2023 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.

Engine Coolant Temperature Sensor

K75, K100, K1 & K1100

Introduction:

The engine temperature sensor is located in the metal coolant exit pipe coming out of the cylinder head at the top left front of the engine behind the lower left corner of the radiator. This applies to all K75s, K100s, K1s and K1100s.

Example: K75 Temperature Sensor Location

(all other Ks are similar)

Note: For Ks equipped with a round temperature gauge there is a second water temperature sensor located at the bottom of the water pump in the drain hole. It only connects to the temp gauge and has nothing to do with engine fueling or fan operation.

Access K75/K100 Coolant Temp Sensor: Follow this link.

Temp Sensor Part Numbers: You can buy the Bosch temp sensor from your BMW dealer but it's usually less expensive to buy it using the Bosch part number.

K Bike Model/Year BMW Part # Bosch Part #

All K75s

61311459197

0280130032

or

0280130069

2 Valve K100s

K100RS4V

13622242184

0280130055

K1

K1100s Model Year 93 & Earlier

K1100s Model Year 94 & Later

12611341602

0280130079

 

How The Cooling System Works

K75/K100: The dual temp sensor sends signals (resistance to ground) to both the L-Jetronic for fuel control as well as to the white temp relay in the relay box to control radiator fan operation and the instrument cluster warning light.

K100RS4V/K1/K1100: The temp sensor sends a signal (via resistance) to the Motronic to control fueling, radiator fan operation (via the fan relay) and the instrument cluster warning light.

Fueling: Based on the resistance from the temp sensor either the L-Jetronic or Motronic controls how rich/lean the fuel/air mixture is. As the temperature increases and the temp sensor resistance decreases the fuel/air mixture is leaned out.

Cooling System States: K bikes go through several states as the engine heats up.

(K75/K100 values in red, 4V values in green, black for both)

State C F   Description
Normal 85 185   Thermostat opens so water flows through radiator for more cooling. This is the usual state when a K bike is moving with the thermostat opening and closing as needed.
Warm 103

105

217

221

  Fan turns on to get more air flowing through the radiator. Usually only occurs when bike is stopped in warmer weather.
Hot 111

115

232

239

  Warning light in instrument cluster illuminates. Usually means that the fan is not working. If you turn the bike off when stopped and then restart right before getting moving again then you can usually still ride until you get to your destination. If the light stays on once you're moving again then pull over.
Overheating 120 248   Radiator cap blows. Similar to a car, K bikes have a radiator cap under the right front of the tank that will "blow" - spewing coolant on the right side of the bike and your right boot. (From the coolant reservoir.) If you see steam and your right boot is wet then pull over.

The radiator cap actually blows due to pressure, not temperature. The cap blows at 1 bar (14.5 psi)

When the system is cooling down the radiator cap lets coolant be sucked from the coolant reservoir at -0.1 bar. (-1.45 psi)

Super Nova ? ?   I've heard that somewhere above 120 C the L-Jetronic or Motronic will shut down to prevent engine damage but I've never tested that and it isn't documented in the BMW service manuals like the above temperatures are.

 

Temp Sensor Troubleshooting

Symptoms:

Poor mileage - If a temp sensor is failing then the L-Jetronic or Motronic will think that the engine is cooler than it really is and the fuel/air mixture will be too rich.

Overfueling: - Fuel in exhaust headers

Dark exhaust fumes - Because the mixture is too rich.

Fouled spark plugs - Because the mixture is too rich.

Overheating when stopped - Because the radiator fan is not coming on when it should. Note that this can also be caused by a failed radiator fan.

K75/K100(2V): The temp sensor is a dual sensor. It grounds via the mounting threads and has two VIO/GRN wires: one to the L-Jetronic control unit to control fueling and another to the white thermostat relay on the right side of the relay box under the tank.

Wiring diagram:

Click to enlarge

1) Test the temp sensor by using an ohmmeter to measure the resistance between the Pin 10 of the L-Jetronic connector and ground. Pins 5 and 13 are ground but any ground on the bike should work.

Just make contact with the connector terminal. Do not jam the ohmmeter probe into the terminal as that might damage the terminal.

2) Test the thermostat relay lead of the temp sensor by measuring the resistance between Pin E of the thermo relay socket and ground.

The blue column in this table shows the specification resistance for each temperature but readings within 10% should be OK.

C F 90% Ohms 110%
0 32  5,306   5,896   6,486
10 50  3,413 3,792   4,171
20 68  2,250 2,500   2,750
30 86  1,536 1,707   1,878
40 104  1,058 1,175   1,293
50 122     751 834      917
60 140     536 596      656
70 158     392 436      480
80 176     291 323      355
90 194     219 243      267
100 212     168 187      206
110 230     130 144      158
120 248     102 113      124
130 266       80 89         98

K100RS4V, K1 & Early K1100s: Test the temp sensor by using an ohmmeter to measure the resistance between pins 21(VIO) and 32(BRN/YEL) of the Motronic connector. Just make contact with the connector terminals. Do not jam the ohmmeter probes into the terminals as that might damage the terminals.

Note: I do not have the data specs for this sensor. If you have them please email me at smithduck@gmail.com to update this page.

K1100s - 94 & Later: Test the temp sensor by using an ohmmeter to measure the resistance between pins 7(VIO) and 14(BRN/YEL) of the Motronic connector. Just make contact with the connector terminals. Do not jam the ohmmeter probes into the terminals as that might damage the terminals.

 

Temp sensor torque - 9 Nm (6.6 ft-lb, 80 in-lb)

Fan Troubleshooting

K75/K100(2V): The fan should run when you put 12V+ to the A2 terminal of the thermo relay socket. You can do this by shorting Pin 15 to Pin A2 of the socket.

If you ground Pin E with the relay plugged then the fan should come on and the temperature warning light in the instrument cluster should illuminate.

K100RS4V, K1, K1100: The fan should run when you ground the BLU/YEL wire to the fan relay. (Pin 85)

 

 

2023 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.