TR 8 Puzzler?

[John Morrison]

Hi all, 

 

I list a question from Tr8 owner and wonder if anyone can help?

John.

Question:

What is wiring link between oil pressure switch, the lamp on dashboard and the fuel pump.

The pressure switch, 3 prong version connections:

P-S = Normally Closed and Opens when pressurised.

S-I  = NO and when pressurised unchanged.

I-P = NO and when pressurised closes.

 

With Ignition On, dash lamp is on. White wire read 12 Volt and White & Brown reads 5 Volt.

What is connection required for the Fuel Pump to kick in ?

 

Thanks

 

John

[John Morrison]

...and a subsequent reply from John, any thoughts anyone?;-

 

I am a member of the TR Action and linked to the Midlands Coventry group.

 

It seems that I have solved my problem, I know what I have done but do not understand the why.

The car reg is LSG 350 a Grinnall conversion to V8.

On the various circuit diagrams I have, there is no mention or detail of a fuel pump.

 

In order to check my oil pressure I removed the Oil Pressure, 3 pin switch.

Ignition ON, Red oil pressure warning light on dashboard on. Car will not start.

Finally I shorted the White wire to the White / Brown wire, normally connected to pressure switch and dashboard light out, fuel pump clicks and car starts.

 

Refit the old pressure switch and connect the White wire to the P pin and the White / Brown wire to the I pin and all works.

 

Of course I have no idea how. With the Ignition ON, the white wire provides full 12Volt and the White/Brown  6 Volts when disconnected from the switch.

 

Thank you for your response.

Feel free to put this to the forum, together with the answer. I would be interested in an explanation.

 

Kind regards

 

John

[RobH]

I have a schematic for the TR8 which does show the fuel pump connected to the oil pressure switch.  

http://www.team.net/TR8/tr8cca/wedgelab/other/schematics/TR8_schematics.pdf

This looks like some sort of interlock and by linking the wires, the interlock was defeated.  I think the purpose is to prevent the fuel pump running if the ignition is left on but the engine isn't running.  There is a feed from the starter solenoid that over-rides this so the fuel pump is running when cranking. Once the engine starts, the fuel pump is powered directly through the oil pressure switch. 

 

I have a feeling from the description above that the wires may be on the wrong pins of the switch.  Unfortunately the schematic doesn't give the pin lettering  But WT/BN (power out to the fuel pump) should be on the switch wiper contact (P ?),  Wt ( ignition switched power) should be on the NO pin (I ?)and  Wt/Or  (power when cranking) should be on the NC pin (S ?).  

[ that would seem to make sense if P=pump, I= ignition and  S= switch ]

Edited October 29, 2025 by RobH
more information found.

[stuart]

44 minutes ago, RobH said:

I have a schematic for the TR8 which does show the fuel pump connected to the oil pressure switch.  

http://www.team.net/TR8/tr8cca/wedgelab/other/schematics/TR8_schematics.pdf

This looks like some sort of interlock and by linking the wires, the interlock was defeated.  I think the purpose is to prevent the fuel pump running if the ignition is left on but the engine isn't running.  There is a feed from the starter solenoid that over-rides this so the fuel pump is running when cranking. Once the engine starts, the fuel pump is powered directly through the oil pressure switch. 

 

I have a feeling from the description above that the wires may be on the wrong pins of the switch.  Unfortunately the schematic doesn't give the pin lettering  But WT/BN (power out to the fuel pump) should be on the switch wiper contact,  Wt ( ignition switched power) should be on the NO pin and  Wt/Or  (power when cranking) should be on the NC pin.  

 

Similar setup on XJ6 series 2, its so there needs to be oil pressure to allow the pumps to run, I got caught out on one years ago with failure of the oil pressure switch that governs that.

Stuart.

[BlueTR3A-5EKT]

Same arrangement on Metro Turbo.

Low or no oil pressure and the fuel pump cuts out.  It is overriden by a circuit from the starter motor, so you can start the engine.  Bit like the pink ballast resistor wire in a post 73 TR6.