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Am I doing a voltage drop test accurately?

diagnostic code indicates trouble with engine coolant temperature sensor. working at the ect sensor with engine on (and feed wires unplugged), the resistance checks out ok over the range of cold to warm engine. testing the voltage ‘drop’ between the two leads (while they are disconnected from sensor with engine off, but ignition switch on), however, shows a drop of less than 1 volt. I understand these sensors typically operate off of a 5 volt feed. Am I testing properly? and if so, could this errant voltage be responsible for this trouble code (and others)??? thanks.

Are you working from a manual that tells you to look for a voltage drop? Myself I always looked for a resistance change (with out voltage applied to sensor)

sorry - I don’t think my wording was very clear.

I did a RESISTANCE CHANGE test across the two terminals of the sensor itself as the enginge warmed up from cold to operating temp. (with wiring leads disconnected). this checked out ok.

I then did a VOLTAGE DROP test across the two wire leads that go to the sensor while they were disconnected from the sensor - with engine off and ignition on. this is what resulted in less than 1 volt. to the best of my reading ability, I’m gathering that the voltage delivered to the sensor here should be 5 volts.

so I’m just trying to confirm that I did the test correctly, and see if I can rightly conclude that I’ve got innappropriately low voltage in this circuit (and likely others) which could be responsible for my multiple sensor trouble codes.

thanks, btw, to all who read and respond to these questions. this place is great.

I think you’re on the right track.
In its simplest form, these circuits are usually a series resistance ladder with a fixed supply side resistor and a load resistor (the CTS). The circuit is biased with bus voltage and as the resistance of the load changes, the voltage drop across it changes. The computer then senses the voltage drop across the load resistor.

Without the load resistor in the circuit, the bus voltage should be present on one of the wires with respect to ground. The other wire is typically ground but there are ways where it might not be. So it wouldn’t hurt to measure between the engine ground and then each wire to see if one has bus voltage on it. An even better test would be to reconnect the sensor and then measure the voltage drop across it while it’s in circuit. Back probe the connector rather than piercing the wire insulation and damaging it.

If the supply voltage is only 1 volt, I think you’re going to have to trace the wiring to see if it’s an open or the supply voltage is bad from the source. Considering you have multiple faults, the latter might be the case.

Let’s back up for a minute. I learned that a voltage drop check was simply the amount of voltage loss through a connection. For instance if you suspect a bad battery connection at the battery, take your volt meter (set to dc volts) and place one lead in the center of the battery post, place the other on the terminal going to the same post. If there is corrosion where the terminal hooks up to the post, you might see some voltage show up when you crank the engine. It could be as little as one volt, or as much as 9-11 volts. This voltage will show up as heat if the drop is significant enough. Once I had an old truck where the battery terminal got hot enough to burn my hand when I grabbed it. I didn’t need to use a volt meter for that one, I removed the terminal and found a ton of corrosion between the two. Cleaned it up, and it didn’t heat up after that. You seem to be on the right tract, but if you suspect you are not getting the proper feed, or supply voltage, I’d look at the PCm or another connection between the PCM and your plug. Have you reset the CEL and tried it again, incase you simply had a bad connection at the sensor and resetting the plug might have cured it?

Voltage drop is the amount of voltage “dropped”. For a specific component the correct way is to measure with the meter connected in parallel to the component. The thing to be wary of is that of there is more than one component in series in that leg of the circuit, you’ll measure only the drop of that component, which will be a portion of the total voltage.

Example: if you have two equal resistors in series being powered by 12VDC, each will show a “drop” of 6 volts across it.

That’s where the schematic comes in. You need to know if there’s more than one resistance in that 5 VDC leg that the component is in. If there is, the component may be only dropping 1 volt even though the leg is powered by 5 volts. This is useful it telling if the voltage being seen by the component is correct, and can be used to detect a circuitry problem. A high resistance connection acts as a resistance in series and will “drop” voltage. The voltage seen at the component will be lower than it should.

In summary, even a bad component in an otherwise good circuit can be showing the correct voltage drop if the supply voltage is good, and a good component in a bad circuit will be showing too low a voltage.

For the record, current is exactly the opposite. Current is measured in series with the circuit and every component in a specific leg sees the same current.

Resistance can get diffficult. If you measure the resistance of a component across the component while it’s in the circuit, you’ll actually be measuring a combination of the resistances of not only the component but also all other legs of the circuitry that are in parellel to the component. The only accurate way to measure resistance is to disconnect one end of the component.

In this case you’re referring to a thermister, a component that changes resistance as the temperature changes. Since it changes resistance and is in series with a fixed resistor, it also changes the amount (portion) of voltage it “drops”. Seeing this without an oscilloscope is tricky. You could do it if you knew the voltage drop at ambient and the drop at operating temp, then checking at both ends. Perhaps an easier way to test is to unplug it and watch the resistance change as the temperature rises. You could even do this on a hotplate with a pyrex vessel and a cooking thermometer.

Mountain bike is dead on. His explanation jogged my memory from a high school electronics class that the voltage drop you’re referring to is in fact, “dropped” across different legs in a parrallel circuit. And that is most likely what the OP is referring to. So there are two different definitions of voltage drop, as least in my (realitively simple) mind. Either way, if the resistance is within specs on the sensor when going from cold to hot, it’s time to look elsewhere if the CEL continues to come back. (I’d like to know what “trouble with the ECT” refers to.) Does the code say it’s out of range? Or does it read high, low? Or doesn’t read at all? I’d suspect a wiring problem between the sensor plug and the ECM.

Checking the voltage while the sensor is disconnected isn’t going to tell you much. You should check the voltage while the sensor is connected to the circuit. The advice that Mountainbike gave is good. By checking the voltage with a meter across the sensor while the engine is cold and warming up will show you if the sensor is working. You don’t need a scope to see what is happening.

As the resistance of the sensor changes with the engine temperature the voltage across the sensor will change also. As others have stated already the other part of the circuit is inside the ECU and the sensor is the variable portion of the load in the circuit. If the sensor is ok you should see the voltage across it change as the engine warms up. You could also measure the resistance across the sensor with the wiring disconnected to it and watch the resistance change as the engine warms up. As always, remove power from any device you want to check the resistance of.

(forwarning: this post is pretty long)

wow. these are all great responses and are providing me with chances to ‘study’ and learn. the level of detail has me scratching my head a bit, but it’s good. In any case, it seems time for me to provide a bit more detail about my situation:

First, the car is an '89 dodge colt. it’s old, the ECM is obviously OBD1, so trouble codes are rudimentary. I’m getting 4 to 5 codes related to 4 to 5 sensors, and all the codes simply indicate that there is ‘trouble’ with the sensor and/or it’s circuit - no further information about readings either too low or high.

I have reset the ecm, and codes keep coming back. Furthermore, these codes started coming on in an intermitten fashion: perhaps CEL ligth would be on for 50% of driving, while other 50% the CEL was not on and the car ran well. Those percentages are getting worse.

These factors together seem to suggest I’ve got wiring trouble (right?). I have checked and cleaned all the grounds I could find (about 6), to no avail. I’ve then been focussing on the ECT sensor and it’s circuit as my first test point simply because it’s easily accessible and has only a two wire feed.

So I don’t think I have the ability or know-how to ‘back probe’ the sensor while plugged in. When I have measured what I’ve been calling the volatage drop is plug my two meter leads into the two wire terminals - while they were unplugged - and measured the voltage difference. this yields less than 1 volt.

As someone suggested, I later measured the voltage on each lead individually compared to the battery ground (engine off, leads unplugged, ignition on). One lead measured ~ 8 volts, the other lead ~ 7 volts. my novice mind quickly asks "if one of these leads is a ground - or return feed to the ECM - why does it have ANY voltage on it when I have the leads disconnected from the ect sensor (and the circuit is broken)??? Is that the right question to ask? And would the right conclusion be that there must be a short in the circuit somewhere between the sensor and the ecm? Are there other conclusions or questions those voltage readings raise? (why are they higher than 5 volts, for example?)

It also seems fair to wonder - since the ecm is reporting 5 codes for 5 sensors/circuits - is if my ECM is malfuntioning. Isn’t the source of all the ‘referrence voltages’ being sent to the sensors the ECM? could it be sending out the wrong voltages to begin with which would produce return results that were beyond usable parameters? I have had the ecm out and looked it over. all connections on the outside look good and clean. I even opened it up, and the circuit board inside looks 95% clean. There is one little ‘transistor’ (probably not right term) that has corrosion around it. Otherwise, it looks surprisingly good and it seems to be a remanufactured one that could have been installed somewhat recently (I’ve only had car for two years).

I"m gonna back up one more time (if people are still reading!!!). BEFORE I ever got the cel light and the resulting ‘limp home mode’ the ecm would kick in to, I used to get periodic electrical trouble that manifested in a different way. while driving sometimes, the car would quickly stall or nearly stall which would be accompanied by a rapidly clicking master relay. usually the car would recover after a 5 minute episode of this, and return to normal for perhaps another month. the relay was replaced - no change. wiring grounds were cleaned and secured - no change. I just lived with it then, sometimes working the electrical system over pretty hard (turning key on and off, on and off pretty quickly) trying to re-establish a proper connection at the times when this problem manifested.

SO… with all that rapid clicking (which, I think I realize, is rapid opening and closing of the whole electrical circuit), and with all my own intentionally induced cycling of the system between on and off, might I have damaged my ecm?

Perhaps, then, I have seperate, but related problems: a bad ecm AND a suspect wiring issue (and a detrimental lack of patience and aptitude when dealing with electronics - but we’ll stick to the simpler problems). This car is old and cheap, but when I have the patience, I learn a lot from having old and cheap cars - which is what this forum is helping me do too!

So thanks all. sorry for the really long story. cheers, and I hope everyone is having a good labor day weekend.

A lenthy post as well written and clear as yours is rare and welcome. I’d rather have all the details.

I don’t have access to a schematic at teh moment, but the voltage for the sensor is probably not being provided by the ECM. It’s likely being provided by a power buss, stepped down to 5VDC.

In rereading your original post it sounds like you’ve checked the sensor properly by checking the resistance change over the temperature range. You can safely rule that out as a suspect.

Your last post leaves me wondering of in fact you have a bad key cylinder, the one in the steering column. If the problem were elsewhere, cycling the key on and off would be unlikely to have an effect.

thanks, mountain bike -

(all you guys and this forum are quite a source of encouragement and information!)

To be fair, I’M not sure if it’s accurate to say that I can affect the ‘glitch’ with my key jiggling. At some point early in this issue’s era, while out of town, a roadside mechanic did have a brief inspection of the problem at which time HE suspected that the matter was in the key cylinder. He also felt like he could successfully manipulate the key (repeatedly) in such a fashion inside the cylinder so as to both reproduce and rectify the problem. (I didn’t have the cylinder changed at that moment since they didn’t have the part, time was short, and the car was working again) From that point forward, however, I’ve never felt like I really affected the issue using his key trick. So when the car would get going after minutes of one of these random episodes and me jiggling the key (out of desperation and frustration as much as anything else), I always accredited it to the inevitable chance the issue had already seemed to be demonstrating for months and told myself that this particular mechanic simply didn’t have the opportunity to really get familiar with the problem.

If I have mistakenly overlooked a simple solution - for nearly 2 years! - just because I don’t have the same deft touch as that mechanic…well, I would still simply be thrilled to have it identified and rectified. Maybe THAT’s where my attention should go next - even if just to rule it out.

Would putting in a new key cylinder be a simple job? if not for me, than for a local mechanic?

It’s doable, but you definitely want to get a manual so that you deenergize the airbag system properly before working on the steering column.

Looking at some info from Autozone on one of the Vista models (1.6 liter MPFI) it shows there is a common connection to 4 sensors. The air flow, TPS, coolant, and EGR temperature sensors are all tied to this lead which goes to pin 24 of the ECU. Your thought about the trouble coming from the ECU may be correct. I’m not sure if that lead supplies power to the sensors or it is the return side of the circuit. The info shows the wire color is green/black. If that jives with what you have you could verify that the wire ties to all the other sensors with that wire color by checking the resistance between pin 24 (connector removed from ECU) and the wires to the individual sensors. You should see close to zero ohms on a good wire connection.

Just as a guess going from what you stated, I would say that the trouble is with the ECU. Since they all seem to have a common source of trouble.