96 Honda Civic possible electrical problem?

civic
honda

#1

I bought this civic recently and it’s giving me a headache. It cranks and has difficulty starting cold, and will also stall quite a few times before running steady. Every time it starts, the battery light appears regardless of if it’s warm or cold until I hit the gas a few times and then it goes away.
It’s throwing three codes at me: p0113, p0118, and p0123. I changed all three sensors that relates with each code, however none of them seem to be working. Whenever I hook up the scanner to get a reading on the engine coolant temperature or the intake air temperature, it just reads 0. (Also I checked my other civic with the scanner and got readings of each sensor) I’ve also checked the ground by the thermostat and that seems like it’s in good shape. Whenever I reset the ecu with the scanner, the Check Engine Light immediately comes back on.

Before I came inside tonight it threw one more code at me: p1108. If anybody has any insight on where I should go from here I’d appreciate it.

Also!! The guy I bought it from has an aftermarket radio system installed with a subwoofer in the trunk. They wired it directly to the battery, but it is disconnected at the moment and my problems haven’t changed.


#2

It may also help to mention this Civic is a DX, Canadian car. Windows are manual and no power steering. Automatic transmission.


#3

Appears the control module isn’t getting communication from multiple sensors. Sensors are probably good. That leaves wiring issues. I believe each sensor has two wires. Regardless of how many wires, one should have 5 volts. I also believe but not 100% sure, I believe the 5 volts to all of these sensors are supplied by the same sourse from the control module. There is probably a short in this circuit. It is called reference voltage. The signal wire is computer grounded and each sensor has it’s own signal wire, so I think your ground is good, otherwise you wouldn’t have multiple sensor faults. Get a wire diagram of the ecu or pcm so you know which wires to check. Might be a good idea to get help from someone with experience cause when testing module circuits damage to module can occur if not tested properly.


#4

Okay, I will look for a diagram for the ecm. I do have somebody helping me with this. We do seem to have power going to all three sensors, mainly looking at the Intake Air sensor and Coolant temperature sensor. However through the scanner I’m not receiving any reading of the temperatures, it just reads 0.
There were two other senors we also checked for current and there was no power going to them, one near each sensor.

Thanks for your feedback. I’ll be looking again tomorrow and I’ll keep you updated.


#5

Do this basic test on the electrical system. Before first start of the day the battery terminals should measure appx 12.6 volts. Now start the engine and let it idle. Battery terminals should now measure 13.5-15.5 volts. What do you measure?

Note: It’s possible the problems you are having reading sensor is b/c a 96 was the start of the obd II years, and maybe your newer scanner isn’t 100% compatible w/the software in that model year of Honda. A Honda dealership or specialist would have a compatible scanner, if all else fails.


#6

Thank you for your input George SanJose. I think the battery was giving off a good voltage but I will check on that tomorrow.

As for the scanner I believe it is working properly. I’m picking up readings from other sensors on this car but not these. I checked with the scanner my other civic to compare because it’s a 1999 and they’re both dx’s, and I am able to see the intake air temp and engine coolant temp on them.


#7

All of the codes you mentioned indicate that the sensors have high input voltage to the ECU. It could be that the 5 voltage reference voltage is higher than it should be. The regulated voltage most likely comes from the ECU. Verify that the 5 volt reference source is good. Also verify that the ground to those sensors is okay.


#8

Both those sensors are probably resistors whose resistance varies w/temperature. Thermistors in other words. they should be easy to bench check. Just place them on the work bench & measure their resistance as you warm and cool them. Use a hair dryer to warm, and some ice water to cool. If you can find their specs you’ll be able to tell if they are calibrated correctly or not by comparing their resistance to what a thermometer says. Thermistors are pretty tough beasts and reports of thermistor failure of these types of sensors here are very rare, so I doubt that’s the problem. More likely a wiring, ground, or ecm problem. Or scanner/ecm incompatible software as mentioned above. But testing for a faulty thermistor is so easy, worth a shot. With the sensors removed you can also check that the wires that connect to them have the correct voltage. You’ll need a wiring schematic to figure out what they should measure with the sensor removed, but usually (with the sensor disconnected) one of the pins will be the reference voltage (5 volts) and the other ground.


#9

Here’s a page from the service manual that I’ve been looking at. Is there a way to know what the voltage should be by looking at this?


#10

I don’t see the applicable circuit on that page. Usually that sort of circuit uses a “voltage regulator” to produce a precise 5 volt reference , derived from the rather imprecise battery voltage. Then the thermistor is wired sort of like this.

Battery — Voltage Regulator ---- +5v ----- fixed resistor ------- (point A) thermistor (point B) ----- ground.

Point A is the voltage the computer monitors, which varies in a known way as the thermistor changes resistance with temperature. Google “voltage divider” for how that circuit works.

So if you remove the thermistor from the circuit and probe the harness pins, point A would measure 5 volts, and point B (the other pin on the connector) would measure ground.


#11

You need to check the ECU diagram for the 5 volt reference source.


#12

Where can I find a ECU diagram?


#13

I suggest you get on Ebay and purchase a factory service manual there for the car. They are pretty reasonably priced and are very valuable to have on hand when problems like this happen. It is money well spent.