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Build A Gauge Tester

Discussion in 'The Tool Shed' started by timgr, Aug 7, 2020.

  1. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Medford Mass USA
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    Part 1: Background and Description
    Part 2: Building the Tester
    Part 3: Demonstration of Testing

    Build a Gauge Tester Part 1

    Testing the dash gauges has come up a few times here and on the Wagoneer forums. I look at the TSM and its obvious to me how to test using some clip leads and fixed resistors. The TSM was written for the dealership mechanics (called technicians today), and they had no qualms reaching into the dealerships' wallets by specifying a "Jeep Corporation Special Tool" for this task, regardless of how simple the tool might be.

    Here's the tool:

    upload_2020-8-7_10-12-58.png
    Impressive looking. Likely a hinged steel box with storage for the captured test leads. The clip in the lid likely holds an instruction card or pamphlet. Of course, it has to stand up to long, rough usage, and be understandable to even the greenest line mechanics.

    Pretty sure all the figures and values came from my digital copy of the 1975 TSM. I'm sure that the '70s models are all the same (the Wagoneers got a redesigned dash in the '80s which changes the gauges, but the CJs should be the same to the end). Don't assume though - compare to your TSM.

    Here's the gist of what this instrument does, from elsewhere in the TSM. For the temperature gauge, there are some standard resistance values that substitute for the sensor/sender immersed in the hot coolant:

    upload_2020-8-7_10-13-38.png
    For the fuel level, there are different resistance values:
    upload_2020-8-7_10-14-5.png

    Here's the gauges' circuit:

    GaugesCircuit (925 x 345).jpg
    The tester connects to the dash at points T1 and T2, with the associated "sender unit" disconnected.

    I thought I might just demonstrate these tests with clip leads and resistors. However, one could build the J-24538 tester rather easily, so that's what I've done.
     
    Last edited: Aug 8, 2020
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  2. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    Part 2:

    Here's the tester - I have to take a break from this ... back later.

    CompletedJ24538.jpg
     
  3. Aug 7, 2020
    Focker

    Focker That's a terrible idea...What time? Staff Member 2021 Sponsor 2020 Sponsor

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    Makes me want to make a box and knob with my 3D Printer.
     
  4. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    You could ...but the parts are cheap. The most expensive component is the switch, likely followed by the box. At work now - I'll post more when I get home.
     
  5. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    Ok, back from work - continuing part 2 about building the tester.

    You'll need to be able to solder electronic parts to build this tester. There's no circuit board - the few parts are hanging directly off the switch, in a style called "point to point" wiring.

    To build the tester you need six different resistance values: there are seven values to test and one value is shared by both fuel gauge and temperature gauge testing.

    {9, 13, 36, 73, 23, 10} is {270˚, 242˚, 171˚, 130˚ and Empty tank, Half tank, Full tank}

    Resistors come in standard values which include {10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82}. Then the series starts again and increases by 10, so 100, 120, etc. This is the "E12" series of resistor values which is in standard use and normally has a 10% tolerance (the value increases about 20% per step). That is, the actual value at room temperature can vary by as much as 10%, up or down. These values and tolerance are what you'd see in old radios and such with carbon composition resistors, for two reasons. First it was expensive to make a more precise part using the technology of the day, and second because in these uses it does not matter if the value is more precise than this.

    We will use modern metallized film resistors, which are both cheap at these values and have a much higher precision (1% tolerance).

    To get the in-between resistance values that we want, we can combine these inexpensive parts by placing them in series or in parallel. Resistors in series add. Resistor values in parallel average by the following formula:

    (R1 * R2) / (R1 + R2) = R

    You can try this with various values. Two identical resistors in parallel give half their value; that is, 100 paralleled with 100 is 50. Intuitively, consider when you put resistances in parallel, the current through them splits between the two paths. In our 100 paralleled with 100 example, the current splits in half and by Ohm's law V=IR that means half the voltage drop across each device. This is the same behavior you'd get from a 50 ohm resistor. When you have two different values in parallel, how the current splits depends on those specific values. Since the ends of the two devices are joined, the current will divide such that both devices have the same voltage drop. This behavior can be calculated by the formula above (which you can derive with a bit of algebra and Ohm's law).

    I look at the above list of resistances and see 23, and immediately I know that a 47 paralleled with another 47 is 23.5. Close enough. There's actually a better combination - here's a table I made in Excel:

    ResistorsExcel (986 x 274).jpg

    Look at 56 and 39 which gives 22.99 … closer than 23.5.

    Note also that 10 and 27 gives 7.3. So 100 and 270 gives 73.

    Fuel gauge: 73 = 100||270, 23 = 56||39, 10 = 10.
    Temperature gauge: 73 = 100||270, 36 = 56||100, 13 = 18||47, 9 = 18||18.

    You need one of the following: 270, 39, 10, 47.
    You need two of the following: 100, 56
    You need three of the following: 18

    If you want to make the Excel table above, here's the formula for a typical cell:

    ExcelFormula (361 x 105).jpg

    Just fill in across the top row with the E12 numbers, duplicate that in column A, type the formula into B2 and drag it across the page. Walla.

    Here are the parts I ordered from Mouser:
    PartsList (805 x 591).jpg

    I made the 23 ohm value from two 47s, because I ordered before I made the above Excel table. So no 39s. I ordered 10 of each; I have a big file box of resistors that I can put them in, and the price drops close to half (11c to 6c) at a quantity of 10.

    The first 6 items are 1/4 watt resistors that will give us the resistance values we need for the tester. 1/4 watt is the power dissipation rating, which determines how much current you can safely pass through the component without risking failure. I did not do any calculation here, but the current through the gauges is tiny and I'm assuming 1/4 watt will be plenty stout. The largest value resistor here is the most heat-generating part, and by Ohm's law P=I^2*R and that's 30 mA at 1/4 watt. No Problem. We could go with 1/8 watt parts, but they are more expensive by a few cents. These parts are "axial" ie, they have two wire leads coming from the body of the resistor. You will also find SMD, "Surface Mount Device" resistors. These are meant to be mounted directly to a circuit board with solder. Don't buy those.

    Item 7 is a rotary switch, 6 positions and two poles. $4.77 is cheap for a rotary switch. All of the resistors will be mounted on the switch. One pole will select between each resistor (or resistor pair) and the other pole will be tied together and used to anchor the other end of the resistors. www.mouser.com/ProductDetail/alpha-taiwan/sr2511f-0206-19f0b-e9-n-w-159/?qs=8%252br4Hz5Xir8Q88xW7j69xg%3D%3D&countrycode=US&currencycode=USD

    Item 8 is a pointer knob.

    Item 9 is the little plastic box you see in the picture above.

    This total order was less than $20, including about $8 shipping. If you are patient, you can order from Digikey and pay in advance (enclose a check). They will then ship your parts at their expense. I prefer Mouser because they don't have a bunch of cookie/log-in goop, and they ship fast, even for small orders. Digikey will be cheaper if you pre-pay.

    I also bought some clip leads from Amazon and cut one to make the clips and cable: https://www.amazon.com/gp/product/B07Z646JRC/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 This was just an easy way to get the clip leads and cable, likely not the cheapest way. It has the advantage that you dear reader can duplicate this part if you wish. I will put banana plugs on the other half of the one I cut and use it for purposes TBD.

    I also used my printer and paper, a 5/16" grommet I had, and some teflon spaghetti tubing. You can probably find a suitable grommet at the hardware store, or leave it off. The tubing is an electronics specialty thing, but you can strip the insulation off some random wire and it'll work fine. The teflon is nice because it does not melt from soldering.

    Ok! We have the parts! Let's start assembling.

    First thing to do is make our resistances.

    sortingResistances.jpg

    You can see that I've taken the paralleled resistors and wrapped the leads around to make a "composite" resistor. I've also mapped out the terminals on the base of the switch using my multimeter: 1,2,3,4,5,6. Pay attention to the rotation, and make your drawing from below, same perspective as you will assemble. I used a Sharpie pen and highlighted the switch terminals I planned to use.The tab is the indexing tab on the front of the switch, that keeps the whole switch from rotating in the panel.


    Ok, I'm going to stop here for tonight. Long day. More tomorrow.
     
    Last edited: Aug 9, 2020
    tripilio, Jw60, 73 cj5 and 2 others like this.
  6. Aug 8, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    We are carrying on.

    Assembling is harder to see in my pictures. There are 12 solder lugs around the perimeter of the switch. There are also two solder lugs in the more central part, one for each group of 6 in the perimeter. The switch has 6 positions, each at 30 degrees rotation from the next. If you look between the switch wafers, you can see the contact between the center lug and the perimeter lugs.

    Now, I sort of improvised as I put this together. I knew what I wanted to do, but it took some experimenting to come up with a scheme for how the resistors are routed. Thus I don't have a lot of detailed pictures of mounting the resistors.

    SwitchAssembly (800 x 600).jpg

    Eventually, I put the resistors going around the outside of the switch and connecting to the opposite side. One side of the switch simply connects all of the resistors together, and the other side does the switching. Here is a bit closer:

    ZoomedSwitch.png


    I've included this in full resolution - hopefully you can see. I put about 1/2" of clear teflon tubing (insulation, seen in the picture) on one side of each of the (composite) resistors. That insulated end has a tiny bit of wire sticking out, and is soldered in the switching order to each of our group of six perimeter lugs. Once you have them all soldered in, take the other bare end and loop it around to the opposite 6 perimeter lugs. Don't cut it off! Solder it to the lug and make it stick straight up.

    I made this after finished using red tubing. Put the insulation on and bend your leads like this, 3 around to the left and 3 around to the right. Use the extra lead sticking straight up to connect that lug to its neighbors.

    resistorDetail.png

    Now you have a switch that can select between any of our target resistances. Connect your multimeter to the two interior lugs, put the knob on the shaft, and check that you get resistances in clockwise order from the front: {9, 13, 36, 73, 23, 10}.


    SwitchAssembly2 (800 x 600).jpg

    If everything checks out, it's time to mount the switch in the box. The hole is off-center, at 1" in from the left-hand long side and 1.5" from the shorter bottom side. The clip leads are at 12 o'clock, and with the knob pointer straight up at high noon, you should measure 9 ohms. The hole for the threaded part of the switch is 5/16". There is also an indexing tab that needs about a 1/8" hole at 3 o'clock (to keep the switch from rotating in the case). The leads go through the grommet in the side of the case, and the knot keeps them from pulling through. Strip the ends of the wires and tin them, then form a little hook of the wire, put them through the central lugs and solder. It's not polar, so it does not matter which wire connects to which lug.

    BoxOnDesk.jpg

    I made this label with lots of trial and error, and glued it to the rectangular recess in the top of the box - meant for that I presume. I used 3M Super 77 to glue it down. You could spray the label with clear lacquer or such, but the solvents will probably dissolve the toner adhesive. Run a test first.

    If you want the label, it's in Powerpoint here https://www.nmr.mgh.harvard.edu/~reese/public_html/Paradise.ppt I recall I set the printer for slides (6 slides per sheet, Landscape). You'll have to experiment to get the dimensions right.

    That's it. I would check it again with the multimeter. The resistances are uniformly a fraction of an ohm higher than the nominal resistances we aimed for. The contacts and cheap test leads are probably to blame. These test leads have a very fine central conductor, but they work ok. I'm sure the result is well within the accuracy of the gauges we will be testing in the thrilling concluding part of this post.
     
    Last edited: Aug 8, 2020
    tripilio likes this.
  7. Aug 8, 2020
    Admiral Cray

    Admiral Cray I want to do this again.. Staff Member

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    What kind of cost to get one shipped to Bainbridge Island, WA?...
     
  8. Aug 8, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    Ya know, it had not escaped my notice in the past that a couple of clip leads and the needed resistors could be sent in an envelope, and profit margin could be huge - though the total profit would be somewhat limited. If I were 15 or 16 that might be a worthwhile enterprise.
     
  9. Aug 8, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    Part 3 - Demonstration/Testing

    Rather than disconnect the sender wire at the back of the cluster, I will intercept the wiring at the senders themselves.

    OnEngine130.jpg

    The Temperature Sender on the 304 is right behind the distributor. I clip to that connector and the nearby coil bracket for ground.

    Temp130.jpg

    Key on, gauge reading at cold.

    Temp171.jpg

    At 171F, bottom of the band.

    Temp242.jpg

    At 242F, top of the band.

    Temp270.jpg

    At 270, hot.

    ClipLeads.jpg

    You don't really need the tester - you can use a clip lead. This is two 47s in parallel, which should be right in the middle of the green zone.

    TempClipLeads.jpg

    As expected.

    Now we move to the fuel gauge.

    Weatherpac.jpg

    I have a Weatherpack connector here because the original connector was damaged. The pink wire comes from the tank.

    OnDoorHalf.jpg

    Tester clipped in.

    FuelFull.jpg

    Full is really full.
     
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  10. Aug 8, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    FuelHalf.jpg

    Half.

    FuelEmpty.jpg

    Empty is close.

    Not much else to show. I hope you found this interesting and entertaining, even if you don't build the tester.
     
  11. Aug 8, 2020
    Admiral Cray

    Admiral Cray I want to do this again.. Staff Member

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    You could sell kits...
     
  12. Aug 8, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    This is not enough? I could mow lawns too.

    This is open source. If somebody wants to sell a kit, all they have to do is cite me in the ads.
     
  13. Aug 8, 2020
    Focker

    Focker That's a terrible idea...What time? Staff Member 2021 Sponsor 2020 Sponsor

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    Great write up... I'm glad you went to the trouble of the hands on tutorial. :clap:
     
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  14. Aug 9, 2020
    termin8ed

    termin8ed I didn't do it Staff Member

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    I have not seen that special tool laying around the shop yet. Im sure it made its way into someone's tool box a long time ago.
     
  15. Aug 12, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor 2020 Sponsor

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    Looking at it - it's probably a 1-100 decade resistance box. Two wafer switches, allowing the selection of any resistance from 1 to 100 in one ohm steps.

    Also - I'd look for better leads than the ones I suggested. The boots over the clips are really slippery, and the conductors are really small. The cable is almost solid vinyl. I have Pomona test lead wire in my stock, and that combined with two alligator clips would be better - though I think you can't buy the test lead wire in short lengths. Shouldn't matter too much considering how seldom this device will be used. www.amazon.com/Pomona-6733-0-Silicone-Insulation-Temperature/dp/B000ODU7HO

    Better to buy a kit of "enhancement" leads for your multimeter and mount banana jacks in the tester box. More multi-tasky.
    www.amazon.com/Sumnacon-Multimeter-Test-Lead-Set/dp/B07F8R3VG9/ref=sr_1_21?dchild=1&keywords=multimeter+test+leads&qid=1597239190&sr=8-21
    www.mouser.com/Cinch-Connectivity-Solutions/Test-Measurement/Test-Equipment-Accessories/Test-Plugs-Test-Jacks/_/N-7wzqnZ1yzvvqx?P=1z0x8gwZ1yzmz9iZ1yzr5xxZ1yuytf6Z1z0z248&Keyword=banana+jack&Ns=Pricing%7c0&FS=True
    Nine colors to choose from!
    New link -
    www.mouser.com/Cinch-Connectivity-Solutions/Test-Measurement/Test-Equipment-Accessories/Test-Plugs-Test-Jacks/_/N-7wzqn?P=1yuytf6Z1z0jwozZ1yzmz9i
     
    Last edited: Oct 6, 2020
  16. Sep 9, 2020
    Admiral Cray

    Admiral Cray I want to do this again.. Staff Member

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    I ran across this day...

    [​IMG]
     
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