[T3] '73 Idle Test

[Jim Adney]

On 31 Aug 2019 at 16:24, William Jahn wrote:

> Jim when I read resistance on #30 & #87 I had both wires disconnected .
> What I saw was it was jumping around and I didn't record the resistance I
> do recall seeing above 25 ohms and the lowest was 9 ohms. If I turned the
> key off then on again it did the same thing

Okay, that was a reasonable way to do that measurement, but flakey 
contacts often give erratic resistance readings. Read on below.

> When I did the voltage drop test it was in mA on both #30 & #87 yet it
> was not steady either. 

If you're looking for voltage drops at bad connections, the best way to do this 
is to use your voltage scale. That will actually tell you the voltage drop, and it 
avoids the problem of damaging your meter if you happen to touch ground or 
find a connection with a large resistance (large voltage drop.)

> Once I took the cover off and manually closed the contacts keeping my
> hands clear of the probes so not to introduce resistance I still got no
> lower than 9.5 ohms and still was higher than 20 ohms just holding the
> contacts closed , I found that odd how the resistance would vary not
> moving the contacts and with no voltage or current to heat the
> contacts. 

Yes, once you get dirt between the contacts, nothing is certain or clear 
again. I wonder how dirt got in there; could the relay have been turned 
terminal side up for some time? Cleaning the contacts, and cleaning inside 
the relay shell, should give you a good fix. You could also use a point file, 
but don't use any kind of abrasive paper, as that can leave behind tiny bits of 
non-conductive grit, which can just repeat the problem.

> Once I cleaned them testing by hand and in the car with the coil
> energized it was a consistent 0.02 ohm , I didn't try the voltage drop
> again after cleaning the contacts. 

I suggest repeating the voltage drop test, using the voltage scale, just to 
confirm and see what you're getting. You'll be looking for voltage drops 
under 0.1 V. With the engine running, it will be interesting to see if you see 
jumps in the drop as the injectors fire.

>  I didn't have good results with both temps sensors connected , I may just
> pull the ECU and still connected engine running see what the voltage is off
> the power relay .
 
That would be another interesting test, but do it on the V scale, with the 
other probe on the battery + terminal. It will be interesting to see if you can 
see much larger drops on the wire that feeds the injectors. I don't remember 
which power input feeds which part of the brain, but you will probably be 
able to tell, but noticing which wire seems stable and which one your DVM 
has trouble reading. If they both have trouble reading, then you're still 
getting voltage drop back in the common part of the feed: in the relay, or the 
wire to the battery.

> Then pull the connector and check to each component for resistance to
> the temps sensors and injectors and see if I can move /wiggle wires to
> see if I can find an intermittent . Then the ECU grounds and perhaps
> with every wire unplugged see if any two wires have continuity , the
> grounds should have but not any of the voltage feeds or the MPS even
> the TPS on the TPS I can leave it connected and see if it reads the
> same as testing at the TPS unplugged.  I know it's a lot of testing yet
> I need to know if anything in the wiring is causing this .  

Yes, that's a LOT of work, and it's probably unnecessary, as those kinds of 
problems don't tend to happen unless the wiring harness has been severely 
abused. That kind of abuse shows up as obvious external damage: torn, 
frayed wires, melted or cut insulation, obvious repairs, etc.

OTOH, the Bosch D-jet testers make most of those checks	quick and easy 
to do.

-- 
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************