[T3] '73 Idle Test

Jim Adney jadney at vwtype3.org
Sat Aug 31 14:37:50 PDT 2019

On 30 Aug 2019 at 12:59, William Jahn wrote:

> I always thought because I've done voltage drops before if I connected
> the voltmeter to batt poss and then probed the relay connected so it
> has current flow I would read batt voltage at both #30 & #87 

Yes, looking for excessive voltage drops is a reasonable thing to do, and it 
looks like you found a problem in your relay. I don't understand how you got 
a measurement of 47 Ohms, as that's so high that the system should not 
have run at all, so I don't believe it. At 47 Ohms, you'd have a voltage drop 
of 47 Volts in a system that only has 12-14 Volts to start with.

I wonder if you were trying to read resistance with the circuit live, which can 
lead to extremely misleading results. 

> I did this without the engine runner just key on and #30 read 0 mA
> telling me the wire was good no drop yet #87 read 2 volts less less
> than battery voltage which would the the resistance in the relay
> contacts. . 

After I sent my last message I realized that I was wrong about this being only 
possible with the engine running. That would be true for the fuel pump relay 
but not for the main FI power relay. Doing this with the key simply ON was 
the right way to do it.

Tests like this, should be done in Volts mode, not current or resistance 
mode. In current mode, if you find a bad connection, you're likely to damage 
your meter. In resistance mode, the extra voltage drop created by the 
external current will give a wrong reading, and the reading will be negative 
(negative resistance!) with the probes in one of the two possible positions.

I still recommend trying a road test, with both temp sensors connected 
properly. It's impossible to go by the gas gauge, because the FI gas gauge 
readings are misleading. The 1st quarter tank is MUCH bigger than the last.

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

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