[Ogre]

After searching the Tech and 70-72 GTO/Tempest/Lemans Tech forums, only a few threads were found which discuss the specifics of odometer (odo) repair/adjust. So I am posting this here to get some feedback from anybody with knowledge of the engineering/production of the odos in our cars.

Cars are: '72 Lemans Sport Vert & '72 Lemans HT. Odos are most likely in all '70-'72 GTO/Lemans/Tempests. I've been in several odometers and their construction is a little confusing (discussed later), but they are easily cleaned up to get them working again.

Recently my vert began to appear to be dragging with slight clicking sound. Upon opening it, I discovered a new "part". The speedo/odo appear to be a virgin, so I am very confident that it has never been molested (unlike some others). The part is shown in the first two posts below, w/broken off missing center piece.

It is a semi-hard, off-white plastic (50+ yrs old) which is installed on the silver "legs" of the odo cylinders to keep them together. The silver "legs" are split to fit over a bar below the numbers (again appearing to be to keep the correct {once set} numbers together). The third pic shows the bar (with open area on left) where the "legs" are fastened, and the 4th pic is my attempt to duplicate the "part" with mixed results.

Does anybody have knowledge of the engineering design/manufacture of the odos to try and clear up some of my questions? I would certainly appreciate any ideas.

TIA,
Ogre

[gtospieg]

Try to contact Pete Serio on this forum. He rebuilds these for all years.... Pontiacs.

[Ogre]

Originally Posted by gtospieg

Try to contact Pete Serio on this forum. He rebuilds these for all years.... Pontiacs.

Thanks gtospieg ... I'll touch base with Pete. I've seen some of his work, and he is great!

BTW, for those interested, I finally found a video which probably better explains the small plastic "guide" than my pics in the 1st post. The video is here: https://www.youtube.com/watch?v=EKbW8jC5w48. About 6:10 in the video he explains the plastic part and then it crumbles and falls apart.

I have made a couple from different thicknesses of plastic with my exacto knife and scissors, and it appears that they will work.

My ONLY reason for posting this was the design of the bar that hold 'some' of the silver legs is not a complete piece and does not hold two of the legs. You can see the bar in the video. WHY is this designed ? There must be a reason, but I can't come up with a logical reason to NOT make a complete bar, and then not have to rely on the plastic guide.

Just call it my "anal" mind worrying about something that doesn't matter. And to wonder why an engineer would design it that way.

Ogre

[Peter Serio]

The sole purpose for that plastic strip and the notch in the guide is to alert a possible buyer of a Used GM car, perhaps some years into the future that someone, at a past point in time had tried to roll back the miles on the car.

In the 1960s and 1970s there was a magazine called consumer reports and in there people would write in letters. That magazine received, many letters which told stories of buying a 6 or 7 year old used car and later discovering the car actually had more miles on it than what the odometer reading indicated.

So GM decided to try and help let people know that the odometer was true to the car, very much the way CARFAX® works modern day. In the magazine they wrote for people to look very closely at the numbers to see that they were all lined up in a row. The plastic piece was made to be brittle from when it was brand new so if a used car lot tried to roll off, say 5 or 10,000 miles on one of their "for sale" cars the plastic strip cracks and the second # does not line up with all of the rest. That is the reason for the notch in the steel bar on the backside.

It would serve little purpose to roll off a couple of hundred miles on a used car, GM figured, so they thought it would be at least one of the far left hand side digits!

The tamper-proof odometers started in new cars first introduced as a new feature on all of the 1969 models and into the future until GM stopped using plastic numbered (painted) wheels to count total miles on their cars.

[Ogre]

Excellent info Peter. I never advance my thinking to the "cheating" stage, so that's why I couldn't figure out the design.

Now it makes perfect sense for the GM engineers to design it that way, and the reason for the two wheels with 10,000 and 1,000 are not anchored except for the plastic piece.

The electronic digital odos of today are probably designed to totally FAIL if ever tampered with. LOL

Thanks Again,
Ogre

[mzbk2l]

Interesting the way the automakers used to try to foil thieves. I remember the screws that hold the ignition cylinder into a 280z were made so the heads broke off when properly torqued, preventing anyone from unscrewing them and sliding the key cylinder out.

I'm not familiar with reprogramming digital odometers, but when I've priced new IPs on ebay to replace one with burned out LEDs, some of the junkyards advertised that they would program the odometer reading to match your existing IP. I'm sure nobody takes advantage of that ability!

[Ogre]

Pete if you're still out there, I have another question.

The speedometer reading seems to be totally based on the spring tension. The lever around the spring can be moved to a different position to change the "reading" only. What do you use to set the speedo for proper reading without putting it in the dash and driving with a GPS, then taking it out to adjust, then, then ... over and over?

As I was cleaning mine up, I noticed a shiny area where the lever was originally, so I set it there.

Thanks for all of your knowledge Pete.

Ogre

[Scott Thelander]

2 different speedos in 72 also
early n late
seat belt light
curcuit board is different also

[Ogre]

Yes Scott, I have one of each. My Vert is early ... no seat belt & 140 mph. My HT is late ... seat belt light & 120 mph. And circuit boards are definitely different. I have the electric page with diagrams from the service manual to make sure which is which.

I'm trying to make sure I don't mix up the correct parts for each. The vert has a set mileage and the HT (bought as a shell w/no motor or trans) will reset to zero, since I don't have the actual mileage.

The innards are so close that I can't tell them apart, so I keep them on opposite sides of my work bench.

Thanks,
Ogre

[Ogre]

Let me make a few corrections before I continue this "chasing-my-tail" exercise.

First to Scott ... I thought I had the diagram for the different 120/seat belt vs. 140/no seat belt. I don't! The diagram I was referencing is diagram 12-39 and 12-40 with difference between the rally gauge vs. the non-rally gauge. A copy is below. Interesting that my hard copy and digital copies of 1972 Service Manual do not show the printed circuit for 120/seat version. And my 1972 Fisher body manual has no references.

Second, the two odometers I am working with are NOT from my vert 140 speedo. Many years ago, I bought a busted up gauge set from a junkyard just to have some spare parts, and it is also a 120/seat like my HT.

After several years of rotator cuff surgeries, spine fusion, both knees replaced, and much more, I am finally getting started again, and I confused myself about what I was working on. My apologies for the confusion, guys.

I am still interested in adjusting the speedometer if Pete can help. The speedo unit 120/seat/no rally needs to go back in the shell of my HT after a lot of welding, cutting and bodywork.

So back to work on stuff I understand.

Thanks for all of the comments,
Ogre

[Peter Serio]

The calibration is set by the spring tension of the return spring, and the magnet strength. At the very minimum I make use of 3 tools to check that magnet and adjust; more magnetic pull or less, as required.

One is a 24 volt DC tool that magnetizes. That one you use first. You actually over-mag the iron so that it reads much higher than true (on the dial). This is the most important first step as you are giving the iron it's (hopefully) long lasting "memory."

Second is a tester made by Stewart Warner (although there are other brands) I like my Stewart Warner, I actually have 2 of them that are identical (one is a back-up).
You can spin the cable on the worktable at various RPMs which correspond to MPH on the dial. The formula for a GTO speedometer is: 500 RPM at the cable = 30 MPH. 1,000 RPM at the cable = 60 MPH and 1,500 RPM at the cable = 90 MPH.

The last tool you need is one that can demagnetize, that one is powered by 120 volts AC.

You run the speedometer at 1,000 or 1,500 RPM and remove some of the strength using the 120 volt AC powered tool, you also adjust the return spring, a little bit at a time. It's tedious work and requires good eyesight as well as an understanding of weights as the speed cup can be adjusted at a few places relative to it's weight. The speed cup (aluminum) and the pointer are viewed as 1 object as far as what the magnet "sees." As work I visualize the circle as the dial of a clock. You have to be able to think in several dimensions at once.

At the end of your spring adjustments you want to be 100% sure that the pointer still returns to zero.

The interior of the gauge housing needs to be a 100% lint, dust and dirt free environment. As they can all affect the speedometer, light touch surfaces and the metals. You have cast potmetal, an oilite busing, aluminum, steel and copper. The speedometers also have a few molded plastic parts, (the odometer gears) which will wear over time.

Originally Posted by Ogre

Pete if you're still out there, I have another question.

The speedometer reading seems to be totally based on the spring tension. The lever around the spring can be moved to a different position to change the "reading" only. What do you use to set the speedo for proper reading without putting it in the dash and driving with a GPS, then taking it out to adjust, then, then ... over and over?

As I was cleaning mine up, I noticed a shiny area where the lever was originally, so I set it there.

Thanks for all of your knowledge Pete.

Ogre

[gtospieg]

Ogre...I had Pete Serio service my speedo when I built my 67 GTO (finished in 2013) and 19,000 miles later it is spot on. If you don't possess the proper tools for this I highly recommend you let Pete Serio do the work. You wont be disappointed.