OK, well that's good to know! But, if the computer doesn't have anything to do with the tranny operation, why do the two wires that are supposed to provide the ground path to the solenoids come from there? I'm just assuming it's a 42RH, since it's a 3.9 engine.

 

The ECU does the go/nogo speed test for lockup and OD; the internal switch can override the go from the ECU for OD.

It's very simple; the ECU can be replaced with a vacuum switch and two pressure switches for all that matters ( see, for instance, the kit at https://transmissioncenter.net/shop/...iring-diagram/ )

RwP

 

Thanks, RalphP, I'll have to check that out!

 

incorrect....the OD solenoid gets its ground from the SBEC on any 42rh to 46rh.

 

This is what I did on my engine swap as I completely eliminated the SBEC. My OD and LU are controlled by two pressure switches and a vac switch using the TCI kit. FYI I also used a relay (repurposed my AC relay since I eliminated AC) and connected it to the transistor circuit on the OD button inside the cab. That way I can still turn off OD using the button. If you get to that point and need a schematic let me know.

But from your follow up tests if both the OD and lockup aren't working I'd suspect a connector somewhere. Using your same test light procedure move further up stream...start with the test light right at the SBEC wires leaving the connector.

From your tests your coils sound ok...you measured a load and confirmed that with the spark...if the coils were bad you'd likely have an open circuit and no load and no spark. If a coil overheated usually the windings short together thus effectively reducing the number of turns of wire in the coil. This could reduce magnetism and not allow the coil to pull in the plunger but you would see this as a reduced imedence....that you measured 30 ohms on both that's not the case.

 

Thanks, Robertmee, that's what I found. Also, the OD switch on the dash also goes to the SBEC. I notice you talk about a transistor in the switch, but I don't see one indicated in the wiring diagram for the switch; it shows an illumination bulb, a push-button switch (NO) and an indicator light to show that the OD is off. When I was trying to get the OD working a couple years ago, I took out the switch and took it apart to see how it worked; whether the push button switch was a toggle, or momentary. I didn't find that out, necessarily, but I DID break the darn thing, so I now just have a hole in my dash. The other day, when I resumed investigating this issue, I hooked up a push button switch (doorbell switch) and a 12v bulb, hooked up just like the schematic shows. The push button switch doesn't seem to have any effect on the operation of the OD. But, when I got in the truck yesterday to do one of many road tests, the light went on as soon as I started the truck. It didn't do that before. But, on my road test, no matter how many times I pushed the button, the light never went off.
Today I will try checking all of the wires that go from the solenoid and the switch to the SBEC, as you suggest.
I think I mentioned in an earlier post that I had connected 3 wires to the wires going to the solenoid connector on the tranny, and ran them into the cab, attached to light bulbs. At no time did the light bulbs light. But another thing I did is touch the two wires connected to the outside terminals on the connector to ground inside the cab as I was driving. Since I grounded the solenoid, shouldn't the OD have kicked in?

 

A few notes:

1) The transistor is inside the control module,which is that assembly with the light and the switch. There's also a diode shown in the 1988 diagrams.

2) How did you hook the light bulb up to the solenoid wiring? I don't see many ways that won't cause possible problems; but if you hook a 12V LED, positive to the center wire, negative to the switched wire, it should light up if the PCM is enabling the lockup or overdrive.

3) Try splitting the wire for OD and the wire for lockup from the connector, then running them into the cab. If you ground the circuit, you should notice a very small or a noticeable drop in RPM for the same road speed (TC lockup won't be but 100RPM or so, but the OD would be about a 1/3 reduction since it's like .7 or .67 OD ... that is, engine RPM will now be 70% or 67% or so of the driveshaft RPM in overdrive). If it's dropping, the lockup or OD is working.

Don't forget to open the circuit(s) before you stop the truck ... TC locked up will cause the truck to stall, and OD will kind of kill the performance for moving from a standing start *grins*

RwP

 

Okay, I did what you suggested, and there has been partial progress. I used two toggle switches, one for the OD and one for the TC lock. I got the convertor to lock, which I hadn't experience EVER before, but the OD still didn't kick in. I tried turning the switch on and off multiple times, thinking that it might break free if stuck, but to no avail. Next I'm going to go back under and see if I can hear the solenoid. And I'm going to examine the flow diagram for the tranny to see if, perhaps if the OD solenoid IS working, then perhaps there could be some kind of block in the fluid "circuit" that could be preventing engagement of the OD.

 

Check the part number on your PCM. I wonder if it is simply programmed for the wrong transmission.......

 

I think I know what the trouble is. The solenoid pack for the O/D and lockup is bolted to the right side of the valve body inside the pan. My truck experienced no O/D and the lockup would sometimes not release. The 3 screws holding the solenoid pack to the valve body had loosened, allowing the fluid to bypass the solenoids. Drop the pan and tighten the screws, and while you're there, change the filter and refill with ATF+4.