Mechanics

DODGE OMNI SENSOR PROBLEM

1989 Dodge Omni • 4 cylinder FWD Manual • 102,000 miles

I have a 1989 Dodge Omni, fuel injected. The car will start with no problem. But once it runs for a couple of minutes it starts to sputter and then quits. When I drive it there is no power as I press the accelerator. I found that if I press the throttle about 3/4 of the way down it will take off again. I found this out when I had to drive it home. So it starts cold just fine and runs find until it warms up and then only runs at 3/4 or more throttle. I looked at the error codes and came up with the following. 12, 24, 33, 25, 15 and finally 55. The first one is a battery disconnect and the last is the end code. Any Omni lovers out there who had to go through this?
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1989omni
February 14, 2011.




Disregard 33 if you don't have air conditioning. All of the other codes have multiple sensors in common. Do you have a digital voltmeter and know how to use it? If so, head to the throttle position sensor to back-probe the wires after the problem occurs. Normal is 5.0, 0.2, and around 0.5 volts. If your readings are different, tell me what you have,

caradiodoc

Caradiodoc
Feb 15, 2011.
Okay, this is what I did. I pulled the plug out of the throttle position sensor and put my probes in to read it. It read 5.0 and.5. That is the same before and after the car started to run badly. Was I supposed to take the reading while it was still plugged in and running? I also went ahead and replaced the throttle position sensor with a new one but that make no difference in how the car ran.

Tiny
1989omni
Feb 15, 2011.
You missed one voltage reading. Since multiple sensors are setting codes, it should be acceptable to read the TPS with it unplugged. I'm most interested in the common ground wire for those sensors. I think that will be a black / light blue wire.

Caradiodoc

Caradiodoc
Feb 15, 2011.
This is what I am doing. I have my digital multimeter set at 20 on DCV, I turn the ignition to the start position. I put the red probe into the A wire or (Blk/red) and I put the black probe on a valve cover bolt for a ground. The A wire reads 5.02. On the B wire or (Blk/Org) it reads 4.57. For the c wire (Blk/Blu) it would not register a number until I set the multimeter to the 200m and then it jumped between.3 and.4.
I had been using an analog reader and I had it set at 10 DCV and the A wire would read about 5.0 and the b wire would read about.5 and I couldn't get the C wire to move the needle. I guess that thing is good for testing my AA batteries only.
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Tiny
1989omni
Feb 15, 2011.
The 5 volts is the feed and is acceptable.

The 4.57 volts would be typical of the signal wire. You should find that one goes down to around a half volt when it is plugged in and you back-probe the connector. There is a "pull-up" resistor inside the computer to force that voltage to go to a defective state, (over 4.5 volts) so it will set a code if there is a break in that wire. There's a break now when it's unplugged. That resistor is so huge, (electrically), that is has no affect on the circuit when it's working properly.

The black / blue wire is reading ok with both meters right now but that is the one I suspect has an intermittent connection based on the fault codes.....

Actually, I have to rethink this. After looking at the code descriptions again, we can disregard codes 12, 33, and 55. 15 has to do with the vehicle speed sensor signal, not the MAP sensor as I originally thought. The manual is a little confusing. Code 25 says one of the four wires going to the automatic idle speed motor is shorted, but it doesn't specify if it is shorted to ground or to one of the other wires. The TPS code 24 doesn't specify whether the voltage is too high or too low, just that it is out of range. That means the problem could be in one of two circuits and we don't know which one, ... Yet.

I guess what I would like you to do is to back-probe the black / blue ground wire on the TPS connector with it plugged in, ground the other meter lead with a clip lead, then prop the meter under the wiper arm and drive the car until the problem occurs, then read the meter. If the voltage stays near 0 volts, try that again with the positive meter lead on the other end of the connector in the 5.0 feed terminal. One of those two, the ground or the feed has to be going away for that code to set.

As an alternative, you can back-probe the middle wire during the test drive. It will vary from 0.5 to 4.5 depending on throttle position. What we need to know is what it goes to when the problem occurs. If the ground wire opens up, that signal voltage will go to more than 4.5 volts. If the feed wire opens up, the signal wire will go to 0 volts.

Caradiodoc

Caradiodoc
Feb 15, 2011.

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