RE: [doc] Re : Wiring Harness

[Nick Tomlinson <nickandkathryntomlinson@xxxx>]

Hi, further to what Rich has said, why not remove the earth (ground) lead from the binnacle and wire in a supplementary test wire from this to the -vebattery post. This would eliminate any further (electrically) weak connections and/ or chassis earth points. If this solves the problem then what Rich has said is proven. If it works then wire in a permenant wire from a goodknown earth point (<0.1 ohms between point and -ve battery post) to the ground on the binnacle.
 
There is usually a voltage drop of between 0.5 and 0.7 volts across a diode. On this car there appears to be quite a lot of feedback protection diodes(70's wiring technology !) and it will only take 1 of these to either go 'leaky' or open circuit to cause maladies like this.
 
To check a diode it is best to use the diode function on a digital multimeter, this will measure the voltage drop (in millivolts so 500 is 0.5v etc). It should read 500 one way and nothing the other. If it reads the other waythen the diode is duff. I know you can check if a diode works by using theresistance reading on a meter but this is not always conclusive and does not detect any OOR (out of range) voltage drops across this.
 
BTW you should always get continuity from the earth point to the battery -ve post regardless how many fuses (if all) are inserted or removed.
 
Good luck, all the best,
 
Nick.

Richard <dickyh11@xxxx> wrote:
Hmmm,

This may or may not be some help...apologies for it's length

It sounds like your binnacle ground wire (assuming it is ground) is
getting voltage via a diode where it shouldn't be (hence the 0.5 - 0.7v)

However if this ground wire were actually connected to the
battery/chassis, then you would have +12v going from the battery via the
diode to ground, which would cause the erroneous diode to frizzle rather
quickly.

This leads me to believe that your binnacle ground wire might not
actually be connected to ground at all.

This explains why you get 0.7v reading between chassis ground and said
wire.
If it were actually connected to the battery/chassis, it should be
impossible to get a voltage reading.

Removing fuse 12 is killing power to a part of the system which is
feeding power via this diode, and turning on the ignition restores it
again. Fuse 12 is used solely to power the interior lights (glove box,
courtesy) and door lights, on the diagram, 41 (driver door switch) is
actually at two locations, the common wire PW between them is connected
to, guess what, the interior lighting circuit, position one on the
ignition switch and the buzzer box. I suspect the diodes on the interior
lighting circuit are the source of the stray voltage, but unlikely the
root of the problem.

I can't tell where the binnacle ground is on the cct diagram since it's
just marked as several ground symbols. Your next bet is to trace this
'ground wire' to find out: a) whether it is a ground wire and b) if it's
actually connected. This may also be causing strange effects on your
other lights since this 'floating' ground could well be finding other
paths through your lights etc.

Rich H



-----Original Message-----
From: jchapelhow [mailto:chapelhow@xxxx] 
Sent: 21 September 2003 8:07 PM
To: doc-uk@xxxxxxxxxxxxxxx
Subject: [doc] Re : Wiring Harness

All.

Further tests/symptoms now as follows.

1. I have discvoered that my battery cutoff switch doesn't! With the 
switch attached to the +ve terminal and in the off postion (key 
removed) there is still 0.5v measured against the -ve terminal. Just 
to make sure I pulled all of the relays except the 
main/accessory/RPM. Fan fail and all others disconnected.

2. As stated before the binnacle has been removed. To this pile of 
removed bits I have added the panel that holds the indicator 
relay/windscreen wiper control/buzzer logic box. The plastic relay 
holder for the indicator relay had snapped where the screw holds it 
down. 

3. The large black cable which attaches to the panel described above 
appears on the wiring daigram to be a ground point, this makes some 
sense to me. If not the following 'problem' may be irrelevant!
With either engine block or the front wheel ground point as the 
reference point for my voltmeter, I get continuity signal to this 
black wire only if the battery is disconnected, when the battery is 
connected continuity is lost and a voltage difference of ~0.5v is 
detected.
I found that if I removed fuse 12 (courtesies/door lights) continuity 
was restored until the ignition swtich was turnedto P1 when 
continuity is lost again. Voltage was detectable ranging upto 0.7v.

4. As a second test the now freely dangling indicator relay unit was 
measured, the black ground point shows the same symptoms as above.

5. The LHS steering stalk control wires were disconnected. Now the 
main light swtich still worked but only alllowed side lights to be 
turned on, main beam was not selectable.


As stated previously in all modes the 'door lock' / Aircon 
illumination lights 'flash' with the indicators. I am assuming that 
if these are still misbehaving if I were to reinstall the binnalce 
the gauges would also still fluctuate. 


Suggestions?

John Chapelhow
DOC 19 : 'Boeing' 0737



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