Hey yall. I have a 1995 940 sedan, non-turbo. While on a 10 hour road trip, my check engine light came on. I had been running 70-75 mph for about 4 hours, everything seemed to be going smoothly. When I stopped for fuel I checked the code & the only one in the computer was 3-1-1. I looked it up and it was something along the lines of speedometer signal malfunction or no signal. Everything has been working fine though, speedo, cruise, everything. Can anyone help me with clearing the code(couldn't figure out how to) or helping with what the cause of this code may be? Thanks in advance.

 

indeed, speedometer signal missing. does your speedometer work properly?

there's a magnetic sensor in the differential connected to a twisted pair of wires that goes to the speedometer. if your speedometer is reading correctly, then that part is all fine.

the speedometer circuitry also amplifies and cleans up this signal, sends out instrument panel 5/1 connector D:1, and this is wired with a violet-white wire to ECU pin 34, which the ECU uses to see how fast the car is going, and fine tune its injection curve.

 

The speedometer does function normally. I did replace the wiring for my rear differential sensor about a month ago but no problems arose until just last week. I have noticed from driving it more with the code being thrown, that my idle is a little random. It surges 200-300 rpm and it pretty rough. Also, highway MPGs are down from 26 to 20. What do I need to do about that code?

 

I doubt that the 3-1-1 code would cause the Check Engine light to appear as I believe the speed signal is just used when transitioning to the idle circuit (ECU delays enabling the idle air controller until the speed is low). Could be that this code is left over from when you worked on your wiring?

What version of FI do you have? Is it a Regina or Bosch? If it is Bosch, is it LH2.4 or LH3.1? My experience has only been with the Bosch LH2.4. My 240 was throwing the 3-1-1 code and the only noticeable change was that the engine RPMs would drop quite low - so much to the point it nearly stalled - when coming to a stop at a light or stop sign. It shouldn't cause surging idle. Have you cleaned your throttle body and idle air controller lately? Also, you might want to go back and revisit your repair work on the speed sensor wiring.

Read this FAQ - in there it discusses how to reset the codes:

https://www.brickboard.com/FAQ/700-9...neOBDCodes.htm

 

I’ll triple check the code again and then try to clear it but the last couple of times it’s been 3-1-1. I replaced the wiring on the speed sensor because it was broken at the sensor. I just went to the junkyard and pulled the entire wire off of another old brick and it seemed to work fine. No codes were thrown for that even when the old wiring was broken. This code showed up about a month later during a long road trip. It does seem more & more like an idle air problem. I’m starting to experience some “wanting to stall” conditions at Reid lights and such. Idle is very rough when cold wants to choke out when I give it gas to go. Today when stopping, the idle is around 1500-2000 for a while until I sit for a bit then it will drop to normal. I will clean my IAC as soon as possible to see if that helps.

 

 

Just to clarify - you can have engine codes without the check engine light appearing. The only codes that must turn on the check engine light are emission related codes. The 3-1-1 is not one of them so more than likely there was another issue that caused the check engine light to turn on. When I had the 3-1-1 on my 240 (LH2.4), there was no check engine light. However, my cruise control didn't work and that led me to diagnosing it as a bad taxi output on the ITT2115 chip in my speedometer.

In your case you have a model car and speedometer so my knowledge may be of limited use. If it were me, I would check which pin on the speedometer runs to the speed signal on your ECU and ohm it out to make sure your getting the signal from the speedometer to the ECU. Since your speedometer and cruise control work, I must assume the signal is making it to the speedometer. The symptoms of almost dying when coming to a stop do relate to a 3-1-1 code. However, you need to chase down the poor idle just to be sure.