P38A Speedo inaccuracy - P38 V8 4.0 auto

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Scotiawhiskers

Scotiawhiskers

Well-Known Member
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Manchester
I've had a look at RAVE, which appears to say the speedo drive is;

ABS sensors > ABS brain > BECM > speedo. So any inaccuracy should be down to some sort of electrical issue, unless a wheel sensor is picking up some sort of wrong signal from the wheel sensors.

Speedo seemed to be reading under by a margin on the P38, so we did some tests, same route, using the Waze GPS speed.
My everyday Fusion (electronic speedo), rock steady 2mph under the Waze speed report at all speeds.
Wife's Ka (cable drive speedo), approx 10% slow on speedo to GPS at all speeds.
P38, tested today;
Up to 20mph - matched to GPS
25 speedo = 30GPS
34 speedo = 40GPS
44speedo = 50GPS
55 speedo = 60GPS
60 speedo = 65GPS
70 speedo = 70GPS

Didn't have enough space to test faster. If the error was consistent at all speeds, I could understand it. If it was a fixed amount, I could understand it. But having a huge mid range chunk reading badly slow, but being correct at the ends seems very weird to me.
Has anyone come across anything like this or can suggest a cure? I'm wondering if it is something to do with the needle damping in the mid-swing, or somehow the signal?
If it is the needle, is there any issue swapping the speedo for a different one? After all, the mileage readings are all in the message centre, not on the dial.
Thanks for your thoughts.
 
Bit odd. I've always found the speedo very accurate compared to my Garmin (within 2 mph).

Yes, you're right on how it calculates it. I seem to recall it gets maybe 4000 pulses from each sensor per mile and takes an average. Even losing the odd pulse from a sensor won't make that much difference.

I don't know how the needle works and if it is damped or just takes whatever voltage the BECM throws at it. I would guess a bad connection but then why would it match at low or high speed? Luck?!

As RRDT says, most issues are wrong profile tyres or wrong diameter wheels. I cannot see tyre pressures making that much difference but I guess if they were seriously high it would read lower. Should be 28 psi in the fronts and 38 psi in the rears.
 
Last edited:
If you change the dash clocks, if the new one has higher mileage your car will revert to that, the mileage is stored in the becm and speedo and if either is swapped will go to the highest, you can get it corrected if you the right diag but that module is very expensive
i dont know if you could actually take the speedos apart and swap just the speedo drive part.
 
what size tyres are you running?
standard sizes are 255/65R16 or 255/55R18 which the speedo is calibrated too, with a margin of error built in
255/55R21 for example, is ~10% taller than a 255/55R16
 
I have noticed with GPS, being under trees affects the reading, when clear overhead speedo reads 2mph faster than GPS,
not in a P38 btw but, would still apply.
 
P38-DFX said:
I know because I tried peddling him my wheels lol.
Click to expand...
And we're still interested.

Yes, previous twit put 20" Stormer copies on there, with (IIRC) 275/45R20s on them. I measured and they come out at the right rolling radius so shouldn't affect more than +/- 1%. And I'd expect consistent error throughout the range.

I'd originally just assumed that it was slow throughout and by the same %age / margin, but obviously it has a mind of it's own! And if it was slow by around 25% first thing I would have been looking for was a loose / excessive gap / missing sensor meaning it was only reading 3 wheels.

There is a slight ABS / brake error which I hadn't had time to search for, but I think that will be to do with the powered side from the way it behaves. Shouldn't affect speed count.

ABS / brake warning lights behave correctly at ignition / set off from start. But they both illuminate for a second or 2 when first applying brakes, then go out again. As fluid level is good, I was guessing that was some failure on the power side and then clears as the pressure from the pedal / servo builds above a level. Typically it doesn't match any of the behaviours listed for the system in RAVE!
 
Scotiawhiskers said:
And we're still interested.

Yes, previous twit put 20" Stormer copies on there, with (IIRC) 275/45R20s on them. I measured and they come out at the right rolling radius so shouldn't affect more than +/- 1%. And I'd expect consistent error throughout the range.

I'd originally just assumed that it was slow throughout and by the same %age / margin, but obviously it has a mind of it's own! And if it was slow by around 25% first thing I would have been looking for was a loose / excessive gap / missing sensor meaning it was only reading 3 wheels.

There is a slight ABS / brake error which I hadn't had time to search for, but I think that will be to do with the powered side from the way it behaves. Shouldn't affect speed count.

ABS / brake warning lights behave correctly at ignition / set off from start. But they both illuminate for a second or 2 when first applying brakes, then go out again. As fluid level is good, I was guessing that was some failure on the power side and then clears as the pressure from the pedal / servo builds above a level. Typically it doesn't match any of the behaviours listed for the system in RAVE!
Click to expand...


I'm in no rush. I've just picked up another set to play with lol.

As for the faults, it really needs a nanocom plugging in and seeing what error codes are listed. See what each abs sensor is reading. The fact the abs light coming up when you hit the pedal shouldn't happen.
 
275/45R20 is an odd size but not massively different diameter vs standard.are you sure your GPS is accurate and working properly? not affected by the heated screen etc
you could try it the old school way and follow someone doing a predetermined speed on a particular stretch of road
 
I figured that with the ABS. And the brakes don't feel quite as powerful / sharp as I had expected, though they are still pretty effective. Next chance I get I was going to check all the power side fuses and relays in case something there has failed and stopped the pump working. I've not heard the pump, but then it does have a bit of a throaty burble to drown out some noise. And the tired EAS compressor as well.
 
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