Need some speedo help

[Dutch Rutter]

Hey all,

I just took apart my dash to replace the lights. That all went well, except this morning with everything back together my speedo is off by +10 mph I believe constantly. I feel that everything went back together like it was supposed to but maybe I had the needle off "0" when I put the cable back in? I'm not very certain.

Before I pulled everything apart it was spot on, so I don't think it is a gear issue.

 

[1987 GMC Jimmy]

Hey all,

I just took apart my dash to replace the lights. That all went well, except this morning with everything back together my speedo is off by +10 mph I believe constantly. I feel that everything went back together like it was supposed to but maybe I had the needle off "0" when I put the cable back in? I'm not very certain.

Before I pulled everything apart it was spot on, so I don't think it is a gear issue.

If it's starting out incorrect at like 10 mph, you can carefully pull the needle off and reattach it pointing to zero. If you need to pull the speedo needle, the best way to do it is with a fork or a golf divot thingy. I use a fork, though.

 

[Dutch Rutter]

If it's starting out incorrect at like 10 mph, you can carefully pull the needle off and reattach it pointing to zero. If you need to pull the speedo needle, the best way to do it is with a fork or a golf divot thingy. I use a fork, though.

Thanks for the reply. that is really starting to look appealing to me. So I pulled it back apart this evening. What I found is that sitting at a light (complete stop) the speedo sat at 9-10 MPH. Now with it apart and the cable pulled the gauge sits at 0. I put it all back together and now.... it does the same damn thing

This might be a dumb idea, but there is a lil white line under 0 just curious, do I need to hold the needle down to that point while I plug the cable in?

 

[Georgeb]

Just a thought..... If it sits correctly disconnected the cable might be pushing on the back of the speeo when you reassemble it. Try spraying some silicone or wd40 on the gommet where the cable comes through the fire wall. That way it slides easily and is less likely to push back when you put the cluster back in position.

 

[Dutch Rutter]

Just a thought..... If it sits correctly disconnected the cable might be pushing on the back of the speeo when you reassemble it. Try spraying some silicone or wd40 on the gommet where the cable comes through the fire wall. That way it slides easily and is less likely to push back when you put the cluster back in position.

There is another great idea, and I don't have to pull the dash apart again to try it. I did notice that the upper left corner of the gauge did not simple set in place it did take some effort. I'll give that a try tomorrow after work.

Thanks guys for the help

 

[chengny]

The cable and the needle never make actual mechanical contact. The cable drives a cupped (it has side walls) metal disk, called the "speed cup". Another cupped disk (facing the opposite direction) is mounted so that it's wall fits inside the outer wall of the speed cup. The shaft that drives the indicating needle is connected to this second cup.

The gap between the speed cup and the other cup is small. Small enough that when the speed cup is spinning - and producing a rotating magnetic field - the needle drive cup is dragged along with it:

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Also attached to the driven cup is a hairspring. The hairspring has two basic functions:

1. It keeps the driven cup's degree of rotation in check - so that upon rapid speed increases, the needle's motion is smooth and steady.

2. When the speed cup (and the truck) begins to slow down, the hair spring returns the needle to its at rest position.

I think your hair spring must have gotten streched somehow and now needs to be tightened a little bit. Tightening will cause the needle's rest position to move CCW - towards the 0 mph mark. At least I thought that was the problem, until I read this:

What I found is that sitting at a light (complete stop) the speedo sat at 9-10 MPH. Now with it apart and the cable pulled the gauge sits at 0. I put it all back together and now.... it does the same damn thing

 

[Dutch Rutter]

Little update.

I pulled another speedo out of my 86 C20 and put it in my 83 K20 (my DD that is having issues) and with this one it reads 10 mph slow. example: gauge shows 55 actual speed is 65. Before it was the other way. It showed 55 and I was going 45.

Now I plan on taking the original speedo apart and checking the spring and everything else out. Or is there something that I can change on the 2wd one to adjust it for the K20?

 

[Rusty Nail]

Dude.
I thought you were changing lightbulbs?
How in the world did you manage to get two trucks torn apart in that process?

Better yet, how's the lighting comin along?

 

[Dutch Rutter]

Dude.
I thought you were changing lightbulbs?
How in the world did you manage to get two trucks torn apart in that process?

Better yet, how's the lighting comin along?

The lights are great!! I ended up getting some cheapos off amazon instead of superbrightleds. They work well just don't dim and some didn't work at all but a pack of 30 for less then $10, it was worth it.

This gauge is kicking my ass tho. Its strange that all I did was pull it then it started acting strange. Now after pulling it so many times I'm sure the spring is messed up. Can I order a replacement one or would I be better off trying to get the one from my parts truck to read correctly?

I just ordered a new cable. Figure I haven't tried that yet so might as well.

 

[Rusty Nail]

Well, you have rousted Jerry so all hope is not lost! I'm paying attention to get some knowledge before I tear into mine when it warms up.

Had you just topped it out or a ythong like that? Kinda weird to mess around like that for no reason...Are you telling the whole story? How long to get to 85?

 

[Georgeb]

I find it hard to believe removing and reinstalling the cluster to change some bulbs would result in the speedo being messed up....

 

[Dutch Rutter]

@ rusty If I were to do it over again, I would have left the speedo alone (assuming that it reads correctly) and just pull the fuel gauge and get at the lights that way. I havent topped this one out but the c20 i might have a few times, that was my DD during my younger and braver days. With the built up motor and the super low gears that were in, it would do 85+ in a hurry.

@ george that is exactly my thoughts as well. There should not have been any reason for it to act up. However after pulling it, replacing the lights and putting it back it went from being very accurate to reading 5mph before moving, then when barely starting to move would jump up to 10 and increase at a normal rate but would read 10 more then my actual speed. After moving and stopping again it would sit at 10 and just increase from there. I'm not sure if the PO possibly moved the needle or not. Seeing how his other fix for a broken vac line was to hollow out a ball point pen and stick the lines over each end I wouldn't doubt that it was a "custom bent" align job for the speedo to match the 33"s. But I honestly have no idea.

I did see a good deal for a 100 mph gauge on Fleebay that looks inviting but i have no idea if that will actually solve the problem or just add to it. I can only assume that the gauge I pulled from my c20 is accurate and its the one reading -10mph but acts correctly everywhere else, so once my new cable comes in, and I lube it up real good we'll see what happens. If all else fails... Throw money at it, right?