2008 ZX10R Race ECU Fan Switch

Temp sensor is ECU pin 25 (O).

Radiator relay control is ECU pin 30 (P/BL) and should be switched to Gnd to activate. Confirmed.

Power Supply to Sensors is ECU pin 7 (BL) and according to the 2006/07 service manual (DFI specifications) should be 'C 4.75 ∼ 5.25 V between BL and BR/BK leads' Confirmed.

Ran the bike on stock ECU until fan switched in at displayed 96c and switched just prior to 97 switched off about 92c.

Measured voltage of sensor wire when ECU connected:

Displayed temp
Measured voltage on temp sensor at ECU connection
 Action on bike
 80  0.67  
 85  0.6  
 89  0.55  
 90  0.54  
 92  0.508  Fan off (approx)
 93  0.495  
 94  0.485  
 95  0.482  
 96  0.475  Fan on
 97  0.464  

So we will need to design a voltage comparator to activate the fan relay switch when the temp sensor output voltage gets below 0.475v and switches it off when the voltage rises to 0.508v

Note: The 2006/07 service manual says the Temp sensor output voltage at the ECU is between 2.80-2.97 at 20c which differs from that measured above.. Confirmed.

So, solution needs to turn on at 0.475 volts and turn off at 0.508 volts (approx). 

Parts required (protoype parts sourced from Jaycar.co.nz & Dick Smith NZ):

    Physical  Datasheets Cost proto NZ$ (DSE or Jaycar Cost bulk NZ$ (excl freight) Bulk supplier/note
LM334Z adjustable current supply limit the current into the LM336 and allows input voltage 3-40v  (expect 12v supply)  TO92 www.st.com  3.60  0.80 es price per 25 units
(digikey US$0.88 ea per 25)
LM336Z25 voltage reference stable voltage reference of 2.5v  TO92 www.fairchildsemi.com  3.95  0.70 es price per 25 units
(low stock) (digikey US$0.71 ea per 25)
LM339N Quad Comparator Comparator, only 1 channel required  PDIP14 www.st.com  1.70  0.20 es price per 25 units
(digikey US$0.31 ea per 25)
BD682 PNP Darlington Output switch.  (Low side). The comparator output will switch on the Fan Relay via this transistor.  Since the LM339 can only sink 16mA max we will use a darlington. We will allow for up to 200mA relay current (expect < 100mA max) so we are well within limits.   www.fairchildsemi.com  2.38  0.90 digikey have TIP107TU US$0.44 ea per 25
(es have no PNP darl suitable)
Trimpot voltage divider so we can set comparator reference voltage      0.50  0.10  
various resistors/capacitors as needed        1.00  0.50  

We will use a comparator on a LM339 (can operate off a single supply 2-36 volts), with a reference voltage set by a trimpot connected to a LM336-2.5 voltage reference chip. 

We will add positive feedback to the reference to get our on/off range.  We add it to the reference as we need to have minimal impact on the temperature input signal else we may effect the reading displayed by the ECU on the dash.

Inverting or non inverting comparator?

VTL = 0.475

VTH = 0.508

Our signal falls as the bike warms up, so we need to turn on at VTL, and use positive hysteresis to turn off at VTH.  Hence we will use Non Inverting (VRef to -, input to +)

This means the comparator output will be low when the input < VRef (set to VTL) and will stay low until VTH is reached (determined by the hysteresis).

Turn on = Low means we will need to use PNP output transistor as the relay circuit requires a low side switch.

First revision:

A regular automotive coil has approx 85R so we will assume 100R for a safe margin.

Load current = 12v/100R = 120mA, LM339 can sink 16mA

hFE(min) >= 5 x (0.12 / 0.016) =  5 x 7.5 = 37.5

The BD682 is a medium power PNP darlington: Ic = 4A, hFE = min 750 so slightly more than is needed.

Base resistor = ((Vc x Hfe) / (5 x IC))

= ((12 x 750) / ( 5 x 4)

= 9000 / (20) = 450R

 IB = 5/450 =


RB = 0.2 × RL × hFE  ??? Where did 0.2 come from?