I have a 94 Bronco, 302 w/ 5 speed. The two issues aren't related so I'll start with the TPS one first. When I start it up when it's cold the idle is high, probably around 2000 rpm and it comes down to normal in about 30 seconds or after you tap the throttle. At this point I have no Check Engine Light (CEL). I can let it sit there and idle for as long as I want and it will never give illuminate my CEL, I let it idle for about 30 minutes while troubleshot and serviced my air conditioning with no CEL. My driveway is on a hill and I can coast it with the engine idling for about a mile before I have to touch the throttle but as soon as I touch the throttle, you can set your watch by it, 10 seconds later the CEL comes on. The Bronco runs great though in all aspects, it even gets great mileage for what it is 15 mpg bouncing into town. It seems I have a throttle positions sensor (TPS) going out and I've inspected it with no glaring issues so I was curious if there is a way to definitively rule it out or as the cause of my CEL?

My tail lights dont work when I turn on the headlights. Parking lights, brake lights, turn signals all work. I repalced the headlight switch months ago so I don't think thats it but it wouldn't be the first time a supposeldy good part died. Any tips for chasing this down? I can't find a circuit specific to the tail lights.

Thanks,
Patrick

 

"When I start it up when it's cold the idle is high, probably around 2000 rpm and it comes down to normal in about 30 seconds or after you tap the throttle."
This is normal operation.

Let your computer tell you what it knows;
Engines are easier to diagnose over the internet when all the codes are listed, and in order of test.

KOEO - (O) Self-Test Codes =
KOEO - (C) Continuous Codes =
KOER - (R) Self-Test Codes =

Make sure your A/C, Heat, Radio, etc. are turned off when testing.

 

Which lights in the rear don't work?

 

I got the tail lights working. I had replaced the switch 6 months ago and found the connector had come loose on the switch. I tried to extract the codes, even with the help of youtube and the manual I never got a definitive " this is it" moment. The code clears whenever you shut the Bronco and won't come back on until you touch the accelerator. I don't know much else in the system that is tied to the accelerator like the TPS is that would cause that.

 

The TPS is one of many inputs to the engine computer. So, simultaneously when you depress the accelerator the computer is reading the TPS, O2, IATS, MAF, ECT, Knock Sensor, Timing, EVP, etc.

The TPS can be tested for 5v supply, vdc output, and the resistance can be checked throughout the range of its potentiometer.

The computer usually records intermittent CEL information in the Continuous Codes, I wonder if you are reading them properly, if there are no codes, you will receive an 11/111 code, and if all test passed you will receive 11/111 codes for all 3 categories. How are you reading codes?

When I read about an acceleration problem and there are no fault codes, I like to pull the Intake Air Temp and Mass Air Flow Sensors, if equipped and check for cleanliness.

 

I'm sure I'm missing something with the codes since I didn't see any to help me troubleshoot this. I'll check the TPS for voltage and check the resistance of it throughout the range of the throttle. The bronco has no acceleration or running issues other than the pesky light coming on. I'll clean the MAF and IAT sensor, I have some MAF sensor cleaner in the shop, and see if that helps it.

I thought if I had a bad sensor I'd get the code for it just sitting at idle and it wouldn't reset. I'll check the TPS and clean the sensors and see if the CEL goes away. I had the intake manifold off rec

 

Like I said, you should get the pass codes if nothing else, if you are not getting those, you need to review your procedure.

You can induce a code by disconnecting the input sensors like the TPS, EVP, ECT & air temp sensors to see if your Self-Test routines are working in your Engine Computer.

What happens when you depress the accelerator immediately after you start the engine during Cold Start-Up Scenario? Take it up to 2500rpm, does the CEL come on?

Check the sensor connectors for tightness & cleanliness as you work through this.

​​​​​​​

 

Great idea, I'll disconnect the TPS and fire it up and see if I still get the code. If nothing else that would tell me if it's at least getting it's 5 vdc.

If I depress the throttle after I start it up in the cold scenario, the CEL light comes on after a few seconds. It does this whether it's warm or cold, same thing, CEL comes on after a few seconds but like I said, it runs great.

 

So, when does the CEL go off?

When you let up off the accelerator?

Or does it stay on the whole time you are accelerating?

What about when you are coasting at 50 mph?

 

It only comes on after applying the throttle. It will sit there for hours and idle with no CEL but touch the throttle and a few seconds later on it comes and it will stay on until you shut off the truck. It will be off when you start it but then comes back on as soon as you touch the throttle. Thats why I'm leaning towards the TPS.

 

there is a code reader for our year rigs that works pretty well. I will have to find out which one it is as i dont remember at the moment. If you are using the paper clip method it can be a pain to follow the proper steps.

 

FYI;


2 lads from Bend, I almost visited Bend last month, but my friend there took a job in Arizona, so I scrubed the visit....

 

ya this above is the code reader you want. will make life much easier. the booklet tells you what to do as well.