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Old 04-29-2021, 02:44 AM   #1
FreeEMSFred
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Default 940 cluster in 960 - Speed and Fuel Level Wrong

Hi all,

When I bought my 92 960 wagon it read lower than the speed we were going by quite a bit, but it also had the wrong size bigger profile tyres on it. I didn't do the math even in my head and put it down to the tyres and then replaced them and forgot about it.

On the way home from the supermarket the other night I got paced by a pig and ticketed for 120kph in 100. I swear I was doing 110 on the speedo, give or take maybe 2kph. It's awkward to keep it at a steady speed without the CC working because it's like 2000 RPM at cruise at 100kph. Same deal in a lower gear at 50-60kph, difficult to not speed even by accident.

So afterward I cracked out a GPS and checked it and 80 was 84 and 110 was 116 and so forth. 116+2 = 118, I guess the cop wasn't too far off, 2-4kph wrong. He said he followed me for 600m and from a distance of 200m directly behind. Anyone with an ounce of physics and math knows that you simply cannot accurately judge subtle speed differences from far away when directly in front or behind with your eyes. Up close, sure, the percent change in distance becomes obvious. The apparent field of view taken up by the car becomes significantly different quickly, etc.

In any case, 111-120 = 20 points and 120 doll hairs so it doesn't make any difference and I already tried to get him to write it out for 110 at the time. So no point in trying to get it changed.

Then on the way home the other night while trying to convince myself to drive at "95-105kph to achieve my usual margin on the motorway I looked at the gas gauge being off the clock high and thought about how I ran it out of gas when the light had only just came on and how the dash/cluster had been replaced by a garage before I bought it and click, is it a 940 cluster?

So assuming it IS a 940 cluster, why is the speedo different? Could be count of slots on the diff carrier, but I doubt it. For odo but not speedo it could be different gears inside the cluster (possible! I know there's 2 for some cars). It could be the electronics tuned differently to the 4 cylinder cars. It can't really be anything else, can it? It's a sine-wave signal to the IC on the board that gets converted to voltage from frequency and moves the needle.

Any insight welcome on either the speed or fuel level thing. I'd like to correct both before I forget again and end up with no license.

Cheers,

Fred.
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Old 04-30-2021, 06:53 AM   #2
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If you remove the cluster, there will be a part number on the rear
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Old 05-04-2021, 04:46 AM   #3
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What will that tell me or you if I post it?
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Old 05-04-2021, 08:26 AM   #4
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Most likely speedometer is set up for different rear end gearing. I have the opposite going on in my 91 245, it reads faster than I'm actually going, which is fine but it's also racking up ghost miles on the odometer, not that that matters much either other than not being able to compute the gas mileage correctly.
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Old 05-05-2021, 05:01 AM   #5
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But that's the thing with these Volvos, the speed isn't determined in front of the rear axle at the trans which could cause that issue on any other car with a swap of ratios, it's measured from the axle so only tyre size and tone ring and dash cluster can affect it. I guess it's possible that a 1041K has a different tone ring to all the other 1041s and 740/940 1031s, but I find that unlikely? I thought there was just the 240 ring and late 740/940 ring. Not sure about early 740s, maybe they use the same number of pulses as the 240 or later cars? I do know there are different gears in the odo, but that doesn't affect speed, however I bet in the clusters that have different gears the speedo is also tuned differently?

FWIW my 240s both read faster than reality, too. The wagon was especially bad when riding on 185/70R14 rather than the correct 185R14 (higher profile) size or 195/65R15 or 205/60R15 or similar. It's on 215/45R17 / Polaris right now and reads closer to correct, but still optimistic.
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Old 05-05-2021, 11:52 AM   #6
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Originally Posted by FreeEMSFred View Post
But that's the thing with these Volvos, the speed isn't determined in front of the rear axle at the trans which could cause that issue on any other car with a swap of ratios, it's measured from the axle so only tyre size and tone ring and dash cluster can affect it. I guess it's possible that a 1041K has a different tone ring to all the other 1041s and 740/940 1031s, but I find that unlikely? I thought there was just the 240 ring and late 740/940 ring. Not sure about early 740s, maybe they use the same number of pulses as the 240 or later cars? I do know there are different gears in the odo, but that doesn't affect speed, however I bet in the clusters that have different gears the speedo is also tuned differently?

FWIW my 240s both read faster than reality, too. The wagon was especially bad when riding on 185/70R14 rather than the correct 185R14 (higher profile) size or 195/65R15 or 205/60R15 or similar. It's on 215/45R17 / Polaris right now and reads closer to correct, but still optimistic.
It's not the tone ring, it's the speedometer itself that's matched to the rear end ratio.
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Old 05-05-2021, 06:03 PM   #7
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Sounds like your new cluster is from a sedan.
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Old 05-06-2021, 02:47 AM   #8
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It's not the tone ring, it's the speedometer itself that's matched to the rear end ratio.
Sorry, but with the sensor in the diff, it's ALL tone ring.

Example:

195/60-15 tires.
Tire diameter = 24.21 inches
Circumference = 76.066 inches = 6.3388 feet
5280 / 6.3388 = 832.96 axle rotations per mile
48-tooth tone ring = 832.96 * 48 = 39982.3 pulses per mile

4.10 gears = 39982.3 pulses per mile
3.73 gears = 39982.3 pulses per mile
2.41 gears = 39982.3 pulses per mile
1.00 gears = 39982.3 pulses per mile

The speedo is only calibrated for the tire size and tone ring tooth count.
Gear ratio makes zero difference.

If you want to look at 240s with a cable drive from the transmission output shaft, that's an entirely different system, and the ratio does matter.
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Old 05-13-2021, 06:16 AM   #9
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Quote:
The speedo is only calibrated for the tire size and tone ring tooth count.
Gear ratio makes zero difference.
Exactly, so it's not ALL the tone ring, it's the tone ring and the speedo, assuming constant tyre size.

Is that assumption wrong? Do sedans run different tyre sizes? I didn't think so, but...

@redblockpowered, what is it that makes sedans different? different tone rings in the IRS rear? 960 and V90 wagons are pretty rare here, so 99% it was from a unlovable 900 sedan of some sort.
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