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Coolant Temp Sensor Issues and ECU Troubleshooting

Then, I'd use the wire. Instead of making resistance checks, I'd connect a voltmeter to the wire and watch the response as the car was started cold and warmed up. I'd compare that voltage to the numbers in this curve, which would verify all of the things you would check with an ohmmeter, plus tell me whether the ECU was suspect.

ectCurve.jpg

Just did this measuring the voltage at pin 13 and grounding to the cigarette lighter. I should mention that the tick marks I am referring to on the temp gauge are because the car has the Dave Barton "R Sport" stickered gauge cluster.

Starting value with the key in the second position: 3.09V
(I live in North Dakota with the car in the garage but the garage heater was not on)

Car started and idled rough at first-eventually smoothing out

Voltage at the bottom of the coolant gauge: 1.23V

Voltage at the second tick mark of the gauge: 1.00V

Voltage at the third tick mark of the gauge: .74V

Voltage at the fourth tick mark of the gauge: .60V

Voltage at normal operating temp (Just below 5th tick mark): ~.48V

The decrease in voltage from start to warmed up was steady. No spikes or drops to speak of.

After completing the test I turned off the car and checked the OBD. On both pin 2 and 6 I got 1-1-1. This is not the reading I received when I parked the car earlier this week and prior to disconnecting the sensor and ECU's to check wire conditions and run diagnostics. Obviously disconnecting the ECU will clear the OBD but after running it for probably 10 minutes or so it should have had adequate time to diagnose an issue?

What does this mean? It seems to prove that the connector, wire and, temp sensor are okay? Does this mean ECU problems? I should mention the car has a weird hickup/misfire at idle that I have never been able to get to go away. This hickup never causes the car to stall but it is noticeable. Obvious culprits such as vacuum system and MAF etc... have been checked so.
 
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It proves the trouble, if come-and-go, has left. Doesn't prove a thing about whether it left for good.

Next thing I would do is examine the condition of the contacts, especially #13, on the fuel ECU connector. They do get hosed by folks probing at them with things, or moisture (look for darker gray on the ECU pins). Your best method to clear codes is not the push-button sequence, but momentarily lifting the engine management fuse, which in '93 may be #6 inside, or if late production, the PAL fuse on the battery terminal. Of course, pulling the plug on the ECU does the same job just fine.

Beyond that, you might try to repeat your voltage test, but prop the meter up where you can see it from under the hood while you grab the wiring harness and give it a shake near the sensor and along its length back toward the ECU. Still, the mystery of why EZK gave you the same symptoms remains, and the presence of that wire makes it look as though it was a mystery you've inherited from someone else.
 
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I'm at work, but I have a sketch of the CLT input circuit on the ECU and EZK at home and will check your voltages later today. IIRC, the input circuit has a couple resistors off an internal 5volt reference point. Offhand, your voltages look OK.
 
I'm at work, but I have a sketch of the CLT input circuit on the ECU and EZK at home and will check your voltages later today. IIRC, the input circuit has a couple resistors off an internal 5volt reference point. Offhand, your voltages look OK.

Looks like this Bob:
EctDwg01.jpg
 
I do terminal drag tests with the proper tool to test for correct terminal tension. I use pins from old components as the special tool.

I would check terminal tension at all 4 corners of the circuit.

For the idle issue, I would start with fresh plugs (gapped correctly. I use NGK copper at 0.028"), fresh bougicord wires, and fresh cap and rotor.

I have seen the ECT code show up after an overheat. I didn't have my eFan on...oops
 
Doh! I was just going to run the voltage-to-temperature numbers in a spreadsheet and realized that Art already added them to his first graph. I'll need to keep a copy of that graph!

So, for the OP, your 3.09V starting temp is ~32°F, 2nd tick 1.00V is ~130°F, up-to-temp 0.48V is ~178°F (ask if you don't follow how to get this from the graph).

[For Art:
The R109/R110/R111 resistors in my '951 ECU, from the faded color codes, are 2.870K, 46.50K, 14.70K, but in-circuit measure 2.65K/3.45K/14.63K.
My '148 EZK box has a 619ohm pullup to +5v, and a 10.0K to a diode to ground, so a bit heavier load than the ECU circuit.]
 
[For Art:
The R109/R110/R111 resistors in my '951 ECU, from the faded color codes, are 2.870K, 46.50K, 14.70K, but in-circuit measure 2.65K/3.45K/14.63K.
My '148 EZK box has a 619ohm pullup to +5v, and a 10.0K to a diode to ground, so a bit heavier load than the ECU circuit.]

:)

That orange/yellow similarity is as bad as red/brown for me with the tiny resistors made in Asia. I have to check my eyes by looking at a standard chart of 1% values, as I don't know them by heart like the old 5%.

In the nearly 20 years I've been trying to help Volvo owners online with the electrical end of things, I've run into several cases where this repeated replacement of the LH2.4/EZK ECT has been frustrating because a blue-bodied (see Ian's #8) instead of the black-bodied Bosch part was specified at first, and then repeating when things didn't work. I trust that isn't the case in this thread, especially that the voltage readings look right on. It is easy to say "the resistance is correct" and only learn after much verbiage that the resistance was being measured by the frustrated owner terminal-to-terminal, which is the way the LH2.0/LH2.2 sensors got wired. This is the Occam's Razor of explanations for both EZK and LH faults of the same sensor. Yet I get arguments about the quality of the ground provided by the head. Yes, someone blamed it on teflon pipe tape, but I find that a, hmm, stretch, unless someone would install the thing finger tight. And, I don't know if the aftermarket sensor vendors are keeping the color part of the FFF requirement.
 
The spooky Coolant ghost has returned.

Just fired the car up today to check wiring and it barely runs. Sure enough check under the hood at the codes and I got 1-2-3 on pin 2. HOWEVER. Absolutely nothing on pin 6. I was checking voltage to the LH ECU by putting a multimeter pin on 13 and the ground to the cigarette lighter while the car was "running" and was getting the steady drop in voltage. Albeit, I wasnt able to monitor it all the way to normal operating temperature because the car would eventually just automatically shut down. But I got a good idea that although the car had massive chop, the voltage was still steady and dropping as it normally should via the charts provided.
 
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If I got a 123 at the same time I saw even close to normal voltage at pin 13 (meaning not wildly different) next thing I'd do is swap the ECU. But having multiple LH2.4 240s in the family means I have spares for these expensive parts to swap. If not, I'd double check the power and grounds to it, trying to catch it messing up (working carefully). No sign of water under the glove box, right? Any burned look around the red leads in the engine-cabin harness connectors?
 
I would run a new circuit first. But if you can't come up with 10ft of wire for a resistance and wiggle test checks....

Interesting story CFT. I have seen lots of bunk codes on these cars, so always consider that there is an issue other than what the code is telling you. This is really old OBD.
 
I would run a new circuit first. But if you can't come up with 10ft of wire for a resistance and wiggle test checks....

Interesting story CFT. I have seen lots of bunk codes on these cars, so always consider that there is an issue other than what the code is telling you. This is really old OBD.

Zach, the voltage readings make the resistance measurements of the blue/red wire unnecessary until they aren't right, and he's looking for the piece that makes them wrong, but the wiggle test works with the voltmeter. Same principle is involved as voltage drop testing you do on power circuits.

Yup it is old, but a 123 is very specific, unless it just happens to be the first sign that pops up when the signal ground rises with respect to the power ground. Those splices I show in the last photo have been a problem hidden under the vinyl sheathing over the intake manifold. Edit: In practice it would most likely be the power ground that rose above the signal ground, not the reverse.
 
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I had a 2016 vehicle recently that would intermittently stall and toss a cam sensor code. Fired a cam sensor at it, didn't fix it. Resistance checked fine end to end of the circuit. The solution was running new terminals at the ECU, new connector and terminals at the sensor, new wires, and even a little solder on the crimps. Fixed the problem.


Something has gotta be very wrong on these cars to toss a code.
 
If I got a 123 at the same time I saw even close to normal voltage at pin 13 (meaning not wildly different) next thing I'd do is swap the ECU. But having multiple LH2.4 240s in the family means I have spares for these expensive parts to swap.

I happen to have a -937 ECU off a B230FT I can slap in and see how it does. In terms of signs of water damage and other possible culprits in the passenger area there are none. All wiring seems to be intact and without any signs of damage, (minus the tampering). Also the ground points on the intake have been cleaned along with other ground points throughout the engine compartment. If the -937 doesn't work I think running a new wire would be the best option or possibly resoldering that pin 13.
 
I happen to have a -937 ECU off a B230FT I can slap in and see how it does. In terms of signs of water damage and other possible culprits in the passenger area there are none. All wiring seems to be intact and without any signs of damage, (minus the tampering). Also the ground points on the intake have been cleaned along with other ground points throughout the engine compartment. If the -937 doesn't work I think running a new wire would be the best option or possibly resoldering that pin 13.

OK, I can see my comments about the voltage checks obviating the need for resistance checks or running new wires, etc. fell short of convincing, so I'm left with nothing further helpful to offer, being unsure whether the 937 substitution (turbo ECU) is going to be helpful beyond checking the return of the 123.
 
The problem with intermittent electrical issues is that they're hard to track down when not in the fully failed state. If the wiring connections are loose, or corroded, or shorting out, sometimes merely wiggling the wires can temporarily "fix" the issue. This is especially annoying because swapping parts also wiggles the wires. You swap in a new part, the problem goes away, and it sure seems like the new part was the fix, but then the problem comes back, so you swap in another new part, nudge the wires around, and "fix" it again. Very frustrating and expensive.

Since you saw reasonable dropping voltage on the CLT pin while "running" poorly, I'd suspect the other side of that circuit, namely the ground wire. Pin 5 on the ECU is the ground used for the CLT sensor. If it's running poorly again, you could check the voltage of Pin 5 to another ground pin -- it should be 0volts difference. Or, shut it off and measure the resistance of all the ECU ground pins (5,17,19,29) to each other. They should all be about 0 ohms (you can hold your meter probes together to get the base meter and probe wire resistance).

Another check would be to remove, or roll back, the plastic sheathing on the ground bundle going to the intake manifold, and visually/mechanically check that the crimps are OK. A warm engine, or a heat gun, would help make the 25+ year old sheathing more pliable. Expose the main 5[?]-to-1 crimp and tug firmly on each wire.

harness1413.jpg



If it happens again, you, or preferably your helper, could try doing the chicken dance around the car, in a clockwise direction, and massage the wiring every 3rd time around. If it starts running better, you have your fix.
 
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