Resistance testing the 5V reference circuit

This whole thread was inspired by the Bad Engine Computer Testing series, which included Paul testing for a short to ground on the 5V reference circuit by using an ohmmeter at the ECM connector. Kinda blew my mind! :woohoo: No other training material or service info talks about this type of testing that I'm aware of.

If you haven't watched this video, you should!



Paul was kind enough to post his rule-of-thumb for this type of testing in another thread :

ScannerDanner wrote: So in short, I want to see more than 1000 ohms between any reference circuit and ground or sensor ground

So this got me thinking - how much of a short does it take? :huh: I used my car as a guinea pig, as usual. I'd post the wiring diagram for reference, but the Toyota versions are confusing and stupid. Short story: Three sensors use the 5V ref. A P/S pressure sensor, fuel tank pressure and the TPS. One wire comes from the ECM to supply all three. They also share a ground, which returns to the ECM on one pin. Very simple overall.

As a baseline, this was the total resistance measured between the 5V ref pin (known as 'VC' in service info) and the sensor ground (known as E2) with the ECM disconnected:



I guess this is a good number? :silly: First time doing this test and all. Also for a baseline, this is the 5V reference circuit KOEO:



So, how much of a short does it take, right? With the ECM reconnected and the key on, I hooked up a variable resistor from the 5V reference pin to ground, and started dialing it down. I started at at 1000 ohms. When I got to 189 ohms, the 5V reference began to notice:



Not much, though, and the car still starts. :huh: I got down to around 6.5 ohms before the reference dropped below 2.00V, MIL went out, and turned into a crank/no start:



So in this case, it took a (more or less) direct short to ground for this 5V regulator to act up. :blink: For this car, anyway. Other vehicles with more sensors and multiple regulators will likely behave differently. I may do this to my Escape in the future.

Comments and suggestions are welcome, as always!

Nice, cool idea
I'd like to see if the Escape is more or less sensitive.