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Pete, Technician
Category: Car
Satisfied Customers: 23900
Experience:  17 years automotive training and experience.
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missfires and speedo connected

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My car is a 1996 Volvo 850 2.4 20 valve with a manual trans. For the record I have all electrical parts are new ( from coil to plugs), fuel pressure good at rail, no vacuum leaks, new o2 sensors, new camshaft position sensor, crank position sensor, new air pump controller and valve, ops check pump and relay, compression are all around 200 psi. I keep getting misfires all cylinders 6 total p0300 - p0305. I have read many post buy my seem unique. When I reset the codes and start driving to get it back to ready status the speedometer dying will trigger freeze frame code p0300 then the mil light comes on even after 3 to 50 miles. If my speedometer don't die or go to zero while driving the p0300 won't show up. The only problem is that the ready status won't come back on; no matter how many miles I drive." even hundreds of mile" . It won't reset so I can pass emissions. Why would the speedometer working or not working effect misfires.

My name is XXXXX XXXXX I am a professional here at Just Answer. I noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and hope I can still assist with this here.

Are any other fault codes occurring and did this issue start before or after you had replaced all of these components?

With your scan tool can you read engine or transmission data with it to see what the vehicle speed signal is showing when this acts up?

Let me know so we can continue,thanks Pete.
Customer: replied 3 years ago.

The ABS light has always been on the whole time I have had the car, which was on before I had any problems. My ignition switch spring also has been week or broken before my speedo had this intermittent problems. I didn't have any other codes besides p0300 - p03005. The parts where change because of the codes. This what I do know. After clearing the codes I drive the car to so that the mil status will come back on so I can get my emission test done. As long as the speedo works the P0300 code won't show up in the freeze frame and I have no pending codes. As soon as the speedo goes off line immediately P0300 show up in freeze frame data then the cars MIL light comes on with in 0 to 10 minutes. If I reset the codes and try it again {and my speedo doesn't work before I even drive the car} I will get the P0300 code right away. If I reset and start driving again but the speedo works I get no codes. The problem I can't drive long enough to get the MIL status to go to on to do my NC emission test. If I drive at night or early morning the speedo works. Once it get hot the speedo doesn't work. So what is common to both. Going to change switch but need to know what else connects the two. Review: Heat affects speedo, and speedo going off line instantly causes the codes. They go hand in hand but don't know why. Hope this helps.

On this vehicle the speedometer receives the speed signal to operate the speedometer properly from the anti lock brake control module.This module/computer receives the signal from the front abs wheel speed sensors.With this speed signal an issue here the abs control unit shouldn't cause the misfire codes unless this is an electrical interference fault causing the P0300 codes to set as I have seen before or the abs control unit is shorting internally causing an issue with the engine computer to set the codes as they use the same data line.

Most likely this is a fault with either one of the front wheel speed sensors or the abs control unit itself at fault needing replaced.

I would check into the abs codes and correct the abs fault now,this sounds like a new fault in the abs system then what you had had before.Then if there are codes set for the wheel speed sensor signals these need checked to see if you have a faulty speed sensor.If all speed sensors test o.k. I suspect an internal control unit fault with the abs control unit itself.

Thanks Pete
Pete, Technician
Category: Car
Satisfied Customers: 23900
Experience: 17 years automotive training and experience.
Pete and 11 other Car Specialists are ready to help you

I am following up with you to see if you still needed help with your question you had asked.If my help didn't satisfy your question please let me know so we can continue.

Thanks Pete
Customer: replied 3 years ago.

I still have the issue but I cleared the codes and drove the car while it was cool (heat caused the speedo to fail which gave me missfire codes) and pass emissions Monday morning. Now I have a year to fix the problem. Thanks for pointing me in the right direction so I can eventually get the car fixed.

Your welcome.Let me know if you need anything else in the future.

Thanks Pete

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