Smiths GPS speedometer problem

[Stevensonjr]

The problem is when I turn off the ignition switch

after a drive, the feedback from the fuel pump and electric engine cooling fan prevents the speedometer from recording the trip mileage for that drive. If I remember to turn off the fuel pump and fan before I turn off the ignition switch the speedo records perfectly.

I think one way diodes would solve the problem but I don’t know exactly where to put them. GPS power? fuel pump? engine fan wiring? 
The most information I got from Smiths in the UK was, “that happens sometimes”.   Any ideas anyone?

[7Westfield]

feed it thru a relay?

[MV8]

Looking at an early seven schematic, the fan is in parallel to the ignition switch. Green speedo wire is for switched/keyed power (white wire) and red/white speedo wire goes to instrument lights (red).

The gps reciever is controlled by the gps harness white wire to the speedo. Is this your harness? Where did you make the connections?

 

 

Edited June 13, 2023 by MV8

[Stevensonjr]

I wired my 7 using aircraft wire with 8 separate fused circuits, didn’t like the stock harness with 2 fuses.

I guess my main question is, do you put the diode on the + wire or the - wire to the fan motor or fuel pump. Which wire would feed back follow? Wired old style, no relays.   Bill

[MV8]

I suggest swapping the main switch for a dual pole to isolate the fan and pump buss from the other loads/main buss that do not have a parallel path to power that can back feed. However, the positive leads from the fan and pump that go to the main power switch for the car can have diodes in series and with the diode stripe toward the fan and pump. The diodes need to be sized (watts) to handle the full fan current with no relay. There is still some reverse current flow commensurate with the diode size. Much better to swap the switch or use standard relays with the existing switch gear.

Edited June 15, 2023 by MV8

[MV8]

A DPST switch. Four terminals.

[Stevensonjr]

I was not familiar with a dual pole switch. Total isolation of the 2 feedback sources  is a great idea, new to an electrical dummy like me. Thanks for your advice and time spent drawing this up.   Bill 

[jbcollier]

I'd suggest starting with putting suppressing condensers on the fuel pump and rad fan power feeds, preferably right at the units themselves.

[Stevensonjr]

I’ll investigate that idea too, Thanks.   Bill