KITAS and Fleet Tachograph Basics: Senders, Speed Pulse Generators and Why the Pairing Matters

A tachograph is only as accurate as the sender feeding it. The sender sits on the gearbox output and turns shaft rotation into an electrical pulse train; the tachograph head counts those pulses and works out speed, distance and driving time from them. Fit a sender with the wrong probe length, the wrong connector or the wrong pulse type and the head either reads the vehicle wrong or refuses to pair at all. On a certified fleet that means a failed calibration and a truck or bus off the road. A sender that bolts up is not enough; it has to match the gearbox and the head.

This note covers what the sender does, how a KITAS tachograph sender differs from a plain speed pulse generator, and how to read the part so you order the right one the first time.

What the sender does

The sender reads the gearbox output shaft and produces a pulse for each increment of rotation. Two details decide whether a given sender suits a vehicle:

• The probe: the length and diameter of the part that threads into the gearbox housing and sits over the rotating element. A 19.8 mm probe and a 35 mm probe are not interchangeable; the probe has to reach the target without fouling the casing.
• The connection: the terminal and pin count the head expects. A 4-pin round terminal is common on commercial senders, but the pinout and the signal type still have to match the head.

Get the probe and the connection right and the head sees a clean pulse train. Get either wrong and the reading drifts or the pairing fails.

KITAS senders versus speed pulse generators

Both parts read the gearbox and both output pulses, so they look similar on the shelf. The difference is what the signal carries.

A KITAS tachograph sender produces an encrypted signal. The head and the sender share a key, and the head rejects any pulse train that does not authenticate. This is the part on a digital tachograph installation that keeps the recorded data tamper-evident, which is the whole point of a fitted tachograph on a certified vehicle. The KITAS II+ Pulse Generator in this range is a 19.8 mm probe, 4-pin round terminal sender of this type.

A speed pulse generator produces a plain inductive pulse with no encryption. It feeds a speedometer, a fleet-management box or a cruise system that only needs to know road speed, not to certify it. The Speed Pulse Generator with the 4-pin M22 x 1.5 female connector and the Integrated Pulse Generator with the 35 mm probe are inductive senders of this kind.

The rule that catches people: a speed pulse generator will not stand in for a KITAS sender on a certified tachograph, because the head will reject the unencrypted signal. The reverse substitution is wasteful, since you pay for encryption a plain speedometer never uses. Match the sender to what the head actually needs.

Reading the part to identify it

Work from the sender in your hand and the head it has to feed:

1. Read the part number off the body. Commercial senders carry a printed number; that number is the fastest route to the exact match.
2. Measure the probe: length and diameter. 19.8 mm and 35 mm are two common probe sizes and they suit different gearbox housings.
3. Identify the connector: pin count, terminal style and thread. A 4-pin round terminal and a 4-pin M22 x 1.5 are both common and they are not the same fitting.
4. Establish the signal type: encrypted KITAS for a certified tachograph, or plain inductive for a speedometer or fleet box.

With the part number, the probe size, the connector and the signal type, the matching sender is straightforward to confirm. Without the part number, the probe size plus the connector plus the head model is usually enough to pin it down.

Why the matched pairing matters on a fleet

A tachograph that reads speed wrong puts the recorded driving hours wrong with it, and a digital head that rejects an unencrypted sender leaves the vehicle without a working tachograph. Both outcomes fail a roadworthy or a calibration check. The cost of the wrong sender is not the sender, it is the day the truck or bus spends off the road waiting for the right one. Order the sender matched to the head and the gearbox, and the install passes calibration on the first fit.

How to order or request a quote

Send the head model, the gearbox probe size and the part number off the old sender if you have it, and we will confirm the matching KITAS sender or speed pulse generator and the lead time.

On Omnipart, the VDO KITAS II+ probe senders and the VDO KITAS screw-on sender for an M22 port are listed with their prices. Browse the Sensors & Senders range, or request a quote with your tachograph and gearbox details.