[T3] '73 Idle Test

Jim Adney jadney at vwtype3.org
Sun Sep 1 08:38:29 PDT 2019

On 31 Aug 2019 at 18:58, William Jahn wrote:

> Jim how did you come up with the voltage drop? I only have two values the
> resistance and the voltage , with ohm's law I just get current.

I meant to include the phrase, "assuming a 1 Amp load", which seemed 
reasonable, and made the calculation simple. I forgot to type that, so that 
sentence seemed to come out of nowhere.

Later today, I'll set my own DVM to DC Amps and simply connect it from the 
battery + to the FI relay output lug (with the key off.) Then my meter will 
substitute for the relay closure and read the current the FI system draws. 
Note, however, that this does not include the current to drive the injectors, so 
it's not the full FI load, but it gives us a minimum, without the additional 
injector pulses.

Okay, now it's later, so I went ahead and did this test. I measured ~0.260 
mA, but as I did the test, I realized that I was also energizing the fuel pump 
relay, so I needed to deduct that current to find the current drawn by the 
brain. Rather than going to the trouble of disconnecting that wire, I measured 
the coil resistance of several relays; they measure ~55 Ohms, giving a 
current draw of 0.225 mA at 12.4 V. So the current actually drawn by the 
non-running brain is only ~0.035 mA.

Nevertheless, the voltage drop across your main FI power relay will be due 
to its resistance and the full ~0.260 mA PLUS the injector currents, when the 
engine is running.

To estimate the injector currents, I'll use 2.3 Ohms as the injector resistance 
and add the resistor that's in series with each injector inside the brain. That 
resistor measures 5.7 Ohms for a total of 8 Ohms. So each injector draws 
about 1.77 A during a pulse. Since the injectors fire in pairs, that's about 
3.55 peak A during a pulse. No wonder Bosch set up a 2-wire power feed to 
the brain, one for the control circuitry and a separate one for the injector 

So the DC current thru the relay is ~1/4 A with pulses up to ~3.75 A during 
injector pulses.

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

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