I spotted a P38 in a scrappy today with a calibrated speedo, which got me wondering what its history was. My first thought was ex-cop car but it doesn't have a "Special Vehicles" plate on it (that I can see). It's obviously had the interior fiddled with because the seat bases and carpets are black but the seats are cream leather, same as mine.

It's got a highline BECM but there's a lowline BECM lying in the boot! I got the VIN if that helps anyone. It's on a private plate but it's probably a fairly early one. The tailgate is badged "4.0 SE".

On mine, being ex-plod, the Special Vehicles plate is on the removable slam panel so if it has ever had that changed it may have lost it. Black, or very dark grey to be precise, carpets and plastic seat bases would be right for the cloth seats fitted to most plod motors so it sounds like someone has swapped the seats at some point. Mine has a highline BeCM as it needs that for the front fogs and headlamp wash wipe although it didn't have cruise or trip computer (that was sorted by fitting a stalk with buttons and telling the BeCM it was fitted). Calibrated speedos were done by Instrument Repair Services of Nottingham and have a sticker with IRS Nottm and a serial number.

Calibrating the speedo is actually very simple, there's two pots on the top of the instrument cluster circuit board that deal with it. The mosfet driver that turns on some of the warning lights on mine started to die. Having no seatbelt warning light I could live with but when it spread to the alternator and oil pressure lights I figured I needed to do something. I got a spare cluster from a breaker and swapped the circuit boards over. The speedo then started to read like the ones in the SE and Ascot, an indicated 70 mph was really only 65 in real life. Called IRS and they sent me some very limited info but enough to allow me to tweak the pots to get mine spot on again.

Gilbert, would you mind sharing the info on the pot adjustment to tweak the speedo reading? Which one does what and how do they influence each other?
Does the adjustment influence the trip computer reading too?

With the larger tyres fitted mine is under reading by about 3-5km/h at the moment. With the standard tires it was over reading by about 5km/h.

One of the P38's I use a fair bit (1998/9 Thor) has a calibrated speedo and a special vehicle plate in the bonnet. It's odd, all matches up but the car is the standard blue (dark interior, grey-ish I suppose - looks black) and looks to always have been oxford blue - outside it's the same colour as my GEMS HSE. Low spec 4L, Lowline BECM, cloth seats, no reverse-dip on the mirrors, badged only 4.0, no SE/HSE etc. Only thing it's got extra is a spare aerial and a couple of drilled holes in the dash which could have been from anything. Always assumed it was configured originally for [whatever reason I haven't figured out] and sold as a lower spec normal car. Nothing interesting shows up in any of its documents.

Undercover MI6 vehicle! :)

James Bond has to maintain standards afterall.

Police spec cars were ordered in whatever spec and with whatever options the particular force wanted. Most were white but some were blue, most had the base model cloth interior but some had the leather. It may well be that if a vehicle was needed in a hurry a standard production one would have been bought and modified after purchase. But a car with Lowline BeCM, cloth seats, no reverse dip (you only got that with the electric memory seats anyway) and simply badged as 4.0 could well have been an unmarked car. Marked wouldn't have any badging on the tailgate as that would have been covered in reflective chevrons. Only one extra aerial would seem to back that up, mine had the cabling above the headlining for 2 additional aerials (which would have been a UHF local pocketphone channel and a VHF force wide channel) where they had removed the aerials and poked the bases through before filling the holes along with 4 more holes where the light bar had been attached. I've got the useful options, front fogs, headlamp wash/wipe and AC but the cloth interior from a base model, no cruise control or trip computer, no radio from the factory and low line speaker install (which I've since upgraded to mid line spec). I've got a pushbutton very neatly fitted onto the blank where the sunroof switch would be if it had a sunroof and the odd hole here and there where things have been fitted inside. There's also a 120A split charge relay under the bonnet and two cables running to the offside boot for an aux battery and some mods to the tailgate wiring which I suspect allowed you to lock the car but still leave it possible to open the tailgate. But best of all, It's still got the big spray stopper mudflaps!

To calibrate the speedo you first need to know how far out it is, by how much and in what way by comparing the speedo reading against a sat nav. The GPS on the sat nav will only be 100% accurate when travelling at a constant speed on a flat, straight piece of road.
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If you have a GEMS and a Nanocom that makes it very easy as in the ABS menu (ABS- Diagnostic-Utilities-Speedo) there is a setting that causes the speedo to read around 50 mph (Nanocom documentation says it will make the speedo read 100 mph but on all that I've tried it on it reads about 50 mph). So you can set that, make a mental note of what it reads, drive holding your speed at that same point and compare what the speedo says against the GPS. Then you have your baseline figures so can adjust from there.

Take the instrument cluster out, break the yellow paper seals so you can remove the translucent plastic over (which will also mean unplugging the speaker) and look along the top of the circuit board you'll see a row of trimmer pots on each side. The ones you need are marked VR3 and VR4.

VR4 deals with Offset and adjusts the reading by the same amount over the full range. So if it permanently reads 5mph over, then you adjust that with VR4. VR3 deals with slope so if it reads OK at 20 mph but the error gets progressively greater the faster you go, adjust the slope until it is correct over the full range. With the instrument cluster surround out, the cluster itself in place but not screwed in and the translucent over partly loose, you can get in there with a very short trimming tool (I used the screwdriver bit from a set of the interchangeable bits) but be careful as you only need to give the pots a tiny bit of movement (many years of tuning radio transmitters for maximum power means I've got pretty good at it).

If you feel like having a play while you are in there, VR1 deals with the Temp gauge reading, VR2 the fuel gauge reading while VR5 and 6 do the same as 3 and 4 but on the rev counter (can't remember which is which though but you have the same offset and slope adjustments on those two).

Thanks for the detail, makes sense now.
I've got a passenger along for a drive later today and I'll get some data for a graph to determine if I only need to adjust the offset, or of the slope needs some tweaking too.

Never mind Rave - all you need is Gilbert - can we get a download somewhere ?

romanrob wrote:

Never mind Rave - all you need is Gilbert - can we get a download somewhere ?

I think that's the first time I've read something on the internet that made my brain actually say 'wow' to itself.

Morat wrote:

James Bond has to maintain standards afterall.

He did drive a 2cv at one point for a few moments I seem to remember.

I seem to recall seeing a speedo calibration function on a Nano?? Gotta go check after coffee...
And, as we are taxing Gilbert's arcane knowledge base, What is the sequence of button pushes to get into the hidden menus of the factory GPS?
I know they must be there, but so far, I have failed to find them......

Re Nano - it's in the instrumentation section, but doesn't let you re-program anything, so not much use day to day.

Good point re GPS - there must be some kind of back door

The only mention of the speedo I've seen on the Nano is the one under the ABS diagnostics, never seen it anywhere else. Might be different on one with the Thor licence, I've only got GEMS and diesel on mine.

Can't help at all on the factory GPS, never owned a P38 new enough to have one.

Just checked Nano.....
Under : Instruments, Motronic, Instrument set up, There is something called "Tyre Coefficient" which defaults to 1.000
Seems suspiciously like a speed calibration for larger or smaller tyres?

Certainly looks that way, try changing it and see what happens. You can always change it back.

That Tyre Coefficient is only for the instrument readout option on the Nanocom - it won't do anything to the instrument cluster.

To get into the "Service Mode" of the factory navigation, go into the "Settings" menu and then press and hold the "Menu" button for about 10s. From memory it doesn't give you much to play around with - just testing of sensor/button inputs and showing hardware/software versions, and checking GPS reception.

Can you get the gps to read out road speed?

Martyuk wrote:

To get into the "Service Mode" of the factory navigation, go into the "Settings" menu and then press and hold the "Menu" button for about 10s. From memory it doesn't give you much to play around with - just testing of sensor/button inputs and showing hardware/software versions, and checking GPS reception.

Marty...our other resident gold mine... someone should develop Trivial Pursuit for P38s

My missus reckons I've got so much random crap floating around in my head I should go on 'Who wants to be a Millionaire' and make use of it. Problem is, they'd never ask questions on the sort of stuff I know about.....

Gilbertd wrote:

Problem is, they'd never ask questions on the sort of stuff I know about.....

Mastermind - specialist subject P38. That'd be worth tuning in to.

Bolt wrote:

Can you get the gps to read out road speed?

Not that I know of, I would expect it to be in the GPS reception screen if anywhere. Otherwise you'd need to use a stand alone sat nav or GPS on a smartphone.