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Speed meter 15% too optimistic

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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Esperado
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Speed meter 15% too optimistic

Post by Esperado » Mon Dec 16, 2019 5:09 am

My speedometer displays values 15% higher than the real one (GPS°.
On my model (Volvo S70 T5 from 1998) the speed information transmitted to the speedometer don't comes come the sensor but from the ABS box.
It does not seem possible to correct this problem via software.
Also, I would like to have an answer on the following question: The information from the ABS box are they a variable frequency of square waves or a DC voltage.
In the first case, I could consider a frequency divider circuit, in the second, a simple voltage attenuator.

Thank you in advance for a lead allowing me to correct this annoying problem in our age of speed limits and cops everywhere.

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erikv11
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Re: Speed meter 15% too optimistic

Post by erikv11 » Mon Dec 16, 2019 8:46 am

Are your car's tires stock size for the vehicle? Check the tires and the label located on the rear facing side of the driver's door.
'95 854 T5-R, Motronic 4.4, 185k
'96 855 NA, 145k
'98 S70 NA, 220k (living out west)
'98 V70, T5 tune-injectors-turbo, LPT engine, 285k
'06 S60 R, 165k
'99 Camry V6 :shock: 125k
gone: '96 NA 850 210k, '98 NA V70 182k

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Re: Speed meter 15% too optimistic

Post by Esperado » Mon Dec 16, 2019 11:20 am

Thanks for your answer.
My tires are standard, but, anyway that is not the subject. My car is how it is.
The question is a technical question: What is the standard of the information transmitted by the ABS module to the speedometer. Square waves with variable frequency, or DC with variable voltage ?
My Volvo garage is unable to answer this question or to tune my speedo, and the OBDII command for the tires circumference (60) do not seem to work neither.
Whatever the cause of this problem, I would like to correct-it.

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Re: Speed meter 15% too optimistic

Post by Stu70 » Mon Dec 16, 2019 1:23 pm

From the wiring diagram - it might be just a generic symbol - but it looks like a square wave signal.
I don’t a graphic meter or oscilloscope to measure for you. But I would think it’s more likely a variable frequency square wave pulse summed directly from the abs sensors.
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1998 S70 10v BiFuel “it’s a what??”
Last of three factory fitted CNG Press Cars :roll:

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Re: Speed meter 15% too optimistic

Post by Esperado » Mon Dec 16, 2019 5:08 pm

Thanks a lot, Stu70.
I have found the following on the CPU schematic:

#B18 Signal speed (from combined instrument panel)
U ≈ Ulowor Ubat (the voltage depends on the position of the front wheels)
f increases with speed f = 40 Hz at 20 km/

But I don't think it is the one used, as, on the ABS schematic,
On the pin 6 of the connector, I see a cable Y-BN with what I suppose it is a connector 24/13C that goes to the A3 input of the dashboard to the speedometer. But no indication of the standard ;-(

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Re: Speed meter 15% too optimistic

Post by Matty Moo » Mon Dec 16, 2019 6:08 pm

My TEVES manual says for the MK20 module it's a duty cycle ground, so it would be variable frequency square wave. That's assuming your 98 is the same as a US spec 98 and has the MK20

But....In my opinion your energy and money might be better spent figuring out why it's off vs a workaround. I've never seen an MK20 have a speed reading going to the cluster that is outside spec with the proper sized tires on the car.

That's the first time I've ever opened that manual up since I bought it 9 years ago.
Image
http://www.midwest-abs.com
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1996 850 Platinum Wagon. ARD Green Tune, OBX.-Gone
1998 s70 ARD tune, EST exhaust, SE/R interior.
1999 s70 Plain Jane.
2000 s70 GLT
2014 Ram
2012 Jeep Wrangler Unlimited

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Re: Speed meter 15% too optimistic

Post by Esperado » Mon Dec 16, 2019 8:11 pm

Matty Moo wrote:
Mon Dec 16, 2019 6:08 pm
But....In my opinion your energy and money might be better spent figuring out why it's off vs a workaround. I've never seen an MK20 have a speed reading going to the cluster that is outside spec with the proper sized tires on the car
Hi Matty Moo,
Thanks for your care.
My tires are standard 205/50/16, and the speedometer/odometer/cruise control are working fine apart this 15% constant error. And I have re-soldered inspected/verified my ABS module pins and welds.
As i'm an electronic engineer, it should not be a huge work to interpose a frequency divider in order to have a more accurate reading. This 15% error really bores me. But It would help me a lot to figure out the frequencies range. I don't have a portable scope/frequency-meter and it should be complicated to drive with a wire connected to my house ;-)
[edit] I found a simpler solution: https://www.brickboard.com/RWD/volvo/85 ... meter.html

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erikv11
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Re: Speed meter 15% too optimistic

Post by erikv11 » Mon Dec 16, 2019 10:42 pm

Cool, hopefully the only thing broken is internal to the speedo, like the resistor you are probably looking to replace. So something doesn't just create another problem in a little while.

Very interested to hear how you adjust the reading, please post up if you figure it out! Pics always welcome ...
'95 854 T5-R, Motronic 4.4, 185k
'96 855 NA, 145k
'98 S70 NA, 220k (living out west)
'98 V70, T5 tune-injectors-turbo, LPT engine, 285k
'06 S60 R, 165k
'99 Camry V6 :shock: 125k
gone: '96 NA 850 210k, '98 NA V70 182k

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Re: Speed meter 15% too optimistic

Post by Esperado » Tue Dec 17, 2019 3:47 am

Here is the schematic of the integrated circuit with is part of the speedo that transform the frequency into DC voltage proportional to the speed that move the voltmeter (speedo).
http://www.radioradar.net/en/files.html?fid=489248

On pin 4 is the resistance that have to be adjusted. As it is 100 Ohms by default, I think I will put an adjustable of 200 Ohms with a shielded wire in order to have-it accessible from outside. Then, the car on a flat highway in strait line (under cruise control at the legal max speed) according to a GPS I will adjust-the speedo. There is an electrolytic cap on the 12V, here, that I will change in the same time. They don't like age.

That is the correct way, but, as I'm lazy to unmount my dashboard, I will try before; from outside, to add an adjustable regulator in order to decrease the 12V to 10.2V and see if it works.
Attached the general schematic with the 12V line in red.
Image

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Re: Speed meter 15% too optimistic

Post by befarrer » Wed Dec 18, 2019 11:22 am

I'll just add in here that my 98 V70 AWD speedometer is also greatly optimistic, it's roughly 10-15% fast. I have P195/65R15 tires installed, the odometer however reads slightly low, maybe 5% slow?
98 V70 GLT AWD
01 Focus Wagon SOHC 5-speed
93 Mazda B2200 with 13B swap
Works on Volvo Trucks

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