Early shifting into OD. Strange readings on TunerPro RT

FourEightZero

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93 K1500 5.7L - 4L60E

Getting early shifting into OD while driving only ~25MPH in my neighborhood. Engine lugs and wont downshift until I almost come to a complete stop.

Tranny shop that rebuilt it said it is fine and they cant figure out what's wrong.

Started firing the parts cannon. Picked up another 16168625 PCM, popped the tuned chip in it, replaced TPS.. still shifts early.

Ran a datalog on TunerPro RT and did a transmission data recording.

Noticed that the computer was showing I was going 60 something MPH in my neighborhood when I was actually going about half that.

My Dakota Digital IC/speedo was showing 25-30 at top speed.

Is it possible that the computer thinks I'm in 4 low when I'm not? Anything else I need to check?
 

kennythewelder

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93 K1500 5.7L - 4L60E

Getting early shifting into OD while driving only ~25MPH in my neighborhood. Engine lugs and wont downshift until I almost come to a complete stop.

Tranny shop that rebuilt it said it is fine and they cant figure out what's wrong.

Started firing the parts cannon. Picked up another 16168625 PCM, popped the tuned chip in it, replaced TPS.. still shifts early.

Ran a datalog on TunerPro RT and did a transmission data recording.

Noticed that the computer was showing I was going 60 something MPH in my neighborhood when I was actually going about half that.

My Dakota Digital IC/speedo was showing 25-30 at top speed.

Is it possible that the computer thinks I'm in 4 low when I'm not? Anything else I need to check?
Trans shifts, are controlled by the speed sensor. If the speedo is off, chances are, you need to replace the speed sensor on the trans. At least, that's was my issue, when I had shift issues with mine.
 

FourEightZero

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Trans shifts, are controlled by the speed sensor. If the speedo is off, chances are, you need to replace the speed sensor on the trans. At least, that's was my issue, when I had shift issues with mine.
I was thinking that, but wouldn't the speed sensor being faulty also throw off my speedometer?

This problem just started in the past couple months while I've had this Dakota Digital cluster for about a year now. I have it wired right into most of the connections of the factory IC harness, including the speedometer wiring.

That 60 mph reading was seen in TunerPro, but the speedometer on the dash was reading the true speed.
 

kennythewelder

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I was thinking that, but wouldn't the speed sensor being faulty also throw off my speedometer?
Yes. Very much so. Mine was fine, until I got to WOT, and it shifted out of 3ed and trying to shift into OD. It wouldn't shift, at all, the speedo would go bonkers, and the trans would just hunt for OD. This happened only at WOT. When I let off the gas, front a stop, and just accelerated normally, it shifted just fine. I just had the trans rebuilt at that time, so I brought it back in, and said fix it. It took about 1 week for them to figure it out, but, He swapped in a new speed sensor, and no more problems. Trans shifted grayer at all speeds, and the speedo wasn't going craze anymore. The sensors are not very expensive. It would be worth a shot. I think there are around $50 or so, maybe.
 

FourEightZero

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Yes. Very much so. Mine was fine, until I got to WOT, and it shifted out of 3ed and trying to shift into OD. It wouldn't shift, at all, the speedo would go bonkers, and the trans would just hunt for OD. This happened only at WOT. When I let off the gas, front a stop, and just accelerated normally, it shifted just fine. I just had the trans rebuilt at that time, so I brought it back in, and said fix it. It took about 1 week for them to figure it out, but, He swapped in a new speed sensor, and no more problems. Trans shifted grayer at all speeds, and the speedo wasn't going craze anymore. The sensors are not very expensive. It would be worth a shot. I think there are around $50 or so, maybe.
Thanks Kenny,

Just went for another spin around the block with my roommate wacthing the TunerPro readings. Confirmed it.

The PCM thinks I'm doing 70mph down my residential street. I guess the cluster speedometer is getting it's signal from elsewhere because it's reading the correct speed.

I'll do one more shot from the parts cannon and replace the speed sensor on the transfer case.
 

yevgenievich

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93 had a drac module. I don't remember exactly, but I believe it is responsible for splitting the signal from the speed sensor between dash and the ecu.
 

FourEightZero

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93 had a drac module. I don't remember exactly, but I believe it is responsible for splitting the signal from the speed sensor between dash and the ecu.
Did more research on the DRAC module and its highly likely this may be the culprit.

Question is... if the cluster speedo is still working fine, but the PCM is getting a signal that is much faster than it should be.. is a new DRAC gonna get the job done? Or do I need to try some other diagnostics/fixes first.
 

Sabinoerc

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Did more research on the DRAC module and its highly likely this may be the culprit.

Question is... if the cluster speedo is still working fine, but the PCM is getting a signal that is much faster than it should be.. is a new DRAC gonna get the job done? Or do I need to try some other diagnostics/fixes first.
I’m outside my experience on the DRAC but my memory from when I was troubleshooting problems in this area on my 93 is that the signal sent to PCM and signal sent to the cluster are different. I also think a noisy speed sensor signal into DRAC can affect one differently from the other. I think there are different DRAC settings for various models so if you replace it make sure it’s set the same.

I had problem where speedo was jumping around but it seemed to shift normally. That was a low amplitude/noisy speed sensor in tranny. Memory was it was easy to find a speed sensor and easy to replace. It was hard at time to find a DRAC for a 93- something like they changed to a different “thing” in later year.

So depending what you find on availability and cost you may want to try speed sensor first.

the tuners on gearhead-efi know this stuff
 
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Schurkey

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My '88 had a faulty DRAC. In '88 it was located in the instrument cluster.

Signal from the VSS goes direct to the cluster, and thus to the DRAC module.

The DRAC processes the signal according to gear ratio, resulting in a pulses-per-mile output.

The speedo--odometer get a certain number of pulses per mile. The Cruise and the ABS get a different number of pulses per mile, and the ECM gets a different number of pulses per mile. I have no idea why all these components need different pulses-per-mile, but they do.

In my case, the DRAC would simply fail to send any output signal at all. When it worked, everything worked. When it didn't work, the speedo, odo, cruise, ABS, and ECM (for the torque converter clutch) all got no signal, so all those systems crapped-out at the same time, and they'd come back to life at the same time three miles or three days later.

But I can imagine a DRAC that failed only on one of it's several output channels. So the signal to the ECM could be wrong, while the signal to the speedo might be correct.

Point being, if the speedo is getting the correct signal, I'd be really surprised if the sensor was at fault. But I've been wrong before.
 

FourEightZero

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My '88 had a faulty DRAC. In '88 it was located in the instrument cluster.

Signal from the VSS goes direct to the cluster, and thus to the DRAC module.

The DRAC processes the signal according to gear ratio, resulting in a pulses-per-mile output.

The speedo--odometer get a certain number of pulses per mile. The Cruise and the ABS get a different number of pulses per mile, and the ECM gets a different number of pulses per mile. I have no idea why all these components need different pulses-per-mile, but they do.

In my case, the DRAC would simply fail to send any output signal at all. When it worked, everything worked. When it didn't work, the speedo, odo, cruise, ABS, and ECM (for the torque converter clutch) all got no signal, so all those systems crapped-out at the same time, and they'd come back to life at the same time three miles or three days later.

But I can imagine a DRAC that failed only on one of it's several output channels. So the signal to the ECM could be wrong, while the signal to the speedo might be correct.

Point being, if the speedo is getting the correct signal, I'd be really surprised if the sensor was at fault. But I've been wrong before.

I'm doing this the expensive way, but I'm horrible with diagnosis.

New VSS didn't correct the problem. I ordered a dip switch DRAC that will show up tomorrow.
 
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