Speed pulse

Hi All,

Anyone know if a '95 850 T5 has a speed pulse anywhere or will I need to fit some sort of sensor in order to connect to a satnav unit?

If there is a speed pulse any idea where I may find it?

If I need to fit a sensor, where do I get it from and where do I fit it?

Cheers

Andy

[warthog]

Andy,

the 850 has two speed pulses that are accessable from the plugs into the instrument panel. I don't however know what effect a different load on them (such as plugging a Satnav unit in) would have on the instrument panel electronics.

The connectors are:
A6 48 pulses per wheel revolution Grey/White wire
A7 12 pulses per revolution Yellow/Black wire

The signals are from 0 to battery voltage.

What sort of nav unit are you fitting - if it's linked into the GPS sat. network then it should calculate speed and position from satelite timings and not from any electronic link to the vehicle.

Regards

Hi Brian,

Thanks for the reply.

It's a Blaupunkt GPS head unit.

So GPS doesn't require a speed pulse? So what systems do then? I thought all Nav stuff was GPS but some required a speed pulse. I know the new portable stuff such as the Tom tom GO don't require it but I thought all head units did. Admittedly I haven't read the destructions yet (I haven't received the unit yet but it should be with me Wednesday) but I just want to be armed with all the info before it arrives so I can fit it straight away.

Like a kid at Christmas me!

I'll have a look for a user manual on the tinternet and see if it requires a pulse.

BTW, any idea why there are two wires giving different pulses? Obvilusly one for the speedo but what's the other for?

Thanks again.

Andy

[warthog]

Andy,

all a true GPS unit does in essence is give you a 3D position (assuming it can track enough satellites) on the surface and in the atmosphere of the earth (not sure about under the water). This position is updated regularly (may be every second or so) and from this position relative to the previous position, a speed between the two is calculated by the unit. All other distance/speed functions that the unit does are calculated from these position fixes.

The declared accuracy for the Garman III GPS for example is 15m RMS in position and speed - 0.1 mph RMS steady state. This is possibly an order of magnitude better than you could expect from a speed pulse signal using as it's reference the rotational speed of a variable diameter tyre on the road.

I'm not familar with the in-car sat.nav kit so can't comment on them, but it would seem somewhat pointless to have access to this GPS level of accuracy and not use it. One limitation of note for GPS is that it must be able to 'see' the satellites so for example in a narrow street in and around high-rise buildings, or in tunnels etc. the signal is likely to be lost, but it would be quickly aquired and updated again when away from the obstructions. So in this respect, maybe some units use car driven speed signals to replace GPS input when necessary. They would then however obviously only be able to note speed and distance, not direction or position.

I don't know why there are two different speed signals, perhaps someone else knows what they are used for.

I'm sure we would be interested in how you get on fitting your new unit, so please keep us informed.

Regards

[jaytee]

Most portable nav.units compute the speed by the change in position with time/distance, most, probably all head unit types have a connection for a speed pulse signal, my Panasonic does and I had a terrible time trying to find out where a suitable connection was!!!
You should only connect accessories to the 12 pulse signal. there is a terminal on the rear of the instrument panel, but maybe only VDO, not Yazaki. My nav unit also has an internal gyroscope which combined with mapping and speed information still show your location even if you are in a long tunnel or a forest where the GPS signal may be lost.

After installing the Pana. nav unit the car has to be driven for 1/2 hour at at least 20mph to calibrate the speed signal (for varying tyre cicumferences)

Hope this helps a bit!

Jim

Cheers Jim,

Yes, my Blaupunkt unit definately has a speed pulse connection so I will have to connect it up I guess rather than relying on the GPS calculating the speed using positioning. I guess this has it's benefits although the initial installation is a pain in the bum.

Is yours an 850 too Jim? You say there's a terminal on the rear of the instrument panel - mine's a VDO panel so I should hopefully have the same. Does this mean I have to remove the instrument panel in order to get to it :-( ? Or can I just tap into the wire somewhere else and run a cable from the wire to the back of the unit?

Cheers

Andy

[KeatV70D5SE]

If the speed pulse is 12 per wheel revolution the headunit won't be able to calculate your speed unless you calibrate it to your wheel size.

The Volvo headunit has a speed pulse because you have the option of the radio getting louder the faster you drive.

I can't imagine any Sat Nav system actually using the speed from an abitrary pulse system when it has the ability to calculate the speed to a decimal of a mile per hour.

I await correction.....

[jaytee]

Without the vehicle speed signal(which also will calculate distance) , a gps only receiver with no signal due to high buildings/forest/tunnel or whatever will probably assume you have stopped, meanwhile you would be driving on and not be given a turn instruction until you picked up the gps signal again, by which time it would be too late and a big detour could be involved. The unit will calibrate to any size of wheel, but should be re-calibrated, as per my previous post if tyres or pessures are changed. The system uses map data combined with gps position, speed signal and gyroscope to calculate the position on the map display. If there is a loss of gps signal the map position display will still be accurate based on the gyroscope and speed/distance information. The gyro can detect a turn of 0.05° per second.

Jim

Thanks for the replies folks. Yes, my satnav unit requires a speed signal and has a gyrothingy so I assume it's posh and can continue to guide my wife even when there are not enough satellites in view.

Thanks also to Jim for offering to e-mail me a pic of the speed pulse terminal on the back of the instrument cluster. I have PM'd you Jim with my e-mail address.

Thanks again folks

Andy

[DieselDoNicely]

The speed is required for dead reckoning by the GSP unit. The loss of signal from the birds is compensated by dead reckoning. Its does have an error margin.

DDN