[T3] 73 T-3 wiring / grounds / voltage drop .

[William J]

 All the years I've owned this car I've looked at the wiring schematic just to get an idea of how it works. 
 
 I never had electrical  issues other than the fuel gauge and a few voltage regulators . I've only really worked on and been through the FI wiring .

 I stared out tightening the FI small connectors on the FI engine componants and it helped a little . 

 Mainly it's the grounds that seem to be an issue. Once I cleaned and tightend the FI to case grounds it helped quite a bit . Especially the one ground that had 2 wires to 1 encased in a now black once clear plastic connector with 1/4" spades we have all over these cars. 

 I have been chasing this no load idle erratic miss as some might know. I also have been in the dash chasing a since I've owned this car fuel gauge jumping once it gets below 1/2 tank , tap the dash near the gauge when it drops to 1/4 and it jumps back to 1/2 , if it gets near the right of the red mark tap it it moves to the 1/4 mark if not I know I'm near empty. And then there is the horn.

 This car still has the original ignition switch and I have a new one I got over a decade ago. 

 Yesterday I decided to do a voltage drop test mainly on the ground side since this is where most issues appear to be. I started at the battery posts engine running DMM set on auto range volts, checking across the posts and cable 0 volt drop as some call it ground zero. You have the poss side to the load and the load to the ground. 

 I then knowing the battery pos and neg were 0 volt drop checked from the battery neg red lead from DMM to battery neg , black lead to engine case had 35 mV or 0.035 volts nothing really. 

  I checked the coil all leads unpluged to see what the primary resistance is 3.4 ohm.

 My  point and thinking is the car starter cranks over fine yet most of the time the ign switch is on the run circuit powering everything . From the ign switch to the coil there is one wire , the only other connection at the fuse box is a connection at the hot side bridging two fuses. 

 I had the DMM connected at the battery neg to the coil #15 pos and read 13.8 volts engine running and the same across coil #15 to the case also 13.8 . across the battery posts it is 14.8 volts a 1 volt drop . I haven't checked across the fuse or the ign switch yet . The coil is not a high current draw . I realize no wire is a perfect resistance free deal. 

 I feel if I have 1 volt drop it's possible the switch has resistance and this might be part of this erratic miss I have. Using an ohm meter is what ever voltage the meter has to push voltage and is not as accurate as a DMM voltage drop test. I also have a pertronix and don't know what it draws . 

 I'm just trying to  rule out ignition before condeming the FI . 
 I do know if I unplug temp sensor 1 all the miss goes away and I no longer hear or feel the erratic miss at the tail pipe and I know the  electric AAR does not always close fully. 

 I also feel if I check for voltage drop by back probing the ECU connector to each componant in the fuel injection engine running where any vibration may come into play rather than using an ohm test engine off I might find issues I miss with an ohm meter. I don't think checking the FI from the ECU checking for voltage drops will harm anything because you are not injectiong voltage from the DMM. 

 Just working in the dash with my big mits to pull the gauges left me with multiple cuts . Since I need to pull the steering wheel to repair the horn contact I might as well replace the ign switch. I know I can be crafty and drill a small hole right where the Ign switch tiny screw is and replace it that way. 

 I did find it odd that if I check for voltage drop on the engine case I see 0.035 volts then at the generator ground I see 0.143 . it could be because it's a rebuilt Bosch and it was painted because it should be a good ground to the case  or it's because it has it's own ground to the regulator to the body which shouldn't matter. My DMM has a 10 Mohm input impedance on the DC volts. 
 I don't like that there are so many unfused hot leads yet without using either fusable links or adding fuses as long as the leads have good insulation they should be fine. 


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