Speedometer sending unit frequency

[Vigri]

Hi,
I have some problems with my Maserati 222SR speedometer. The problem is that the speedometer shows distance correctly but speed ~20% less.
I took it to one specialist and he asked me to find out what should be the signal frequency from the speedometer sending unit. To calculate it I need to know how many impulses the speedometer sending unit sends out with every propeller shaft rotation. Does anyone know it?
I have the ZF 5 speed gearbox.
I tried to measure it also when driving but I couldn’t get any consistent results.

BR,
Vigri

[pantera874]

Is it 20% at all speeds? If so, you can easy pull the needle and plug it back in at another position. I did this with mine years ago. It is a bit of try and error.

[Thorsten Ruge]

It can be a general offset like Roland mentions it but I believe in missing power to "pull the needle up". Get the cable connectors down from the impulse transmitter at the ZF gearbox, clean/scrape everything very seriously incldg. the covering "rubber condome", look that the cable plugs are strong in place and not bridging by contamination at both sides, that should be it for most cases.
If not you have either electrical restrictions inside the dash unit plugs, bus foil or pin connectors into the speedo unit at the dash. Last not least there is an adjustment poti at the speedo unit itself, but like I mentioned above, the cleaning at the ZF gearbox switch only solves it.

[lindi-02802]

Hi Vigri,
normaly you have the Information on the backside of the instrument.
In my case I have 4196 imp/km. With this information you can calculate the frequenz for any speed.
sample:
speed = 100 km/h => 100km/h x 4196 Imp/km= 419600 Imp/h
this are
419600 Imp/h / 3600 s/h = 116,6 Imp/s = 116,6 Hz

best regards
Georg

[Vigri]

Thanks Georg, I will check tonight if mine has this information left on the instrument.
It cannot be the needle because on 50km/h it is ~10km/h wrong but on 100km/h it is ~20km/h wrong. If it would be the needle, then it should be the same km/h wrong all the time?
Previously in addition to wrong speed the speedometer needle also jumped quite a lot and after redoing the connections of the impulse transmitter on the ZF gearbox this problem was resolved but the problem of incorrect speed still remains.
As the speedometer distance counter shows 100% accurate distance, then my only guess is that the problem must be inside the speedometer unit itself.
BR,
Vigri

[pantera874]

Take it as this: It's an italien car with an italien speedometer. All this cars and speedos are only made to impress the spectators.

[lindi-02802]

Hello Vigri,
if it´s the needle on the pin you have every time the same difference. I think there is a problem on the electronic of the speedometer. Most problems are broken solderings.

The car is from italy but the Jaeger instruments are from france.

bb
Georg

[Thorsten Ruge]

Of course, you can guess what you want Vigri
But if the speedo unit does not have enough power it will deviate exactly like yours.
So clean the darn contacts at the gearbox induction unit like m.a. first, that's a 5 min. job, drive the car for a test, and if it is not solved, dig into the more time consuming, delicate things.

[Vigri]

Thorsten, I have already checked the connections on gearbox side and I cleaned and redid all the connections there. It resolved the problem with bouncing gauge but it didn’t resolve the accuracy problem.
Also the odometer gives 100% accurate reading so my only guess is that the problem needs to be inside the speedometer unit.
On the back of my speedometer unit it was 3548,5 Impulse/km.
I’ll try to drop the speedometer off to my electrician to see if he can now find the fault or not.

[Thorsten Ruge]

You see Vigri, half the way is done, thx. for the update!

There might be some metal-gunk coating at the rear end of the induction unit inside the gearbox, so that it does not catch high frequent impeller pulses anymore. Screw the sending unit out and clean it.
Also, the axle bearing at the bottom of the speedo unit might have dried out. You gotta drop very tiny drops of acid free oil, I always prefer Jojoba mechanical oil, on the top of the pin, and let'em run down. After that, turn the pin from hand from 0 to full engagement, that should finally do it.