Driving '91 GM electronic speedometer from a US Shift Controller

[Gary Gable]

I had a chance to get things apart tonight and have a look, and in the meantime the tech from US shift got back with me. Both of you are saying essentially the same thing, except he identified the 4WD wire pair as the one to jumper onto and you're identifying the 2wd pair, which oddly enough lines up with the color coding on the wiring diagram he sent along with the advice. So there is a discrepancy I'm going to give him the chance to answer tomorrow. Really the hardest part here is getting a representative wiring diagram. The one I have for the A/B & C/D pinouts is very close, but either it's got a typo or something else is up. Thanks again for your help.

 

[tayto]

the ones i identified are for a 2wd application.

 

[Gary Gable]

Last night I got a chance to work on it again. To follow what the tech at US Shift recommended I spliced into the two wires in their loom that go from the transmission speed sensor to the port on the Quick 2 into the two they said I needed to splice into, which were wires B5 & B6 at the PCM connector. These two are identified on my PCM Connector ID diagram as "Transmission Output Speed Signal (4WD)" and "Transmission Output Speed Sensor Ground", and are LT BLU and DK GRN/YEL respectively. The tech said there was no polarity to observe, just slice them together one for one. This didn't work. The transmission immediately began shifting oddly, the readout on the the Quick 2 became unresponsive, and the speedo still didn't work. I don't know if there actually is a polarity requirement, or if this is simply the wrong pair to splice into. I definitely don't have a separate DRAC, it's built into the cluster, so I'm sticking with finding the correct wires to splice into at the PCM. Wires C7 and C15 at the connector are identified as IAC Coil "A" High and Injector #2 Control, so I'm sure they don't go off to the instrument cluster Drac connection. The only other pair of wires at the A/B connector that seem remotely connected to running the speedometer are B10 and B11. They're identified as "Transmission Input Speed Sensor Ground" and "Vehicle Speed and Transmission Output Speed Signal (2wd)" and the wire colors are DK BLU/WHT and BRN. For reference, my truck is 2WD and all the connectors remain plugged into the PCM. My old transmission harness is zipped tied out of the way near the transmission and I didn't cut it out of the rest of the loom. Thanks for any tips at this point! The US Shift tech hasn't answered my questions after repeated requests. I'm not real wild about their tech support, they don't seem to know at all what their talking about and they've given me several bum steers so far.

 

[Gary Gable]

I maybe figured this out, and maybe Tayto you're right, possibly even the tech at US Shift is right. I thought all along when you guys were talking about jumpering wires you were talking about those that terminate in either of the two connectors that make up the PCM connector set, but now I'm wondering if your talking about jumpering to those found in the harness that makes its termination in the instrument cluster connector. I've attached 4 PDF's. The first is the US Shift Quick 2 wiring diagram. The second and third show the wiring found at the PCM connectors, the forth shows the Instrument cluster connector. Can any of you tell me which two wires coming into the Quick 2 controller (identified as Trans Speed Sensor) jumper to which two wires on the other remaining schematics? Before I went down the road described above, the transmission shifted great with the new controller but I had no Speedometer at the dash. This is the last bit of the puzzle. Thanks! Gary

 

[tayto]

C7 and C15 are NOT on the PCM! they are the wires on the DRAC unit. The US shift tech is wrong. I have not mentioned ANY wires that goto the PCM.

put all your speedometer wiring back how it was and take the old output speed sensor wires and splice them into the new output speed sensor wires down by the transmission tail shaft.... you will NOT be splicing anything in the interior, under the dash, PCM, speedometer, DRAC, engine compartment, etc.

 

[Gary Gable]

Hi Tayto. Given I'm somewhat beholden to at least try what their tech suggested (that is if I intend to keep them linked into supporting their product, which I do) I went ahead and tried the latest wiring connection he suggested. It didn't work, no speedo output. I believe your suggestion above is correct, and I will follow it but another little wrinkle appeared. The controller, I think I mentioned, behaved erratically when I connected up as this same tech suggested the time before last. I'm pretty sure following his advice then damaged the controller, as now I have error code indicating shift solenoid B (SSB) circuit is open. I know the wiring from the transmission I completed during installation using the US Shift supplied harness is correct, and the shift solenoids are new, the internal wiring is new (93' & up style), the plugs are new and all other sensors are new too. Pretty much all of it is new. All the connections made at the Quick 2 controller are fine, no damage or compromise. I've asked them to send me a new controller. We'll see how it goes. There is no way it shouldn't be picking up signal from SSB, but the controller isn't reading it. I'll post when I hear their reply. Thanks again. Gary

 

[tayto]

I am not surprised it didn't work and that it harmed the controller..often is the case when guessing. again, not to sure what the fixation on getting the controller to drive the speedometer is. i see no real benefit and the cons out weigh the pros (such as the PCM no longer seeing a speed signal, cruise control not working etc).

I am not to sure what the reasoning is to tap into the (4wd) output sensor wires. were you trying to hook the controllers speedometer output here??? I am honestly confused by what you and the us shift tech are trying to accomplish especially when the transmission worked initially. this pcm does not send ANY signal to the speedo. The speedometer gets it's signal from the DRAC and the DRAC gets it's signal from the output speed sensor. like i've been saying from the beginning, feed the truck the output speed sensor signal again and your speedometer will work.

 

[Gary Gable]

Hi Tayto,

At this point I'm sure you're right, and I'll test that theory on the weekend. I needed to follow this guys advice first, it's their product. As it happens the US Shift controller magically, after a week of sitting, managed to clear it's error code and shift normally. It didn't respond to any efforts to clear the error code a week ago, but evidently it needed a longer "power off" period. Anyway, that was a pleasant surprise. The tech's final idea was hooking up their speedo output wire (5V) to B5 on my PCM because he was sure that would drive the speedometer. I tried it, obviously nothing. What he doesn't understand and I frankly I didn't either is that while the original OSS wires weave into the loom and head up into the cab, they don't route to the PCM, they evidently split off and weave into the loom segment that plugs into the instrument cluster where the DRAC is. It's easy enough to jumper leads from the OSS wires that came with the US Shift wiring harness right down at the transmission over to the existing pair that still go up to the drac. I'll do it on the weekend and report back. Thanks! Gary

 

[Gary Gable]

Tayto, you were exactly right. Jumping the OSS wires that came with the US Shift controller kit over to the old OSS wires that I had hanging zipped tied out of the way near the transmission allowed the speedo to work again. That, and the fact that error codes the US Shift controller were throwing went away after a period of power disconnection means the transmission is now shifting normally again. There are probably about 9 things I would have done differently in hindsight. The first would've been to really drill down on the differences on the 88-91 wiring vs later models, as that led to a whole bunch of confusion initially on where the drac is located. Second I would not have assumed the US Shift technical advisor could read a schematic or really understood the product the company he works for sells. Nor would I assume they'd understand much about this particular vehicle. Third, in purchasing the controller I should not have assumed they'd have a pre-printed ready to go, wire for wire installation guide for my vehicle. That's fairly common nowadays, but not the case with US Shift. I realized now they are much more geared to those installing a 4l80E in a pre-electronic control vehicle or hotrod. They just either don't get the request much to do what I needed the controller to do, or they just don't see it as worth the investment in time to figure it out in advance. Either way, good solid tech support would've solved that, and in that regard they're sorely lacking. Fourth, and this really goes back to the initial diagnosis by two separate transmission shops that were hopelessly lackluster, one should never assume that shops that have been in business for decades would necessarily have seen or have the institutional memory to correctly diagnosis what the problem actually was. Which, drum roll please, probably lies in the original wiring harness. Not the drac as one shop was sure of. Not the PCM, which another was too. I actually hunted high and low for a new external wiring harness to run from the transmission up to the speedo and PCM. I simply couldn't find anybody who sold one. Having one custom made was more pricey than the new US Shift controller which came with a wiring harness, so in the end that's the route I took. But cripes, what a saga.

Thanks Tayto, who clearly has a solid understanding of these transmissions and their electronic control. Thanks to others who chimed in, there was some good advice there too. It's all appreciated.

 

[tayto]

excellent good to hear!