I hope someone can help.

I just bought a replacement 145 speedo off ebay because the needle assembly on mine broke. My problem is that the speed is way off when compared to my GPS. It definately feels off too. The car is a 1986 IROC-Z and originally had a 145 speedo so this is not an upgrade. I have the stock tranny, gears, and tire size. I am the 2nd owner and I have had the car for 15 years and I know everything is original. It always seemed to work before the old one broke.

The speedometer has a warranty, and the seller says it had a 0 -60 accuracy check done. Before I contact the seller I would like some advice on what else might be the problem. I don't want to complain to the seller if it might be something else. The car sat for like 6 years and I just started driving it again. Could the cable be bad cable to the speedometer? At 25mph it is off like 5 mph, at 45 it is off like 10 mph, and at 55 it is off around 12 mph.

I would appreciate anyones help asap as I have 3 days to return it if it is bad.

Thanks, Steve

 

there are stickies (2 to be exact) by Atilla the Fun in this forum topic. They show what you can replace to make it accurate.

 

Perhaps some ideas here:

https://www.thirdgen.org/forums/inte...eedo-woes.html

JamesC

 

Thanks guys.

James I read through the link you supplied. I will pull the cluster back off, YAY, and try cleaning the cable to see if this fixes it. I only have a couple days to send the speedo back if one might think it is bad. I will not be able to get to the cluster for awhile since I just seperated my shoulder. Really wish I thought of cleaning the cable first.

I guess my main question is, what could be bad on the speedo? It is more than likely one of the other problems as mentioned in the link provided? I am not sure what can go bad other than the spring maybe attached to the needle. I did not receive a filter on the speedo, I just reused my old one. Not really sure what this does. The seller did say it had an accuracy check done and it did come with a warranty, so I don't want to send back a perfectly good speedo. What are the chances it is the speedo?.....

Thanks

 

James,

I also took your advice from one of your post's on Xenon lights from Macewen. I orderd some and changed them out when the cluster was out.

What a difference. Thanks!

 

Find a straight highway and do 15 or 30 miles *of milemarkers* @ 60mph per the speedo.
If it doesn't take 15 or 30 minutes respectively, the speedo is off.
Also, compare the odometer to the milemarkers.

If your GPS and the road test both say the speedo is off, it's off and you should return it.

It's possible for only the spring to be bad like you said in which case the odo should be spot on but the speedo be off.

Some sellers bluff to sell parts.

 

I thought about doing that, not sure how fast I will be going at 60. Also according to the GPS I went 2 miles further than what my speedo said I went. I reset them both when I took it for a drive last night. I went 13.2 miles according to my GPS, and 11.2 off the speedo.

I take it that the odometer not directly driven off what the speedometer reads. So maybe if the odometer is off, and that is driven by the gears in the speedometer assembly, which is driven by the cable to the transmission, maybe the cable or something else is bad?

 

Right. The speedo cable spins the input shaft which has a gear built into it and flying magnets.

That gear (looks kind of like the gov. gear on the output of a 700-R4) spins gears that turn the odometer cluster(s). So the odo should be dead-on.

The shaft's flying magnets turn a magnet cup which turns the speedo needle spindle against the hair-spring.

I had a trip-odo cluster lock up in a GM speedo. The main odo was fine. Replaced the trip-odo and went on a long trip. Odo was less than 1/10mi. off after 30mi. w/ 3.73/ 235-60R15/ 15 & 45 speedo gears.

IMNVHO, your speedo was checked to see if it "worked" without any concern for accuracy.

 

Maybe it was only checked to see if it worked. Here is how it was listed on ebay....

This why I don't like buying things sight unseen. But, I bought this one because it had a warranty, and the seller has a 100% positive feedback. I am going to contact the seller and see what he says.

Am I to believe that if the odometer is off, that it might be my cable that is bad. I switched the actual odometer that had been rolled to 0, to my original mileaged one, without taking off the needle. The trip odometer does work also.

I am going to take it for another test drive now and see what I can come up with.

Thanks

 

Ok. I did some testing and this is what I found. I compared the GPS with my company truck and it was the same. So i believe the GPS is right.

60MPH on GPS = 50mph on speedo.
60mph on speedo = 72mph on GPS.
11.1 miles on GPS = 9.3 on car.

I also determined that the ratio between the speedometer and odometer in the car is spot on when figuring for MPH. I timed a fixed distance and calculated for MPH and it came out to within 1 mph of what my speedo was reading.

Now I know that the speedo reads slow, but does this sound more and more like a cable problem? I am awaiting a reply from the seller who seems really reputable. I don't know, anymore ideas?

 

A cable problem might cause up & down wavering, not a consistent % amount off.

What broke on the old one?

You could clip just about any 80s GM cable driven speedo into your car to see if it matches the GPS.

 

I hope you get this resolved- seems strange that it was all fine before the new speedo. Update us on how it went as mine is also under reading.

 

well it does not waiver up and down. The needle is definately consistant and steady while driving at various speeds. The reason I had to change it is because about 6 years ago I tried to put white face guages on and attempted to remove the needle on the speedometer. I broke the needle and the pin to which it attaches. Also over the years of it being out of my car I lost one of the gears in that drive the odometer.

Let me ask this. Is it possible that someone put a different face plate on the assembly? I would think it would read really high if this were the case.

I just don't know what could be wrong? Maybe the spring on the speedo is stretched out or something from too many hot suppers, or the magnets that drive it are bad. I guess I will have to wait to hear back from the seller. I just don't want to send it back so I can buy another one and have it do the same thing.

I will definately keep you guys updated. hopefully I can figure this thing out. Keep throwing ideas out here as I am up for trying anything now. Maybe I will take the cable out of the tranny just to make sure on the gearing.

Thanks for the help so far.

 

faceplate swap wouldn't cause the odo to be off.

You really don't have a pedigree of any sort to indicate that the speedo is correct so it's not really worth keeping, even if you needed parts to repair yours. Sometimes the magnet cup & needle spindle are the same PN between an 85 and a higher speedo. I'll have to take a look.

 

I didn't think a faceplate swap would act like that, I was just throwing out ideas.

I just heard back form the seller and he apologized for the inconvenience. He is willing to take it back, but I want to do some more tests first.

Are you definately thinking it is the speedometer and nothing else?

 

The only other thing it could be is speedo gears but that would be all too peculiar since they were correct for you for such a long time.

All I can say is having an 86 and an 87 makes it easy to swap parts and figure out what's causing the problem.

I'm only going off of what you said in your first post. I've owned my cars similar amounts of time and know what you mean.

Find a 1980 something GM speedo you know is good, clip it in and go for a drive. That's the only test you need to do at this point.

 

I'm posting this not to suggest you try and find the parts new but to show that you could probably gut another speedo and fix yours *if* your original was calibrated correctly.

The spindle & speed cup assy. (the piece you broke w/ the needle) is the same between the Berl, and the 85 and the 145. (there's one on eBay now but you still need a needle. I have none of that in my box-o-parts) The 1st worm/magnet assy. is probably the same for the 85mph. The hairspring is unique for each. They did basically the same thing to calibrate the 85/120 speedos in the montes for '87, which is why I knew this crap.

So again, I'd toss in any old 1980something mech. speedo from a GM car and check it against your GPS. If it reads with the GPS, send the speedo back. If it is off too, then you need to dig into the speedo gears.

 

Thanks JMD. I did just find out from the seller that it has a 90 day warranty rather than a 30 day. So this gives me some extra time to do the testing I need this weekend. He is also thinking that maybe it is calibrated wrong. I am going to try and do what you said.

The gear in the trannsmission is directly related to the gearing in the rearend and not specific to the speedo right?

Thanks.

 

Yes, it's related to the rear in that GM speedo gear setups left the factory to spin the speedo cable 1001rpm @ 60mph.

 

Good information thanks for that. I will be doing some testing today.

 

Any updates on this? I reckon my 145mph speedo has under-read since I've had the car. How is this possible with no drivetrain changes?

 

I don't have any updates from my original trouble. I haven't had much time to work on it. The only thing I did find out is I timed 1 mile according to the tripometer, at 60 mph, and got 1 minute. Which to me tells me the speedo is calibrated. If you go by the actual speedo reading, it seems to be correct, even though it reads slower than my actual speed.

The next step for me is to do the cleaning of the cable, and check the gears in the tranny, just to make sure. I will update what I find out.

 

In the T5 case thread, you mentioned a NWC (non-original) T5 so how are there no drivetrain changes? Start a new thread with speedo gear tooth counts / rear gear ratio / tyre size.

 

jmd- you should be a private investigator

Well, when I still had my 700r4, my speedo under-read. This was verified by the police, whom I overtook at an indicated 50mph in a 50 zone. He pulled me over and told me I was doing 70mph! That was when my suspicions were first raised. I then changed trannies with the T5 and it reads about the same.

I know for a fact that my car would not have originally come with a 145mph speedo so wonder whether the speedo is actually faulty.

Cheers

 

Update: Anymore help or ideas would be appreciated. I am putting the car in storage for the winter this weekend. I wont have access to it for 5 months and was hoping to have some more ideas before I do.

So I sent the first replacement speedo back, and bought another used cluster. This one has the exact same results as before. The speedometer reads slow and progressively gets worse the faster you go. I tried interchanging the VSS on back of speedo with my original and the one that came with the cluster. No change, and I don't believe the VSS affects the speedo reading anyway. The seller of the first cluster retested it and said it tested fine. So at this point, I am pretty sure it is something with my car. And let me tell you, I am getting really good at changing clusters!

Before I put this one in, I cleaned and lubed the cable, and made sure it was seated in the transmission properly. I would think that if it was a some sort of gearing issue in the transmission, it would be consistently off by the same amount no matter how fast you go.

Again, I am the 2nd owner for 15 years now. Stock 700r4 transmission, gears, rear end, tire size. I do however have a bigger stall converter. Could this potentially be the problem?

Thanks. Is the transmission board the best place for this? Maybe should be posted elsewhere also?

 

Did you change out the torque convertor? Wouldnt be the first time a Speedo gear fell on the floor, got kicked across the shop, and was replaced with whatever the guy found in his toolbox.....

What size tires and what ratio rear end do you have?

have you double checked that the gear is right?

 

Yeah, I put a new torque converter in with a higher stall. I had the motor and transmission out of the car for like 6 years. Isn't the speedo gear in the tail shaft? Never really had the transmission apart, with the exception of a corvette servo.

The tire size is the original 245/50/16. Rear end is original 3:23 as far as I know. I have never done any gear work on it, and don't believe the previous owner has. It always worked properly before the motor was apart and put back together. Like I said before, it sat for 6 years in my garage with no drive train in it, but original motor and tranny was put back in.

 

EDIT (the following info is for a T-5): If the tires are OE sized, I'd verify the gear ratio (42 : 13 = 3.23 (RPO GU5) / Approx. RPM’s at 70 mph--2133) and then the drive and driven gears (3.23's require a blue 7-tooth drive gear and a brown 18 tooth driven gear.

JamesC

 

Thanks as always James. You're right I will have to verify this to be absolutely sure. It is just baffling that it used to work perfectly and now it is so far off. So if the gearing happens to be wrong, will this coincide that the speedo is farther off the faster you go?

Thanks, Steve

 

I verified my rear end gears according to my build sheet, and it is a GU5/3:23. Still need to check the car to make sure.

I was reading through the transmission gear stickies. It looks like the drive/driven gears are different in the stickies than what are mentioned. Am I not getting something here? It says for my specs to use a 38:15 or 43:17(driven/drive).

 

My bad. I confused you and ZZ42, who has a T-5 (which has a different tooth count than the 700R4). Edited above.

JamesC

 

Is there a fast and easy way to verify what drive/driven gears are in my 700r4? I thought I thought I saw something on here once, but can only seem to find how to replace them.

Thanks.

 

remove the retainer screw & clip. Pop driven speedo gear & sleeve out. Count those while they're out; look into the tailhousing to see the drive gear. Colour is not a reliable way to count the drive gear teeth inside, but a sharpie and the trans. in neutral / rear on jackstands should do it and you're only probably going to have a 15 or 17 tooth, though there are others.

 

Thanks JMD! I actually just took my car to storage and won't get it back until April. I didn't have a chance to do this, but it will definitely be my first priority come April. OH well, I will pick this topic back up in the spring when my other garage is done and I am able to work on it. Damn, I wish I could have gotten my other garage built in time. I hate cold weather!!!!