SA68SS

I collected my new 93 Z28 in Redding California 9 days ago, and changed out the plugs, oil and #5 plug lead since it was burned through.

The plugs are difficult to access, but all were done without any dismantling of engine parts.
Shimmed the starter as it was hard to start and showed some tiny metal particles in the small inspection port.
Drove 3 1/2 hours down to Reno next day, got some Nitto 555 tires and an Optima Battery from the Sparks Nevada Summit store. 315/35/18 rears and 275/45/18 fronts. Pep Boys did the tire change for 67 dollars. Balanced, fitted, old Sumitomos taken away. they had gone very hard. Drove back up to Redding for the drag races that night.

So, 383 stroker with forged internals all set for a run at the Redding Drags. A better driver would have got in the mid 12 seconds, the engine was very strong. Last run the tacho was jumping and all electric power was shunting on and off. After 20 minutes cool down it started up.
I suspected the alternator and old battery, so next day got a new alternator and fitted the Optima battery. The old alternator tested good, so problem not really solved or identified.

Left Redding for Michigan, drove 3 hours and got the tacho jumping and loss of power again near Reno. A cool down got it running smoothly so I continued on to Elko Nevada for the night. Bought a re-man coil from Oreillys "just in case" Could not get an ICM at the time.

Next morning the car crapped out again on the Bonneville Flats. After a cool down I changed out the coil and drove on for next three days without any further problems. It is kind of stressful waiting for that sort of problem in high speed traffic in remote places. Could not find a ICM in any Chevrolet Dealership or parts store anywhere along I-80. Nearest was in Denver CO.

So, for what it is worth, a bad coil can be indicated by bouncing tacho and intermittent total loss of power, with full recovery after a cooling down period. I bought some heat sink compound from Radio Shack yesterday and will probably put it on my current ICM and see how it goes.

Hope this helps someone.

will be posting questions on all the other little issues with this car.

The car is a blast to drive, easily spins the tires in first three gears.

Injuneer

Interesting results. The advanced planning really paid off.

You might want to correct your speedo for the oversize tires. Stock tires were either 235/55-16 (26.1") or 245/50-16 (25.7"). Your new fronts are 27.7" and the backs are 26.7". Your speed is indicating 2% - 4% lower than actual speed.

The bouncing tach due to coil problems will only happen on the 93, where the tach is driven directly off the coil, via a filter. 94 and newer, the PCM drives the tach.

SA68SS

Good to know about the coil driving the tacho. Little tips like that really do pay off when diagnosing problems.
I am just grateful that the ICM had not failed. Coils are 30 dollars and available everywhere (almost literally)

Funny you mention the speedo. I WISH it was only 4% inaccurate. At present the speedo is one of a list of little issues that I will be trying to correct. Currently it shows 20 miles per hour above actual speed. That is a lot. It starts at zero, but is wildly inaccurate, obviously.
I note that builder machined a 'ring' for the ABS to work with the Ford 9 inch rear end. I wonder if this ring is also the drive wheel or counter for the speedo?
The speedo is inaccurate and the ABS INOP light comes on above 50 miles per hour and stays on. I think there is a link between these two items

Thanks for the reply and info injuneer

lt1-xjs

Search "4l60 speedometer calibration". You'll need to pull the tail shaft and change the gears depending on the tire diameter and gear ratio. PATC has a chart to help choose the correct ones.

SA68SS

This car has manual T56 trans. Thanks for the reply.

I am hoping the 93 has a simple trans counter, rather than a rear end counter for speedometer signal.....

lt1-xjs

I see that now, I blew right past your sig. Good luck.

Injuneer

The 4L60 4-speed auto has the drive gear on the output shaft, and a driven gear that turns the electronic VSS sensor. The T56 had an 11-tooth reluctor gear on the tailshaft, and an electronic sensor that reads it. The 1993 factory manual shows the VSS signal wired to the ECM, with a 4000 pulse/mile output from the ECM to the speedometer. I would appear the correction has to be made in the ECM chip. Or you could get something like the Dakota Digital SGI-5 speedometer interface box.

Keep in mind that the 1993 is unique in many respects. The trans selections are all 1-year only. There were two different T56's, neither of which matched the gear ratios of the 94-02 T56. As a result, the 93 T56 has a lower torque rating. Contrary to popular belief, the 93 F-Body T56 is not the same trans used in the Viper. Even the reluctor gear selection (11-tooth) is unique to the 93. All other years of the T56 have a 17-tooth reluctor.

SA68SS

Thank you Fred,
Interesting. a fix via the ECM might be the easiest method.
Do you mean that the output shaft of the transmission holds the reluctor gear, or the actual tailshaft (drive shaft) of the drive train?

I suppose there is the obvious reason, being the 4.30 gear set, or perhaps the PO put in a late model T56 with a 17 tooth reluctor gear?

Injuneer

The output shaft (tailshaft) of the transmission. Not the driveshaft.

A 94+ T56 has a "skip shift" solenoid on the driver side of the trans, the 93 does not. I'll post a picture in a while.

The 93 T56 would have been coupled with a 2.73 rear axle ratio (RPO code GU2 on the service parts tag) or a 3.23 rear (GU5). Check the math with old vs. new gear ratios, and old vs new reluctor gear tooth count. See if any combination works out to what you see on the speedo.

SA68SS

I just spoke with the builder in CA, he recalls his brother putting a WS6 T56 transmission into the car. I think he wanted the Viper T56 with the stronger output shaft but didn't pony up for it.
So, I am thinking it will have a 17 tooth reluctor in that transmission.

I will ring a few of the chip burner companies and see if they can offer a fix.

Injuneer

You can't just bolt in a Viper trans..... shifter will be in the wrong place. Also an issue with the LT1's unique pull-back clutch design. But there are Viper upgrade parts that can be installed in the F-Body T56. There is no such thing as a WS6 T56.... same trans used in every LT1 from 94-97. Early models were made by Borg-Warner, but near the end of the LT1 era, B-W sold the design and production facilities to Tremec in Mexico.

SA68SS

Right, thanks for the info Fred. I should have said the PO installed a T56 from a WS6, which I assume would be 1997 or later model. I am looking at this page for trans am production numbers
Trans Am Production Numbers

A correction to my tire sizes. I made a stupid error on the wheel sizes, My actual tires are 315/35/17 and 275/40/17 Which gives a tire height of 25.68 and 25.66 respectively. Essentially the same size front and rear. Apologies for that misinformation.

Injuneer

First 4th Gen WS6 was 1996 model year.

We're running the same tire sizes.... match my original factory 245/50-16 diameter.

My issue with the speedo was going from the T56 (17-tooth reluctor) to a TH400 (41-tooth reluctor). Rather than reprogram the PCM as an automatic, and have it throwing codes for the missing transmission interface (a problem a 93 would not have), I used the Dakota Digital interface box. I used LT1_Edit to correct for the 4.11 gears.

SA68SS

I have been in touch with Solomon at LT1 PCM tuning, and he cannot help with Speedo adjustment on the 93 ECM. Does not have the ability.
So, I am looking at the Dakota Digital SGI-5 C.
I have found a few wiring tips on the third gen sites.

some basic questions -

Where is a good position to mount the unit?

What wires are readily available for splicing into?

Injuneer

I put mine under the console. Tough to remember the details from 13 years ago.....

The VSS has two wires - signal (yellow) and ground (purple) - that are easy to pass up through a hole/grommet drilled in the floor, connect through (signal, sensor ground) the box as required, and then back under the car to the PCM. The box requires 12V power and ground. There is a switched power pin (1/4" female spade lug required - I'm assuming the 93 is similar to my 94) in the fuse panel at the end of the dash labeled "IGN". That is protected by an adjacent fuse. Good source for switched 12V power to a low amp load like the SGI-5.

Be interesting to see if the 93 T56 is the same as the later years - I know the 93 won't have the skip shift feature.

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