Before I removed my gauge cluster, my speedometer was off by 5mph at 55 (55=60). Yes, it was a percentage, but where I live all the roads are 55mph, so its what I use as an example. But after I removed and re-installed the cluster, it is off by almost 15mph (now 47=60). I really don't want to have to pull the cluster out again, but have gotten a little better at it lately (been in and out 6 times now). I just find it tedious to get the speed cable disconnected from the back of the cluster.

Any thoughts as to how to attack, or is the cluster needing to come back out again? What would I look for or do?
Thanks!

 

Unless you swapped out speedometers I can't imagine how you would have affected the accuracy. The calibration is changed by swapping out gears at the transmission. There is a sticky thread on this and it's useful, but doesn't account for variable degradation of the speedometer magnet. I would buy a pack of various speedo gears and start experimenting. You will probably find each gear change causes about a 2mph change in the speedometer reading.

 

My guess is a binding cable or the wrong speedo gears. It's kinda' uncommon for the speedo head (cluster) to be the cause of an inaccurate speedo. Sure - it happens, but more often than not the issue is the wrong (or worn) speedo gears, or a cable that's "binding" inside the sheath causing the reading at the cluster to be a little slower than it should be.

I'd start your investigation at the VSS on the tranny. You can remove it from the tranny using a 10mm and pair of pliers in about 2 minutes. Once it's out,... Spin the square end of speedo cable and feel if there is any resistance, or does it spins freely ?? Next; (while the VSS is removed from the tranny ) temp install the VSS onto the cable and spin the plastic gear on the end of the sensor,.... does it still spin freely ?? Then; make note of the color and tooth count of the plastic speedo DRIVEN gear ( on sensor) as well as the color and tooth count of the DRIVE gear (in the tranny). FULLY inspect BOTH of those plastic gears for damage / worn teeth. Now; reinstall everything - at least for now !

At this point you'll know what speedo gears are installed, if they appear OK or damaged / worn, and if the cable spins free & smooth (into the cluster, and thru the VSS to the cluster ) properly.

Final step is to buy some gear oil (and a diff gasket if you are so inclined) and drain the rear end so you can verify your rear end gearing. ( probably time for new rear diff fluid anyway ! ) then check to verify the correct speedo gears are installed.

 

John, good info - my question is what color of the gear on the VSS relate to what ratio of rear end gears?

 

THANX !

I've got a few gear charts,... so do a couple other members. If I don't have the right one,... I'm pretty sure this group can find what-ever I don't have.

 

[QUOTE=John in RI;6541034]THANX !

I've got a few gear charts,... so do a couple other members. If I don't have the right one,... I'm pretty sure this group can find what-ever I don't have.

[/QUOTE

NICE !! Exactly what I was looking for - THANKS

 

I have an 84 Camaro , 305 HO,T5 transmission. Isn't the VSS on the back of the gauge cluster?
My two thoughts are
1) when I was taking the cluster in and out, I somehow damaged the yellow box on the back or the paper circuitry
2) when I pushed the speedo cable out of the way to keep it from snapping back into the cluster as I test fit it multiple times caused the cable to bind.

Are either of these valid? I have not touched the tranny or rear diff. The rear tires are oversized and so I assumed that is why the speedo was off a little. But for it to get worse was surprising.

 

A binding cable won't cause the speedometer to read "slower" per se. It can't, it is a direct drive from the transmission gear. Whatever speed that gear turns the cable MUST turn (or break). The only caveat to this is a binding cable causing speedo needle bouncing, which is the cable binding and slowing for a split second, then being overcome by the force of the gear and speeding up (bouncing). It averages out to the same actual speed due to the direct drive, but a bouncing needle indicates binding which is split second slowing and then corresponding increase in speed above the actual speed. The slight springy quality of the cable allows this to happen.

The charts are an okay starting point if you are way off, but using a chart to find your speedometer gear based on rear gear ratio and tire size was only very accurate the first 10 years or so of these vehicles' life. The NEWEST thirdgen is now 32 years old. Any car that uses a magnet in the speedometer (such as the OP) is eventually affected by the degradation of the magnet itself, causing a drop in speed as shown on the speedometer face. This is inevitable, as all magnets degrade at different rates. you can take two speedometers from the same make/year car and both can degrade at a different rate. This is why a pack of speedo gears will get you where you want to be.

OP, the only OTHER reason I can think of for a sudden change (other than speedometer swap) is the cable not fully inserted and literally slipping somewhere. If so, it probably won't be long before something is worn out and complete and total failure occurs.

 

I wonder if I didn't get the cable "all the way" in. I tucked it out of the way when I was pulling the cluster, so that when I test-fitted it the cable would not automatically connect. Once I was ready to connect it, that was a pain to get lined up again.
That could mean trying to take it all out again, pulling on the cluster or may just push more on the cable from under the hood?
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