Series 1 temperature warning lamp

robertf

Well-known member
Jan 22, 2006
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103
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We all know the temperature gauge is useless. Adding an auxiliary temp gauge or a scan gauge clutters up the interior, and reworking the factory gauge to more linear would likely cause a panic attack when the needle isn't exactly where its been for the last 25 years.

I'm about to fire up a new engine so making certain it doesn't overheat is top of mind. I started to design a circuit to tap into the temp sending unit and compare it to a preset voltage and make a light on the dash blink when it hits a certain sending unit resistance. When I got around to assembling the circuit on a spare gauge cluster I discovered Land Rover had already designed a similar circuit but for whatever reason decided to not populate the circuit board.

Its a mix of diodes, resistors, and transistors. I salvaged most of it from another spare board, but I'll put together a BOM once I test it on a real vehicle.

They left off a couple of traces on the flexible circuit board on the gauge cluster, you need to run wire from the ground side of the light and from the temp sending unit. I tried soldering and I'm really glad this is a spare gauge. I'll be using some conductive epoxy on the next one. And for wire strain use silicone. I first used some MEK based adhesive and it started to dissolve the flexible circuit board.

Below is a video of its operation. I've exaggerated the needle movement by shorting it. I have it set to go on around 29 ohms which is about 230F. At that temp the needle is still parked in its normal slightly below middle position.

The images are the flexible circuit board jumper wires, the unpopulated gauge driver board, and the populated board. There are a few items I left out of the board, and the brown wires are for testing. They would be a solid jumper in normal operation.



flexiblecircuitboard.JPGunpopulated board.JPGpopulated.jpg
 
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robertf

Well-known member
Jan 22, 2006
4,288
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LOCATIONDESCRIPTIONQUANTITYPRICEEXT PRICEPART NUMBERMOUSER PART #
C5,C9Ceramic Disc Capacitors 1Kvolts 33pF 5% .25LS NPO
1​
0.85​
0.85​
561R10TCCQ3375-561R10TCCQ33
C12Ceramic Disc Capacitors 1Kvolts 10pF 5% NPO/N750 .25LS
1​
0.77​
0.77​
561R10TCCQ1075-561R10TCCQ10
R9Metal Film Resistors - Through Hole 4.7Kohms 1% 50PPM
1​
0.22​
0.22​
270-4.7K/REEL-RC270-4.7K/REEL-RC
R10Metal Film Resistors - Through Hole 1/8watt 1Kohms .1% 15ppm
1​
1.05​
1.05​
PTF561K0000BXR671-PTF561K0000BXR6
R3,R4Metal Film Resistors - Through Hole 1/8watt 10Kohms .1% 10ppm
2​
0.88​
1.76​
PTF5610K000BYR671-PTF5610K000BYR6
R6Metal Film Resistors - Through Hole 10K OHM 1/4W 1%
1​
0.12​
0.12​
MFR-25FTE52-10K603-MFR-25FTE52-10K
R5Metal Film Resistors - Through Hole 1/4 WATT 22K OHM 1%
1​
0.19​
0.19​
MF1/4CCT52R2202F660-MF1/4CCT52R2202F
R7Metal Film Resistors - Through Hole 1M OHM 1/4W 1%
1​
0.12​
0.12​
MFR-25FTE52-1M603-MFR-25FTE52-1M
D3, BULB CHECKDiodes - General Purpose, Power, Switching Hi Conductance Fast
2​
0.10​
0.20​
1N4148TR512-1N4148TR
ZD2,ZD5Zener Diodes Diode Zener Single 7.9V 3% 500mW 2-Pin
2​
0.20​
0.40​
NZX8V2A,133771-NZX8V2A,133
TR3Bipolar Transistors - BJT NPN Epitaxial Sil
1​
0.43​
0.43​
KSC2328AYBU512-KSC2328AYBU
TR1,TR2BJT PNP Prebiased 10K OHM
2​
1.76​
3.52​
NTE2356ORDER FROM NTE DIRECT
TOTAL
9.63​
 
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robertf

Well-known member
Jan 22, 2006
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I plan on adding this to it as well, that way if the coolant gets low the dash goes crazy.

C219-2201/4" spade connector
2​
tank cap
AC69-15000106-TSumitomo 090(2.3mm) Female Contact 0.3-1.25mm
1​
fuse block power tap 94-95:C208-9 96-99:C208-1
C261rubber sleeve
11​
tank cap crimps, ground crimp
c306male bullet connector
1​
sending unit intercept
c307female bullet connector
1​
sending unit connector
c2523/16" eyelet
1​
ground
c820relay
1​
c7044mm ID sleeve
4​
ground to junction, power to relay
c7055mm ID sleeve
4​
float switch to junction. Power to relay
c7066mm ID sleeve
4​
sending unit tap to relay,ground and float junction
c71010mm ID sleeve
4​
ground and float sw junction to relay
c71614mm ID sleeve
2​
relay to all the above
c128white/green 14 gauge
6​
power to float sw
c128green/orange
3​
float sw to relay
c128black
3​
eficonnectiongreen/blue wire
2​
sending unit tap
FG21-51153378-Hred relay base
1​
relay
AC57-42100-Trelay terminal 14-18g
4​
gnd,pwr,floatsw
 

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robertf

Well-known member
Jan 22, 2006
4,288
103
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instead of the above circuit for the warning lamp, I'm going to use the blank warning light (TC on 94/95) and wire it to a relay that is activated by the float switch. Copied the LR3 warning lamp over to it. The RRC lamp was a little grungy, and the newer one fits the rectangle better

I can only find the terminals necessary to add the lamp from UK sources, so waiting on that.
 

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robertf

Well-known member
Jan 22, 2006
4,288
103
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Wired up with the conductive epoxy. I accidentally swapped wires, thats the reason for the splice in the middle. Would have been an easier fix if the ribbon could take the heat of real solder

I also missed R8. It needs to be a 4.7k 1/8W resistor. Its used for the 5v pull up on the op amp output.
 

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