My cherokee 91, shows ca.15km/h wrong(to low) speed.. How can i fix this? Thanks Johnny

how much is the RPM showing in the datatlogger while cranking?

HI,
Strongly suggest taking to the dealer as it should still be under warranty.

Code info I googled:

OBD II Fault Code P1F68 :Description of OBD II Fault code P1F68

TH2TNK Sensor Value Offset Shifting
Possible failuer of OBD II Fault code P1F68

• Hydrogen Tank Internal Temperature Sensor (TH2TNK) offset shifting
• Inlet Air Temperature Sensor (TA) offset shifting
• Offset shifting of both Teperature Sensor (TH2TNK) and Inlet Air Temperature Sensor (TA) in Hydrogen Tank
• RTC function error (Miscalculated Soak Time)
• FSVSCM failure
• FCCM failure

Diagnostic Procedure using AutoHex auto Scan tool tester

• The rear average wheel speed > 15km/h and vehicle speed on the meter<1km/h
• If the rear average wheel speed > 15km/h and VSP<1km/h lasts for 0.3 seconds, the malfunction is determined.

Test Conditions using AutoHex Scan tool tester

• Vehicle speed / wheel speed is normal
• Brake is out of action
• The automatic brake is out of action

Clear Fault codes Conditions

• The rear average wheel speed > 15km/h and vehicle speed on the meter<1km/h
• If the rear average wheel speed > 15km/h and VSP<1km/h lasts for 0.3 seconds, the malfunction is determined.

MIL Status

• Glow ON

Note:

• AutoHex is a professional OBD /OBD II diagnostic scanner / scan tool tester.
• This fault code is a GERENRIC fault code, some cars may change the meaning of this fault code.
  To get the accurate fault code description you may use AutoHex OBD scanner

Has to be calibrated on a rolling road with the correct software ,or you have the wrong size tyres fitted ,they are low profiles making the wheels go round faster in relation to the speedo calibration

Your Jeep has limited slip diffs.The last time you had it serviced,is probably when the problem started.Think.Was it?What the problem is,,the wrong diff oil was put in.At slow speeds,the plates in the diffs are shuddering.You can get some additive called friction modifier additive and in each diff you need 2 fluid ounces of the stuff.Or change all the diff oils..
Once the additive or new oil is in,,drive the vehicle in very tight slow turns for about 5 mins.A shopping centre carpark is good.This helps spread the oil throughout the diffs and should very well fix the problem.the additive is the cheaper way to go and does work.Hope this helps you

sounds like the oil pump
install a gauge and measure pressure
do not rely on the dash gauge
most of the time they won't show you a problem until you have less than 5psi

likely you have the wrong gear in the speed sensor there are different ones available for different tires and differential gear ratios.

P1682 - Charging system voltage to low.

is there any play in the transfer case output shaft

Thats a bad fan switch
as you stated it works at lower speed....
replace the switch
LB