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Thread: Speedo Calibration

  1. #1

    Default Speedo Calibration

    I replaced my instrument cluster with another cluster and put a new speedo cable in that I bought from Ebay. While driving today it "felt" like I was going slower than what the speedo was reading.(speedo read 80 km and I "felt" like i was going no more than 50km) So my question is are there differences in clusters where they cant be interchangeable without modification? or Is it possible for the cable to read faster than it is. Keep in mind that I did change the speedo gear in the transmission last year, but I used the exact same tooth count (19) If anyone has any idea let me know.

  2. #2
    Wagon Addicted Tiggie's Avatar
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    Instrument clusters are delicate things that don't really like going through the postal system. I'd wager the calibration of one or both of them are off. They should be the same. The speedo cable will have no effect. The gear will change things, but it sounds like you ran the newer gear with both clusters which would mean that's not the problem either.

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  3. #3

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    I got the second cluster because of the circuit board, it curled over and kept blowing the fuse for the backlight display.

  4. #4

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    and the cluster i removed from the wreckers myself, only the cable i bought through ebay.

  5. #5
    I'm an air-conditioned gypsy gadget73's Avatar
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    got a smartphone? Get you a speedometer app and see what its actually doing.

    The clusters are all the same. Off the top of my head I forget the number, but there is some RPM of the cable that equals 60 mph, and thats true for all the speedos. The cable is also just a cable. The gear is what determines the calibration. The speedo itself can be damaged internally and cause it to read funny but you wouldn't have multiples reading off by the same amount. That would be the speedo gear.

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    I had the same problem with a bunch of mine. Seemed like each one was different.
    I was able to locate one of those speed radar trailers that warn drivers of their speed. At least it was a gauge as to how far off the speedo was. I finally replaced it with a nos. Those circuit boards sheets are a replaceable item. Just need part number. I have seen one recently.
    Best thing is to take it to a speedometer shop.

  7. #7
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    I never found the 85 MPH units to be particularly accurate, whether cable or electrically driven. I've used both between my cars, and the error margin increased past 55 MPH it seemed. Using the '90-'94 style in the '97, 78 MPH actual read as 85+, but it read fine at 60 and under. The '87 was similar, and was true to speed at 55 MPH, but beyond that, it got way off. I think 75 MPH read 85 MPH. I did check odometer mileage vs. time, and found that to be accurate, just an over-reporting of speed at the head.

    Now the police units I've used have not had the margin of error issue with higher speed. The unit in the '87 is usually within about +/-1 and the unit in the '97 is roughly fast by 3 MPH, but that's just thanks to me not putting the needle on square.

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    You talking about Ford/Mercury stuff? The speedo's on our Town Cars are dead nuts accurate...
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  9. #9

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    This morning I took the speedo head out of my original cluster that came with the car and swapped it onto the unit I got from the auto wreckers. I took it for a drive and it seems to read a lot better than yesterday. I might try that speedometer app and see just how accurate it is.

  10. #10
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    Sometimes the lubrication in the speedo mechanism gets cruddy. A little WD40 followed with some 3-in-1 oil or some other lube that will have some stick-to-it properties but isn't real thick seems to work pretty good.

    If digital cluster, sometimes just reseating everything to clean off the contacts sorts out all manner of stuff.

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    Quote Originally Posted by 1985crownvicltd85 View Post
    This morning I took the speedo head out of my original cluster that came with the car and swapped it onto the unit I got from the auto wreckers. I took it for a drive and it seems to read a lot better than yesterday. I might try that speedometer app and see just how accurate it is.
    That works too. I was going to suggest just swapped the good known printed circuit onto your original cluster.
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    There's no adjustment in there for speeds? I figured at least with the police clusters there would be a little "slower/faster" lever to adjust it, like in a clock.
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    I'm an air-conditioned gypsy gadget73's Avatar
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    I think they bend the tabs on the magnetic coupler, or maybe its a spring tension thing, bending the anchor point to adjust. Its not an adjustment clearly and obviously marked "calibration" though. Prob one of those things where if you know what you are looking at its cake, but I don't know what I'm looking at in there. Its got a thing that spins and the needle is connected to a C shaped thing that sits over the spinny part but beyond that its voodoo.

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    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

  14. #14
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    Be wary of those roadside radar things. I've noticed a disturbing trend lately of those things showing speeds higher than actual speed, (conspiracy theory:vision zero b.s. is starting to creep deeper to force their agenda of people driving too fast). Use your own gps to verify.

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    Quote Originally Posted by Brown_Muscle View Post
    There's no adjustment in there for speeds? I figured at least with the police clusters there would be a little "slower/faster" lever to adjust it, like in a clock.
    31 years on, we never adjusted calibration as there is no user friendly way to do so. We just checked the existing unit with a chassis dyno at headquarters in Richmond at first issue sometimes, sometimes with the garage's 5th wheel, then with a 5th wheel every 6 months for a number of years, then they stopped with that if a shop local had a dyno, like Diesel Injection in Roanoke that we used. That didn't last long as it costs $$$ and took the Trooper off the road too long, so since radars were checked every shift at start and end with tuning forks and speedo comparisons, after every violation there for a time, we started using radar to check speedo calibration too. Tuning forks went to a lab every 6 months to get checked for frequency vibration, and they were certified if OK. Cuts or bends could affect them I was told. If a speedometer didn't check out within 3 mph, it was replaced.

    About the radars, I never saw one that turned on to not calibrate. I have seen them not work due to failed component or misuse ... like pulling the plug out by the cord or dropping the antenna on the pavement or simply by a stepped on plug. One buddy hit the wrong window button at speed, the window went down but the antenna window mount stuck for a few seconds, then dropped out of the window channel and fell outside the car, destroyed that antenna. In 31 years, never once saw a radar that worked not check perfectly with tuning forks or against the car speedo.

    In all speedometer calibration checks, the Trooper read the numbers and the mechanic, etc, just wrote down the numbers as the Trooper lined up the needles. The Trooper was the one who also signed and made oath to the speedometer calibration sheet. We were assigned cars that we kept at home off duty. One learned real quick whether to read the left or right edge of the needle or if dead center. Only time exact counted was on pace, and I just always wrote lower than speedo reading on a pace, like if I wrote you at 85 on a pace, I was really seeing 90 or 95 on my speedo while matching your speed over a distance, which depended on how far behind you I was. I've paced from a half mile back, but did so for 5 or ten miles and at points, saw the violator pulling away and then if in violation significantly, a stop was made. I have paced just a few yards like if I merged beside of or immediately behind a high speeder.

    Quote Originally Posted by GM_Guy View Post
    Be wary of those roadside radar things. I've noticed a disturbing trend lately of those things showing speeds higher than actual speed, (conspiracy theory:vision zero b.s. is starting to creep deeper to force their agenda of people driving too fast).
    Might b be reading that guy coming up behind you ... or the guy in the other lane going the other way. I've never noticed them being off when I was the sole target. Doppler Speed Radar can only track the strongest signal generator so for enforcement, it's absolutely imperative that the operator be paying attention and making sound speed judgements independent of the radar that agree. A 40 mph smart car near the unit can see a 50 mph reading which is in reality that 50 mph Kenworth an eighth mile behind.

    Quote Originally Posted by GM_Guy View Post
    Use your own gps to verify.
    Best bet there, although I did put my issued radar in my personal cars, even on my Gold Wing maybe to check them a few times.

    Quote Originally Posted by gadget73 View Post
    I think they bend the tabs on the magnetic coupler, or maybe its a spring tension thing, bending the anchor point to adjust. Its not an adjustment clearly and obviously marked "calibration" though. Prob one of those things where if you know what you are looking at its cake, but I don't know what I'm looking at in there. Its got a thing that spins and the needle is connected to a C shaped thing that sits over the spinny part but beyond that its voodoo.
    Manufacturer did that adjustment new, nothing for the user.
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    Last edited by tbear853; 04-19-2021 at 11:45 AM.
    No ... I'm not arguing with you ... I'm just explaining why I'm right ...

    Now go ... and whatever you do ... have a safe trip!

  16. #16
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    Quote Originally Posted by tbear853 View Post
    31 years
    Might b be reading that guy coming up behind you ... or the guy in the other lane going the other way. I've never noticed them being off when I was the sole target. Doppler Speed Radar can only track the strongest signal generator so for enforcement, it's absolutely imperative that the operator be paying attention and making sound speed judgements independent of the radar that agree. A 40 mph smart car near the unit can see a 50 mph reading which is in reality that 50 mph Kenworth an eighth mile behind.
    A lot of these are county locations entering small towns, I'm the only vehicle more often than not. (I cover ~8000 square mile service area, on the road everyday, so I come across my fair share of roadside radars and little towns.) I used to use them as a secondary speedo check as they always agreed with GPS, but what I'm seeing, I'm suspcious of ulterior motives behind them (that being the big push to post slower speed limits), and if your data shows lots of speeders, the town council agenda gets pushed further along. Unlike the U.S. prevailing road speed is not something that is used around here, and what I am seeing is significant increase in bullshit speedlimit drops--much like the stereotypical U.S. speed trap towns, where the speed limit is 50 or 55mph and for no good reason other than the pavement changed color, the mayor farted, or somebody howled at the moon it drops to 30mph for 100 yards, and then pops back up. And we also have photo radar getting expanded since that shit got green lighted by the anti-car politicos that seem to dominate local positions. My own councellor pretty much blows me off anytime I bring up anything illogical I see with regards to these limit drops in my area.

    Alex.

  17. #17
    I'm an air-conditioned gypsy gadget73's Avatar
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    un-manned speed cameras are bullshit. Have the same feeling about red light cams.

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    Quote Originally Posted by phayzer5 View Post
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    Quote Originally Posted by GM_Guy View Post
    A lot of these are county locations entering small towns ... etc ...

    Alex.
    Well, there was that day in west Texas I was traveling along at the posted limit 65 east of Lubbock, then we came to Guthrie, slowed at the 45 ... or maybe it was a 35 sign, then I got stopped shortly. I was really surprised, he started explaining his radar but I stopped him by saying I teach radar, believed it was right, but that I never saw a 25 sign. He warned me, let me go, then I circled back to look for a sign, saw the 25 sign was bent over like it had been hit so I pulled up to where he was and mentioned it. Then I headed on east.
    No ... I'm not arguing with you ... I'm just explaining why I'm right ...

    Now go ... and whatever you do ... have a safe trip!

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    Quote Originally Posted by GM_Guy View Post
    ... (I cover ~8000 square mile service area, on the road everyday...
    Correct me if I’m wrong, but wouldn’t 8,000sq. mi. be like patrolling all of North and South America and about 3/4 of Russia? Daily? Lol

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    Quote Originally Posted by sick88tbird View Post
    Correct me if I’m wrong, but wouldn’t 8,000sq. mi. be like patrolling all of North and South America and about 3/4 of Russia? Daily? Lol
    Square root of 8,000 is a hair over 89.44, so 89.44 x 89.44 is 7,999.5136. That's still a pretty big county there ... but I understand "covering" as my county is only 600 odd square miles. Some shifts I saw only a very small narrow part of it, some shifts I felt like I had visited it all (but of course ... I hadn't ). Some shifts, I'd drive 120 miles within my county, and I do recall some where I ran calls from end to end and racked up 400 miles. Was nights we'd get a fight call in the north west corner, get there, assist, etc, then get a call way down in the south east corner and get there only to find it was near nothing, then get called back to the last call as things heated up again, then there was 100 mile lap of Interstate to check at least twice per shift if possible, plus any other calls. Coffee, don't forget coffee.
    No ... I'm not arguing with you ... I'm just explaining why I'm right ...

    Now go ... and whatever you do ... have a safe trip!

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