Accord loss of power with headlights
I was urged to post this job I repaired. The shop that called me to do the diagnosis on this one stated the vehicle would lose power when the headlights were turned on at about 30-40 mph. I test drove and confirmed the symptoms. It would lose power and wouldn’t respond to more throttle request. Turning the headlights back off would restore the vehicles performance. It was very consistent.
I verified this problem would not happen with other loads ie rear defroster. I scoped the ground cable at multiple points looking for a voltage drop. Nothing out of spec. I checked the alternator output volts DC as well as AC. No spikes or dropouts were recorded when the headlights were turned on.
At this point I felt in my gut the problem was a throttle valve issue. Scanning the vehicle I looked at APP and TPS as well as electrical load sensor. I noted the APP would remain constant but the TPS would return to idle when the headlights were turned on. No major change in ELD. At this point I am convinced it’s a throttle valve issue.
I decided to scope the motor power and ground wires. I connected 1 lead across both wires. First video is the result of this test while driving. The second video is the same connection done in park revving the engine. The pattern is the same when it is at idle as it is when the condition exhibits. I am attaching two videos showing these tests. This told me the problem is a logic issue and not a control issue. So now why is the pcm commanding the valve closed when the headlights are turned on?
I looked in Honda service information for conditions that might cause the pcm to close the throttle. I came up blanks. Basically the only thing I found was a note about the existence of a fail safe mode. Nothing that I found said what conditions would set it in the fail safe. I’m sure it’s my inexperience with the service information system that failed here. If anyone knows where or how to locate it please show us.
A couple phone calls were made to good friends in the industry. One thought the brake position switch state was a good place to look. I returned to the vehicle with wiring diagrams for researched and a fairly good idea how the brake switch and headlight systems work. I started by turning the headlights on with the engine off. I then walked to the rear of the vehicle and noted all of the brake lights are lit. Both right and left as well as center high mount. I decided to start by removing brake light bulbs. The first one I removed was the RR. It uses a 7443 bulb. This is an all glass bulb with two filaments. I notice the tail light filament is broken. It was laying across the brake light filament. It was not permanently attached. With the bulb removed the other brake lights are off and tail lights are on. New bulb installed and test drive proved the car is functioning correctly.
The vehicle wouldn’t code for this because it sees the brake light switch as a normal input that could happen if a driver hits both the accelerator and brake pedals at the same time. It responds to that by shutting the throttle off.
Great find. That’s probably the best paying bulb change you’ve ever done lol
Had a sonata with the same issue. Was awesome when i figured it out.
Gotta love drive by wire. Thanks for sharing.
Justin, your title says it's a Camry! Good post, thanks.
Oops. Corrected. The vin and vehicle info was correct. Sometimes these jobs blend together. I had a Camry today also.
Careful you might set off a snowflake
Remember the old Camry with the dash indicator that would light up with a bulb failure....and sometimes you would have to switch all the bulbs to make the sensor happy? But sometimes the sensor-module was the issue.......Bet that could have wreaked havoc on a TAC/ECT system like this too.
Not just Camry, I worked on a lot of European cars and they had them too. I replaced a lot of the brass base bulbs with the correct shiny chrome ones to make those lights work. I heard a lot of, what do you mean you had to replace the bulb, I just put it in there, and, how much? For one bulb? Those were fun. I had a car with the same problem as the original poster's only it was pre computer…
That's beautiful. So, just to get it right in my head, the taillight filament was shorted to the brake filament, and whenever the lights were turned on the computer would interpret the voltage shorting to the brake light filament as a brake switch input, forcing the throttle motor to idle position?
Great find, thanks for taking the time to write it up.
After the Toyota runaway throttle fiasco some makers started to incorporate this strategy where brake pedal operation would close the throttle. Nissan has had issues with mis-adjusted stop lamp switches causing this same symptom if the driver touches the brake pedal even slightly while driving. There were also some cases of water intrusion into the tail lamps creating shorts between the tail…
Good write-up and explanation ! I've run across two filament bulb caused issues over the year's enough times to where tail and brake lights are one of the first few things I check, complaint dependent. I'm in the habit of checking all exterior lights and for current license plate decals before I test drive a car and usually if I see a tail or brake light inop and it's an easy bulb…
That brings up another issue I didn’t put in the write up. This vehicle was at the dealership before it went to the shop that I was at. The dealer tech decided it was an issue with the transmission. Printed a TSB that showed many steps involving valve body service as well as programming update for the ECM. If you look closely in the video just to the right of my laptop there is the vehicle…
Excellent point, I’m sharing this with my staff at the next shop meeting. It would suck to have the issue disappear after visiting the service rack prior to being dispatched to a diag tech.
If you fixed the car, then the ultimate goal was achieved. So I wouldn't say that "would suck".
What sucks is not knowing what was done to solve the problem via validation.
This just shows the credibility of dealership shops these days.I have come across similar issues with customers bringing their vehicles to the dealerships before coming to me.
And then we get customers questioning our prices; this is why we charge what we charge experience knowledge and patience.
Excellent work Justin, thanks for sharing. Can you share how much time start to finish you had invested in this job?
If you include windshield time it’s over 4 hours.
Thank you. Do you have a “day-rate” that you would typically charge against for something like this?
I don’t have a day rate. I charge by the hour.
I'm curious, and you might not know....did the SHOP ultimately figure out that the lights had to be turned on at 30-40 mph (before you saw the car), or did the customer actually have a complaint that (awesomely!) specific to begin with? I can see a whole bunch of us getting an R/O that said "lacks power" - in the morning, when we show up for work, having no indication that the lights had to be…
I don’t know if the customer defined the complaint or the shop figured it out. The shop is the one that called me and described the conditions for the issue.
Just think how much fun it would be to figure it out if the car had automatic headlights. Customer; my car loses power every time I drive under a bridge, it needs to be a bridge longer than two lanes wide. Or in a tunnel, I can't drive through a tunnel. My wife's car has auto on/off headlights and you can tell when they turn on because the dash lights dim slightly, so how about this, Customer…
There was a story I heard so long ago I can't fully remember it, but it involved a dual filament bulb laying sideways internally like Justin found. It was pre-TAC era obviously. May have been an instructor that would rig cars for advanced students.
Justin, Excellent report, excellent case study. I'm thinking that a lot of shops would have tossed a new throttle assembly at it. :0)
Fantastic Work Justin! This just proves that when we stick to it, use our resources and the customer is willing to pay, we can fix about anything. This was a simple fix but a difficult path due to the inadequate SI information.
Just doing my part to enable the 80%
Heck of a catch. One to remember.