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Either there's a short in my brain, or my car. help me decide?

I’m still troubleshooting my CEL issues on an '89 dodge colt. at this point I’m focussing on the voltage on the two wire leads that connect to the ECT sensor and am wondering if I"m right to be mystified by my readings.

with the ignition ON, engine off, and wire leads (2) disconnected from the ECT sensor, I"m reading ~8 volts on what must be the feed wire. The return wire shows ~7 volts (!#$!%???)

in my novice understanding, the feed wire should be providing a constant

‘referrence’ voltage to the sensor, while the return wire should be carrying the ‘modified’ voltage after it’s passed through the resistor of the sensor (right?) But if the wires are DISCONNECTED from the sensor (and the circuit is consequently open), THEN THE RETURN WIRE SHOULD HAVE ZERO VOLTAGE, right?

any opinions appreciated. thanks a lot.

You’re reading the current voltage drop of a circuit power source that was formerly also feeding the sensor. And you have the meter connected in reverse polarity.

The sensor circuit is in parallel to other items on the power buss. Disconnect the sensor and you still have power to the buss. That 7 volts suggests that there’s also other things in series in that line that you’re now reading the voltage drop of one of.

Since you really don’t know how the circuit is designed inside the ECU it is hard to say what voltages you should see on the two leads with the sensor is disconnected, unless you have some service data that tells you what to expect when testing that way. If the sensor operation is having a problem I recommend you check the voltage across the sensor leads with it connected as it normally should be and while the engine is warming up. As the coolant temperature changes the voltage across the sensor should change also.

Making sure the wire connections between the sensor and the ECU is a good thing to do so checking the resistance of each wire (both sides of the circuit disconnected) between those points would be good to do if you think that is a problem.

as always, thanks to everyone who lend’s their time and knowledge to those of us perplexed. if anyone’s willing, the following is another set of ‘clues’ that I would hope could help further narrow my search for the root of my 5 coinciding error codes… and again, it’s lengthy.

SO… since my last post, I have checked the resistance on the two wire leads to the ECT sensor (per Cougar’s advice). They check out ok. I have also tested the RESISTANCE across the sensor itself while the engine warmed up from cold to normal. That also seemed to check out ok. *I did the resistance test because I don’t have a simple means to ‘backprobe’ the circuit for a voltage test. (as for mountain bike’s closing ‘suggestion’, I would also just say that I checked my wiring diagram and it indicates that the ‘return’ wire goes straight from the ECT sensor to the ECU without connecting anywhere else - for whatever that may be worth).

In any case, I’ve moved on to inspect another sensor that’s listed in my trouble codes - the MAF sensor. And again, I’ve seen results here that seem very odd in my novice mind.

With the ignition ON, the engine OFF, and the wiring harness disconnected from the MAF sensor, I tested the voltage at the green/black feed wire measured against the negative battery terminal (i’ve concluded this is the ‘feed’ wire because my diagram shows two green/black wires exiting the ECU and then ‘merging’ into one, off of which comes 4 seperate green/black wires in parrallel and going to 4 seperate sensors - including, ofcourse, the ECT and MAF sensors. that is the only wire common to 4 of the 5 sensors with reported trouble). Anyway, the voltage here at the MAF sensor is ERRATIC. In sharp jumps, it goes anywhere from less than 1 volt to as high as 5 volts, and to all points randomly in between. Furthermore, these jumps are co-occuring with ‘actuations’ by my idle control actuator - which, according to my diagram, is not on the circuit (unless, ofcourse, they are linked inside the ECU itself).

These erratic jumps are also present at the green/black feed wire of the ECT sensor so long as the MAF sensor wiring harness is disconnected. Re-connect the MAF sensor harness, however, and the voltage at the ECT sensor feed wire goes back to 8 volts and is stable. All these findings are duplicated when the voltage is measured at the ECU end of the green/black wire.

Do these observations offer any more clues as to where I should focus my search? I"m still suspecting my ECU itself is bad (as I’ve read they are prone to be for this model car), but I’d like to shore up my suspicions before I send it off and get it repaird/replaced at a cost of ~ $200. any and all opinions welcomed and studied! thanks, and have a good evening!


Most likely not. It’s a little hard to explain, but you need to “load” the circuit with the same resistance it would see when operating. If you don’t, you will see something fairly close to the input voltage on the output wire. The easiest thing to do usually is to hook the sensor up normally then either backprobe a connector or push a pin through the insulation and measure the actual voltage the ECT is seeing between the return wire and ground.

Since your checks have proved the wiring to the ECT sensor is ok and the sensor is working like it should be then it appears your suspicions of a faulty ECU are correct.

The other wire going to the sensor may or may not be tied to ground. You could try checking the resistance of that wire to ground while the power is OFF. If it is a grounded return lead you will see a very low resistance.

! this could prove handy!

anyone want to comment on the appearance of the circuit board of my ECU in relation to my suspicions about it’s functioning? you’ll see the one area I"m particularly concerned about.

thanks a lot

(here’s the broader view)