Hi all, I have a new mystery Camaro.

I say mystery because what was changed from stock is a mystery. I inherited this vehicle and everything is a learning process.

1989 5.7 TPI, Auto, 255-60-15 tires.
According to my brother and the VIN, a 5.0 was stock. The engine and transmission were rebuilds.
I also understand he had the rear end rebuilt (broken axle), I have no idea which rear he installed, is there a way to tell?



Current issue:

code 24: VSS SES code.
Basically the Christmas light glows at particular times during acceleration. The car under throttle runs fine, but upon a sharp stop the engine has idled hard and stalled (not good for winding off ramps). My speedo and odometer are both off.
GPS indicates a speed difference of 1.25%. (example - speedo:gps --> 100:80, 75:60, 45:36), as you can imagine, this fouls with the ECM and the engine.

Now, obviously changing the rear end and tranny might effect the speedo/odometer, do I have to drop the tail shaft to switch gear ratios? Or do you guys think this is strictly electronical with the VSS.

If this is a gear ratio issue, how can I figure out the correct ratio I need?

How can I figure out which rear end I have?

Thank you for your time!

 

Same way anybody else can.

Change the fluid, like you need to be doing anyway, given that this car is "new" to you.

While you're doing that, make a point of looking at the gears; and check out the carrier for clutches, so we don't have to deal with yet another pointless "do I have posi" post.

Looks to me like AHELLUVALOT more than any 1.25% of speedo error. Looks to me like it's 25%; for example, 100 indicated vs 80 actual = 20 mph error; error/actual = 20/80 = 25%.

Since the ECM has no way of evaluating that, then it will NOT cause a code to be set. However it might make the converter lock up at odd times or that sort of thing.

Yes you will probably need to pull the extension housing, because you will probably need to change the drive gear. Go here for help with gears. Third time I've posted that link in the last 2 days; people must not be reading posts that keep asking the same questions.

 

yeah, I realized when I said 1.25% after I posted that thats the difference when I divide the two. 100 mph (as per speedo) divided by 1.25 gives me the correct speed of 80 mph (as per gps).

So the actual percentage is the difference of the speedo and gps, divided by the actual speed. (example - speedo:gps --> 100:80 = 25%, 75:60 = 25%, 45:36 = 25%), gotcha!

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The fluid was changed by my mechanic three weeks ago, because when the former owner changed the rear end, he used shoddy seals and they leaked oil all over the brake drums. I wonder if he could tell me from memory.

I read you can rotate the tires and count the revolutions of the drive train to one full revolution of the tire. This make sense?

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I think people, like myself, appreciate specific questions answered because they are catered to our needs.

You showed me how to correctly figure out the wrong percentage of my speed, I haven't read a post through our search feature that correctly explained this.

Thanks for your time! Have a good one.

 

The "count revolutions" thing doesn't work real well, because the tires MUST rotate EXACTLY the same amount.

Neither does any method involving the tach, because they're so inaccurate.

Best way to calibrate it is with the odometer. Since that's nothing but a turns counter, and it takes 1000 turns of the cable to make it read one mile, it's kind of a no-brainer to work it that way. Plus, it CANNOT become inaccurate, because it's nothing but a bunch of gears; nothing that can change.

And of course, now that you know that 1000 revs of the cable = 1 mile, it's equally a no-brainer to calculate the gears you need. I know this, and it is my specialty, because I have no brain.

Look at your rear end gears, and get the ratio; and then measure from the EXACT CENTER of the wheel to the ground, with the car sitting on a level surface. That gets you the REAL tire radius, should be around 13 or 13½". Using the "trade" size of the tires (235/55-15 or whatever) IS NOT sufficiently accurate. There's more variation from one model of tire to another in the same size, than tehre is from one size to the next.

Once you have the 2 pieces of info, multiply the tire radius by 2, and multiply by pi (3.14159265359 etc.). Should come out to somewhere around 80. Divide 63660 (12 x 5280, the # of inches in a mile), by that number. That will give you the number of times the tire rotates in one mile, should be around 700-800. Multiply that number by the # of teeth on your ring gear and divide by the # of teeth on your pinion. You should come up with a number between about 2500 and 3500. Divide that by 1000 (# of speedo cable revs per mile), and that's what your speedo gear ratio should be. From there, it's just a matter of selecting gears that you can actually buy from off of that page, that come closest to the ratio you determined.

Like I said, no-brainer. So easy even I can do it. And I'm dumber than about any caveman.