The speedometer in the IROC has been a Pandora’s box. The first parts of it, were logical and lineal, the last parts have been anything but.
The original problem was simple, it seemed. It was constantly reading a lower speed than it should, while the odometer was reading significantly faster than it should.
I first verified and replaced all mechanical gears to ensure those systems were proper for the setup of the physical car. This removes all of the problems up to the speedometer itself.
Originally, I assumed I could just adjust the needle and resolve the problem, resulting in me breaking the needle. Little did I know then, but that was a fatal mistake. Breaking the needle, broke the shaft the whole assembly connects to, which includes a clip that holds that shaft in the correct place, a clock spring to take the needle back to zero and lower the speed when slowing down, and a “black dish” that caps the assembly. The black dish butts up to but does not connect to the actual assembly that connects to the speedometer cable. This works through an eddy current system to display speed and record distance traveled.
My original fix was to replace the needle and shaft. To keep as many original to the car as parts as possible, which has been my charge on the restoration. Over a couple of test drives with only the speedometer in the car, I learned that the “black dish” was not staying in the same position on the shaft, causing it to then make contact with the eddy current system.
I had purchased 2 complete clusters out of other Camaros to use as parts to rebuild this speedometer. Because these parts are rare and ever expensive (a 145mph speedometer alone on eBay is $254), I only used one of these clusters initially. Yesterday is when I made the realization about the black dish and eddy current system touching. The only logical reason why that black dish refuses to stay in position is due to the cheap and thin material it is made from. The removal of it from the shaft, changes it’s ability to keep it’s position on the shaft.
Last night, I grabbed that 2nd cluster and found some pleasant surprises. It was an 85mph speedometer, from a base model car. The only gauges it has are the speedometer and the fuel gauge. The rest of the items are warning or “idiot” lights as they are known. I decided to transplant the odometer reels, face plate and needle I want to use to this one.
The pleasant surprise was that the 85mph speedometer did not have a trip odometer. The clips that hold the trip odometer in place on the other two speedometers I have been using had broken “J clips” which hold the axles of the trip odometer assembly in place. Since the frame of this speedometer had never housed a trip odometer, these clips were in place and never used. The only thing I had to do was move a simple gear that operates the trip odometer with the regular odometer, which was easy. I didn’t have to make any repairs to the assembly for the trip odometer to function. I had used zip ties on the one I was using.
The last two evenings I have been up until past 2am working on this speedometer. I have not had an opportunity to test the functionality of this as of yet, but it has the best hope of all of the things I have done so far. The largest question on this attempt is that spring shown at the beginning of this post. In each one of the speedometers I have, the collar on the spring is a different color. I’m unsure if these springs have different rates, this could play in the accuracy of the speedometer. Especially at higher speeds.
I have learned a lot on this journey. There isn’t much documentation I can find on the speedometers of these cars. The largest thing is the Trip Odometer fix from the 3rd Gen Forums. With my faithful girlfriend by my side, often having serious, heated debate on the issues with different points of view on how to resolve each problem that was faced, I think we did it. I’m so happy I have someone who stands beside me and is always up for helping me on things neither of us fully understand. That itself is worth it’s weight in gold.
The test drives I have taken have brought up two issues that I need to remedy however. The air conditioning is not working. I will need to convert it to 134-a refrigerant. There seems to be a significant gas smell from driving, I believe this to be the charcoal canister needing rebuilt/refreshed. The car also died on me several times out of the blue with no warning, which could be associated with the charcoal canister.
UPDATE
This attempt did not give the results I were going for. Speed would not be indicated until I was going about 20mph, but would be accurate through about 40mph. Once I would reach highway speeds however, the speedometer would read much higher than I really was traveling. I’m leaving this image for evidence.
That said, I did finally reach indicator nirvana and the fix was quite easy. I changed the clock spring (pictured near the top of this post) to the one that was in the car originally. I had to make 2 slight adjustments to the needle position, and voila!
I have reinstalled the speedometer into the cluster. I just have to reinstall the entire assembly back into the car now and this wormhole will have been traversed. It’s been an adventure in learning about a subject I had no real knowledge on.