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Speedometer error, Volvo 122S BW35 to M40.

spock345

Well-known member
Joined
Nov 3, 2016
Location
Livermore, CA
At some point in its life my 122s was converted from a BW35 to an M40. The speedometer is a bit off. Using a GPS speedometer on my phone I have found it reading a bit fast. It will read 35mph when in actuality I am going 25mph. The error is significant enough that I have decided to try to fix it. I replaced the speedometer cable because mine was sticky and the rubber casing had melted away (someone let it sit on the exhaust), that fixed how jumpy it was. I verified it has the 16 tooth yellow gear for a 4.10:1 rear end ratio, which should be the same between an M40 and a BW35.

Is there anything different about the speedometer on automatic cars or is it just badly out of spec? Is there anything in these that can get stuck or need lubrication? I have found information about stuck or slow speedometers but nothing about then running fast.

I am running 165/80r15 tires on the stock 15x4 wheels.
 
Don't know a lot about 122s but 410s doesn't sound right since 430s were in 140s till 70.

:e-shrug:

I know for sure they put 4.10 and 4.56 rear ends in 122s but I wouldn't be surprised if 4.30 gearing ended up in some cars. There is a little metal tag on one of my differential bolts that says 4.10.

Mostly I just need to know if there is some common problem with these VDO ribbon speedometers or if I am missing something.
 
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Do you know the colour of your speedo gear
nylon drive unit?
Yellow = 16 teeth
Green = 17 teeth
Blue =18 teeth.
The higher the no of teeth, the slower the speedo.
1 tooth changes speedo reading by 5 to 6%.
2 teeth " " " " 10 to 12%


Informations above from the Volvoforums

Good luck, Kay
 
Last edited:
Do you know the colour of your speedo gear
nylon drive unit?
Yellow = 16 teeth
Green = 17 teeth
Blue =18 teeth.
The higher the no of teeth, the slower the speedo.
1 tooth changes speedo reading by 5 to 6%.
2 teeth " " " " 10 to 12%


Informations above from the Volvoforums

Good luck, Kay

Mine is yellow, 16 teeth. Which should be correct for 4.10 rear gearing if the Volvoforums are to be believed.

I am going to double check today.

165/80r15 is the closest to the stock tire you can get now.
 
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165/80r15 is the closest to the stock tire you can get now.

I stand corrected. For some reason I thought you were talking about a wagon.

122S sedan, original tire size was 5.90 X 15 which is approx. 165/80 X 15.
122S wagon, original tire size was 6.40 X 15 which is approx. 195/75 X 15.
 
That should have the correct metal speedo drive gear so that's not the problem.

I have to take out the cluster to repaint the bezel. While I have that off I'll take a look inside and see if anything looks off. Maybe something is rattling around in there or some piece of plastic broke.
 
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Does odometer give reasonable values? Einstein figured out that that there is correlation between speedo and odo error.

The trip odometer behaves oddly. So there may be a problem there. I'll know more when I take the bezel off for a repaint.

The electronic speedo also seems like an interesting idea. What do you use to trigger it on the drivetrain end? I don't like relying on just GPS for the speedometer.
 
You don't, which is kind of the point. If you had a trans that supported an electronic speedo, then you could use that as well. I haven't had any issues with GPS, other than it takes a bit to lock a signal after starting. You shouldn't be driving your car right after you start it anyway :)

Tire size changes, rear end differences, speedo sender gearing, none of that makes any difference to the GPS box.

The electronic speedo also seems like an interesting idea. What do you use to trigger it on the drivetrain end? I don't like relying on just GPS for the speedometer.
 
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I just got the cluster out of the car, not a fun job.

The speedometer looks fine, nothing fell out while I was taking the faceplate off. Nothing rattling around. Took at look at the odometer. It was a bit sticky due to old grease. Cleaned that up and put some new grease on the little gears. Also lubed the little piece that the speedometer cable sticks into.

The only abnormality that I could find is that the 100s digit on the trip odometer doesn't work. It doesn't tick over or reset properly.

The thing moves freely when you turn the hookup for the speedometer cable.
 
Popped it back in the car. Hopefully just lubing a few bits will fix it. If not then I will be in search of another cluster or sending this one out to a shop after taking some readings for the exact error.
 
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