Wednesday, November 13, 2013

Fuel Gauge and Wipers - Wiring Again!

Unfortunately, I think I must enjoy torturing myself, as I've decided the next two jobs that simply MUST be done are to complete the wiring for the fuel gauge and the wipers.

Last night I sat down and read through the E21 wiring diagram again (available ex and worked out how the E21 wiper motor works; except no easy way to identify which pin was which until I found the following diagram: - this helped me work out which wire was which.

Then, following the basic wiring from this ( youtube video, I drew up a diagram for my application. As my existing wiper switch is on the toggle board, and is only a two pin switch fed ignition switched positive and simply outputting 12v when toggled, I'm going to need to implement a 5 pin relay to pull this off.

 I did a test run by simply supplying ground and hooking the switch to either the "fast" or "slow" pins on the wiper motor, and this showed that I had the right pins for those functions, but definitely that I need to wire in the "park" function. This is what the relay will achieve. I've got the relay ready; just need to wire it up and test - I need to find the easiest spot to pull an ignition switched positive from (it's probably best, if not easiest, to pull from the back of the wiper switch as this way the one fuse behind the control board covers all wiring related to this circuit if the wiper motor stalls and it won't cut out any other circuits.) 

Switching focus, I'd researched a little (per previous posts) on the fuel sender. The GM sender uses 0-90ohm, and the E21 is purportedly 3-74ohm. I was hand delivered a 390ohm resistor to run in parallel with the GM sender in the fuel cell and proceeded to solder it in to the wiring:

And crimped on a new ground (sharing with the fuel pump) 

A better shot than previously posted showing the fuel pump in situ - taken with my Canon 5D / EF 16-35mm f/2.8L 

Temporarily wired to the fuel gauge on the cluster for testing. The cluster has a common 12v feed (ignition switched) and the signal wires ground through the various sensors which either trip to ground (i.e. oil pressure switch) or supply varying resistance (fuel gauge)

I perhaps should have guessed something was up when the gauge was displaying empty with the cluster on and no sensor connected.

Connected, and with an empty tank, the car shows a full gauge! I pulled the filler cap on the tank and operated the float manually - the gauge is inverted!

At this point I've got two options - pretend that F on the gauge means "fill" and R on the gauge which I assume means "Reservekilometerzähler" or Fuel Reserve means "Ready" or "Race", or make an inverting circuit using an operation amplifier 741 and adding potentially unnecessary complexity to the system.

At this point I'm thinking a simple sticker stating "Tankanzeige invertiert" stuck at the bottom of the gauge will suffice to remind me of the situation - "Fuel Gauge Inverted".

Tomorrow I'll neaten up the wiring, solder and heat-shrink, and connect the relay for the wipers and test!

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