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This may sound like something that's been posted 1000 times before, but despite extensive searches of the forums, I cannot find a complete, proper answer.
A 1991 TBI Firebird I recently picked up had a rear gear swap at some point. Car left factory with a 3.08 (GU4) in it, but it appears a previous owner swapped in 3.73 gears (GT4) with no other changes to sort out speedo and speed limiter issues. In addition to the speedo mis-match and the engine cutting out at 80mph, the engine is not adjusting itself terribly well in closed-loop mode, especially when stopping and idling- so I need to get this sorted out.
The problem is that there is no DRAC/VSSB unit in this car. There is the "4 out" frequency modulator between the speed sensor and the ECM, but no DRAC or the like appears to exist, I have searched behind the dash and removed the gauge cluster to see if there were resistors/jumpers there like on many 1988-1992 trucks (i.e. DRAC built into cluster electronics) but this appears unlikely based on what I'm seeing without completely disassembling the cluster.
Is there something I'm missing here, or is the only way to properly calibrate the rear gears and the speedometer to re-flash the ECM or use some sort of aftermarket programmable speed sensor? If these are the only options, is there a programmable speed sensor available that will work with the factory speedometer?
Just to be clear you have checked to make sure that the transmission has the correct speedo drive system? I bought an 88 gta and was having the same issue. Come to find out the gears and the trans had been changed out at some point. The trans had a mechanical gear that fed into an electronic speed sensor. Was a very odd combination.
Every third gen used selected speedometer gears in the transmission to match tire size / rear gear.
Models with a VSS at the trans. used a sensor (like below, a 700-R4 sensor, and T5 sensor with gear) with driven speedometer gear installed on it.
The majority of third gen models with 3.73 rear gears and normal "performance" tires and wheels for the year used a 1:3 ratio drive and driven speedo gearset.
700-R4 15 drive / 45 driven
T5 7 drive / 21 driven
So if your trans. has speedo gears and not a reluctor-wheel VSS setup, change the gears to correct it.
Is there something I'm missing here, or is the only way to properly calibrate the rear gears and the speedometer to re-flash the ECM or use some sort of aftermarket programmable speed sensor? If these are the only options, is there a programmable speed sensor available that will work with the factory speedometer?
You're not missing anything. Changing the VSS gears is one way to correct this. Only issue is being able to find the correct gears to use. Not easy.
The other way is to use a SGI5 box by Dakota Digital. It is like a DRAC and is wired between the VSS and the 4 OUT BUFFER box. Magnetic input from the VSS with a magnetic output to the 4-OUT box.
I have been looking into the speedo gear swap idea, but am having difficulties locating the right gears for this.
The Dakota Digital SGI-5E looks like a viable solution- but it also appears when I look over the instructions that it would actually replace the 4-out modulator- it has a sensor signal -in- and a -power- and a -ground- to connect to the speed sensor (I'm assuming the 2-wire speed sensor is a signal out and power, and it is grounded through its housing), and 5-outputs.
Anyone around here install one of these? If so, did they outright replace the 4-out buffer completely or how did they wire this in the system?
That page is hard to undestand based on its labeling- but the drive gear does not appear to be one that will fit a T5, but a 700R4.
Originally Posted by jmd
Read RBob's post one more time.
I have, multiple times- the problem is the VSS has two wires coming off it into the 4-out and the "4 out" actually has 5 wires coming out- one to the ECM, the others for things such as the speedometer, cruise control, etc.
How is a two-wire in (from VSS to Dakota box) two-wire out (Dakota box to 4-out box) supposed to vary the signal to the 4-out box? The 4-out box and the Dakota digital unit both do the same thing it appears- modulate the frequency off the VSS for the ECM, speedo and cruise control- the difference being that the Dakota unit is variable/tunable. How does it wire between the VSS and the 4-out box? It appears to be that the 4-out would be redundant.
Didn't see T5 in your information or this thread.
For that matter, in order to eliminate the difficult to find speedo gears, you could also get a 96-02 Camaro T5 tailhousing, VSS, reluctor and use the SGI-5.
Regarding the wiring, that's not a problem at all; the SGI-5 can provide one output or multiple. If I remember correctly, their installation instructions direct you to eliminate other devices, replacing the 4-way with the SGI-5 but it's not mandatory.
I prefer keeping the 4-way for "what if" reasons I won't bother stating.
There is less re-wiring to do when keeping the 4 out buffer box. The VSS ppl & yel wires goes to the SGI-5 box as the speed input (ppl) and the sensor ground (yel).
Then wire the SGI-5 "calibrated 'AC' output" to the 4-out buffer. It goes to the purple wire. No need to do anything with the yellow wire on the 4-out box.
Just an update in case anyone else comes across this issue;
I used a Dakota Digital SGI-5E box (this is the current version). It turns out on my setup that the Yellow wire to the 4-out box is the sender signal, the Purple is ground (perhaps someone reversed the wires somewhere in the harness at some point, I didn't go digging through everything). So;
As per the "Application #1" in the Dakota Digital documentation (High-High signal)
-Yellow wire from speed sensor cut by 4-out box and run to "Signal In" on DD unit.
-"Out 1" (calibrated ac output signal) on DD unit wired back to Yellow input on 4-out box.
-Power (positive, ignition 'on' only) and ground connections for DD unit spliced off from stereo wiring harness
-Purple (ground wire) left in place on 4-out box (i.e. not cut-it was left as it was found).
-everything tied up behind panel on dash, DD unit then calibrated as per instructions for the modified rear gear/speedo setup and everything is working A-Ok.
-lower front dash cover reinstalled- no one will ever know there is a modification unless they go looking for it.
There is less re-wiring to do when keeping the 4 out buffer box. The VSS ppl & yel wires goes to the SGI-5 box as the speed input (ppl) and the sensor ground (yel).
Then wire the SGI-5 "calibrated 'AC' output" to the 4-out buffer. It goes to the purple wire. No need to do anything with the yellow wire on the 4-out box.
RBob.
I have a 1990 c1500 Silverado and am trying to locate the DRAC. I've looked at the instrument cluster circuit board but do not see any wire jumpers for changing ratios. Is it part of the ABS module mounted at the brake booster and if so, Can I just use the SGI-5 to just change the ratio of the source signal coming from the transmission speed sensor so that it corrects the A/C signal being fed into the DRAC so that all things that the DRAC supplies are recalculated?
