Speedometer Repair

About 6 months ago I pulled out the old truck and went for a drive.  Suddenly I heard a strange noise in the driver side wheel area.  It sounded like a rock stuck in the tread then it sounded like it was hitting the fender. After pulling over I looked and found nothing.  I started up again and the noise returned.  This time the noise seemed to be coming from the speedometer area.  In addition to the noise I noticed the needle was bouncing and not reading anywhere correct.  When I stopped it also would no longer return to zero. 

Removal of the gauge panels are fairly simple, 6 phillips style screws.  The challenging part is how to remove each panel without breaking a wire or scratching things up.  I found that if I remove the right side one first and pullout the light socket that goes to the blinker I can then twist it so it lays flat in inside the dash.  This allows me the ability to reach behind the left panel and remove the high beam light socket and unscrew the speedometer cable which allows the panel to move more freely.  Then with careful twisting, and taking care to not bend the copper oil pressure tube you can gain enough access to the metal retainer plates that hold the speedometer in place.  There are two metal plates and four screws.  Start by loosening the screw farthest from you, but don't remove it.  The metal plates have slots for each screw so you will be able to slide it out once you remove the screw closest to you.  Make sure you are not putting pressure on the gauge during this procedure as once released from the metal plates it can easily pop out and go slithering into the open dash area or worse onto the floorboard and break the glass.

I drove this summer without a speedometer gauge as I pondered what to do as well as searching on YouTube and various GMC blog sites to find potential answers.  Surprisingly I was limited.  One gentleman in Oregon called Elderly Iron has a video which helped me learn a few good things.  However the opening and closing of the metal case was the most daunting part.  I also found another person on a different model speedometer which might have inadvertently helped me as well.

We can break this down into three basic steps.  1. how to open and disassemble the speedometer without breaking it.  2. inspect and fix. 3. reassemble.   I still had no idea what was wrong with the speedometer, if parts are available, and what a replacement/repair would cost.  However I did find that it would cost me a min of $200 to have the speedometer looked at plus more depending on what was wrong through a speedometer repair shop recommended through the old GMC discussion board. 

 

Disassembly

The Elderly Iron video shows him trying flat blade screwdrivers, a rounded chisel, and a paint can opener, in various configurations to "bend" the crimped lip of the front metal ring of the speedometer out so the main body housing and the front ring can be separated. It seemed like a very rough and messy operation.  So just like him I used the tools I have in my tool box.  One of those is a small pair of carpenter nippers.  Like magic they effortlessly allowed me to unbend the crimped lip.  The front ring also has several indents or steps that allow a perfect hold for the nippers on the lower jaw.  The curved edge of these small nippers also does not rest against the green glass that surrounds the housing for light to shine through.  Your motion to open that crimp is also away from the glass so no concern about breaking it.

Now you can remove the ring from the main body.  Depending on the ease of removal you should note that the ring is followed by the glass, then a black thin gasket then a black metal ring then another gasket.  When I pulled the outer metal ring the glass followed suit.  Then using a metal razor blade I was able to insert it between the metal housing and the last gasket and simply "break" the seal from the sandwich of parts without any damage.

At this point the needle is exposed SO BE VERY CAREFUL.  So now I placed the face down on a soft rubber mat to prevent slipping and removed the two straight slot screws in the cable end.  This allows the speedometer mechanism to slide right out.  I recommend that you take the last screw out with the speedometer facing upright so nothing can slide out from the housing.   AGAIN BE VERY CAREFUL WITH THE NEEDLE.  The face and needle may or may not be sensitive to fingerprints it depends on the age and condition of the speedometer, but be aware.  

Needle removal is actually very easy.  The needle is attached to a thin metal rod, and there are no threads or special attachments you need to remove.  The metal rod connects to the back of a metal cup, called the speedcup.  There is another metal housing over that speedcup to hold it in place.  Now that housing and the back of the metal cup both have protrusions that work together to provide a physical stop. So Zero on the dial is one stop and if you continue to rotate the metal rod clockwise far enough it will hit the other side of the stop, or waaaay past the fastest speed your truck could ever go, even over a cliff.    This is all important because if you use the stop to your advantage you can simply rotate the needle counterclockwise so that it hits the zero stop and then keep going.  The stop holds the speedcup from spinning which in turn is connected to the metal rod which is connected to the needle.  So as you continue rotating the needle it will loosen from the metal rod very easily with no drama.   Use the side of your finger close to center for some leverage, NOT the far end of the needle.  I'm not sure if this is a bad idea but it worked for me by accident and gave me control later when I reassembled it to zero out. This will not work if any part of your metal stop is broken. Which means you know what your problem might be.  Another approach I saw was to take a piece of thick paper and make a cut from the edge.  Use that slit to slide between the needle and the dial with the metal rod in the middle.  This protects the face.  Then using a fork place the tines under the needle and leverage it up to pull up on the needle to remove it.  Both methods will create tension on the metal rod but I liked the "feel" I had with over rotating the needle.  It allowed me to get a sense of how stuck it might be. Also with the fork method I was concerned about touching the needle or letting it fall down.  My needle was very sensitive to any rubbing and would lose paint quickly.

At this point I was feeling alot more confident as the hard parts were done.  In my mind. There are two small straight slot screws holding the face to a metal flat bar.  Mine was rusty and as suspected did not move with a screwdriver.  I found that using the nippers again was perfect or a pair of vise-grips to slowly turn the screw.  I did not want to spray WD40 or anything else that might dissolve the rust and the face dial. The image shown is from my junk speedometer that had a missing needle and glass.

With the face dial out of the way, you can now unscrew the two screws on the metal flat bar.  Be gentle as the metal rod goes through the middle of the flat bar and may or may not have a fragile tiiiiny washer on the backside.  Mine broke when I tried to slide it down a bit.

Turn your attention to the odometer next.  On the left side of the odometer (non gear side) is a flat plate.  Using a jewelry screwdriver you can slide underneath the flat part and pull it straight up which is used to secure that metal shaft end of the odometer.  Again the numbers on the odometer might be fragile so try not to tough the dials.  You will notice BEFORE removing the dials that the bottom has five metal connections that straddle a thin metal piece.  When you reassemble you need to make sure those line up.  Now slide the odometer to the left until the gear end is released from the shaft hole on the right side and then pull up and out sliding now to the right.  Carefully set aside.

Pay attention to the orientation of the gears along one side of the chassis and in fact photos might help as you go, just in case. There are two fairly robust screws to remove the speedcup housing but before you take them out, let me explain what is underneath.  The speedcup has no physical connection to the speedometer cable.  That's right the odometer and the various gears have no effect as to the speedometer.  Two separate items in a single housing. The speedcup is rotated based on magnetic eddies created from a magnet that spins via the speedometer cable.  The mechanism to force the needle back to zero is a weak clock spring that provides enough resistance once the magnetic eddies stop to return the needle to the metal stop found in the housing.  So when you take off the speedcup housing the speedcup will come with it, but might slide down some on the metal rod which might make it fall away from the metal stop and rotate because of the clock spring force.  So now you are facing a less than strong clock spring from the original settings.  That means you might want to turn the whole thing upside down to keep the metal stop and speedcup in the correct position or go through trial and error to find the right setting.  Which is what I had to do. Images are from the old speedometer so the speedcup shows lots of corrosion.

You should now see the part which spins via the speedometer cable  and looks like a squashed letter "U"  This is what creates the magnetic eddies.  I was not sure how to remove this part and to be honest was not looking to.  However I assumed that there had to be be a way to oil or clean it.  Sure enough on the outside of the chassis near the threads for the cable is a small round indent.  That is actually an oil

port.  Use a dental pick or a small screwdriver to slowly put pressure along the inside of the cup and up.  Go back and forth on each edge until it comes out.  Make sure you are slow so that it doesn't want to fly out and hide down by a wheel of a lawn mover covered in grass and dirt.  Don't ask. Inside is a foam plug.  Again use a needle or small nail to work out the plug and you will see the inside of the mechanism.  I added a few drops of sewing machine oil and put the plug back in and the small port cup.  The worm drive is kinda hidden under the "U" and blocked by another gear.  You might have some grease or it might be bone dry.  Mine has grease.  On the other end of that gear is another gear that is held in place by a small round aluminum disk.  Using your nippers again you can easily grasp it and pull it straight out.  This allows the gear to come out. 

 

 

 

 

 

 

 

Inspection

As I have been disassembling the gauge I of course am looking for the smoking gun.  A broken gear, broken speedcup, metal rod missing, etc.  As I mentioned before the actual speedometer portion is fairly simple and so there is only a handful of parts to its success.  I found NOS speedcups online via ebay but could not find a single part number or identifier on mine.  I figured that if a part was broken or I did not know how to repair a part or replace it I would send the whole thing to the repair shop and let them deal with it!  For me there was no smoking gun.  I oiled a few parts and cleaned a few things but that was it.

Reassembly

At this point everything went back into in reverse order I decided to wind the clock spring one extra spin, because one of my issues was that it was not returning back to zero. So more tension on the clock spring seemed reasonable.  Before I put the metal housing back on, I had the wisdom to use my drill and put in a #2 square bit.  I did not know which direction the drill was to spin but knew that because of how the mechanism works I could not screw this up. I discovered later that when looking at the end of the drill it should spin clockwise.  If looking at the back of the speedometer housing/back of drill looking forward it will spin counter-clockwise.  I was at full rpm on the drill and got up to 20MPH on the speedometer dial.  That seemed right.  I never thought much about fast my drill spins.  

The last thing to do was put the housing and the front metal ring back together.  Again I discovered that using a metal punch or a small hammer is NOT the way to re-crimp the metal ring onto the housing.  It knocks debris inside the housing and sticks to the inside of the glass. It can also easily crack or damage the green glass windows if you are not careful.  Again don't ask.  I learned that using a pair of channel locks is again the perfect tool.  The bottom lip of the channel locks attaches to the outside metal ring and does not come in contact with the front clear glass and is eventually covered with the grey plastic trim piece when you install it in the dash panel so no worries about teeth marks.  The top part of the channel locks has a forward and down movement which allows for great control as you crimp the lip.  It is the perfect tool for this job. Notice again that the top jaws are curved and have a low profile so they do not interfere with the green glass.

Testing

I put the speedometer back into the truck and found that it did NOT work.  Nothing!  So I did some final research and found a guy doing his own speedometer repair to save $200+ dollars like me and he said that the repair shop told him that most often its grease on the speedcup, speedcup housing or "U" that causes the issue.  Apparently the setup is very sensitive.  

Disassembly Part 2

So back to the tools and opening the housing once again.  I found that I had a clean speedcup and housing.  Kinda wished I found something.  As a last thought with the comment about the speedcup is sensitive I decided that maybe I try different clock spring settings.  Then test with the drill again to see what happens.  Well low and behold I found that a less wound clock spring worked the best.  Now using my drill I could make that needle get up and go to normal ranges, even faster than I would probably ever feel safe driving the truck at full throttle.  

Reassembly Part 2

Many of the tips and suggestions were because of the second disassembly and things I learned.  Also I happen to have another speedometer that is missing glass and a needle that I practiced on.  So technically I've done this three times.  

Testing

It works!