describe how they are wired or how to to test the units. I'm pretty sure the brake off sensor is working since I can get the lock to disengage when I tap the brake pedal, but it kicks right back on when I let off of the pedal. I'm thinking the temp sensor is working since my high idle and cold advance are working and the problem doesn't show up till the temp rises a little. But the vehicle speed sensor and throttle position sensor? Not much to go on there. My speedo has been fading in and out lately and out right quit working two weeks ago, sounds like a good place to start, but how does it operate? I can't find a speedo cable so I'm guessing it's electric, but again I need some insight into the wiring. The thottle position sensor is mentioned in a couple places but none of the manuals address in any depth how it works and how to test it for proper operation. Is the TPS on a diesel strictly for tranny control? Is there a resistance range to measure to check for a malfunctioning unit? I did take a wild guess and found the two wire plug on the drivers side of the tranny and pulled it loose, now the tranny runs normal. Problem solved except that the convertor lock up isn't functioning now. At least I'm half sure that the problem is external of the tranny. Anyone have a wiring diagram or some experience to pass on about how all the elements interact to control the lock up? Thanks in advance for the help.

[libtec]

I forgot to add, the lock up problem just started a couple days ago, prior to this the lock only happened in OD 4th gear like it should. I hope it isn't supposed to happen any other time because the truck falls flat on it's face now.

[TimK]

You said you have the 700R4 tranny. Are you sure? I thought the 700R4 was strictly a mechanically controlled tranny (controlled by the TV cable)and the 4L60e was it's electronicly controlled brethren. Similiarly the TH400 was a mechanically controlled tranny (controlled by a vacuum unit) with the 4L80e being its electronicly controlled brethren with the added overdrive.

I just replaced a 4L80e in for co-worker in a 2500 series Sub with the 454cu motor. The tranny had all kinds of problems. It would give you whip lash going through 1st and 2nd and then OD would slip in an out over 55 mph. The speedometer was also erratic. We talked to the GM dealer and they told us the 4L80e had some inherent problems internally and with the electronic control system. We replaced the tranny with a rebuilt and supposedly with all the factory recommend updgrades. We didn't have to mess with the computer however the tranny did come with a new wireing harness to be spliced into the old one. Since the tranny replacement it works great.

TimK

[convert2diesel]

Libtec:

Just went through the same scenerio trying to convert my 91 Buick to 6.2 diesel (By the way it is running and will post some pics soon). Turns out that the last variant of the 700R4 treats the lock-up a little differently then previous versions.

Instead of relying on a 3/4 pulse switch, or for that matter the better seperate 3rd and 4th engage switches, OBD2 equipped GMs just assume that if you are going over 35 MPH, then lock up should be available. They do this by monitoring the VSS output. When the VSS indicates more then 35MPH, the engine temp is above 100 F and the TPS voltage is above 1.25 volts and in a "steady state", the CPU grounds out the TCC solinoid. As the TCC gets it's oil from the second gear circuit, then that means it is possible to get lock-up in second gear. There is also an overheat switch that grounds out the circuit, but only in 4th.

I am trying to find a local nerd to help me design a simple logic circuit to accomplish the same thing without routing it through the ECM. Seems when I disconnected all the gasser connections, the computer went into full default and won't allow any lock-up, not to mention no AC.

If you invest the 20 bucks with "All-Data", all the wiring schematics you will ever need are in there. Have found them to be a varitable font of information and gives you on-line access for the whole enchalada.

Hope this helps

Bill

[libtec]

Thanks Bill! I think I figured out what part of the problem is. It's not that the lock is happening at the wrong time, it's that it never worked before and now it's coming back to life. The truck is a recent purchase and I'm still learning about it. Any how, I plugged the tranny wires back in today and I couldn't get the thing to lock in in any range so I'm guessing I have a loose wire some where. Once I do get it figured out I'm going to put in a cut out switch so I can turn off the lock anytime I want. I guess a question to ask would be if I can eliminate the ECU unit that gathers the sensor inputs and decides when to turn on the lock. If all the ECU does is control the lock-up I'd rather install a switch to send a 12v signal to the solenoid when I want it and not anytime else, avoid all the computer and extra wires and make it simple. Besides I have proof that the system isn't going to work for me, to get to my home I have to put the truck in 2, and hold it at a constant throttle setting up a long hill with a consistant slope for about 4 minutes. This is the conditions that tell the ecu to lock-up, problem is I'm pulling about 4000lbs plus moving the truck and it simply can't go solid to the axle and still turn the motor. Anybody know if the ecu is needed for anything else on a bare bones besides the converter lock up? Thanks!

[convert2diesel]

Libtec:

Just reviewed your original post and I guess I missed the fact that you have lost your speedo. This is a little more important then just avoiding the local constabulary. The speedo is driven by the afore mentioned VSS (vehicle speed sensor) located where the speedo cable used to connect to the tranny.

More then likely it is either this sensor or a thing called the VSS buffer (located usually close to the steering column), neither of which is very expensive, but both are required for proper operation. Check the wiring before you rush out and buy these things, especially the vss. Usually your Haynes manuals will have a wiring schematics toward the back of the manual. This should have your TCC circuit clearly indicated, at least mine does (The wiring diagrams are the only usable things in a Haynes manual from my experience)

The vss sends a pulse signal at I think (please correct me if i'm wrong) 4000 pulses per mile. The buffer then splits this pulse so that the speedo and cruise control get the 4000 pulses but the ECM gets only 2000 pulses per mile (I guess the poor little computer can't handle big numbers :D ).

As far as loosing the ECM altogether, some mechanical considerations should be addressed. If your engine is equipped with an EGR valve, this is the only other thing that should be controlled by the ECM, unless by 1990 they were using the computer to control the glowplugs, but I doubt it. My concern is that the 700R4 was designed around having a lock-up converter and it has been my experience that prolonged high speed with the converter un-locked literally cooks the tranny with the heat.

Another concern is that when the torgue is locked up, there should be someway of unlocking it when the throttle comes back to idle (de-acceleration) such as when you are going down hill (the clutch was designed to hold on power, not overrun) and it should un-lock again on acceleration to give you torgue multiplication climbing the afore mentioned 4 minute hill. All of this is what I hope to achieve with my little logic circuit.

Again, I hope this helps.

Bill

[libtec]

Thanks a bunch.

I am concerned about running the tranny in OD without a lock and making extra heat. I hope I can locate the speedo problem when I go into the dash to put in a stereo this weekend. I am getting power to the speedo, just no signal.

As for putting in a switch to turn on the TCC, I'm holding that idea. I'd rather find the root of the problem rather than use a band aid to get around the situation. I'm going to get the system working the way it was intended in the first place, then I'm going to put a switch in line with the solenoid feed wires to shut the sytem off for the long hill climbs.

I wish my Father-in-law was still around, he could answer all the questions I've posted in the last month in a few minutes. The last thing we did was get this truck going out of hibernation. He test drove it, told me I got a good deal but there were a couple of bugs to work out and said we'd work on them later. That was the last time I saw him, he had a bad accident a couple of days later. Anyhow, I really appreciate all the advice people have been giving me since that guy never mentioned what he had picked out as the "bugs".

Thanks again, I'll let you know how it goes.

[convert2diesel]

Libtec:

Try the plug going into the tailstock of the tranny first. Clean it up well and use some connector grease (Used to be called DC4 compound). Might be that simple. While your at it the forward plug to the internals woudn't hurt either.

Bill