My son was in an accident this past summer in our '90 240. Since fixing the body damage, the cruise control doesn't work anymore. Checking through my Haynes manual, it doesn't seem to offer anything with regards to wiring diagrams for cruise control.

Does anyone out there have a wiring diagram and/or troubleshooting information for this?

Thanks in advance...

 

attached.

 

Thanks Bubba. That's exactly what I need. Something to keep me busy this evening.

 

I hope this does not hijack the thread.

My cruise control does not work either in a 1990 740 turbo wagon.
It has been that way since I bought it for some reason.

There is no way this is not going to sound like a dumb question but here goes.
Does anyone have pictures of what the cruise control unit and vacuum unit look like? If looking for them would they be on the driver side near the front of the strut tower?

 

I can't say I know anything about a 740 as I've only owned 240s. On the 240, the vacum unit is mounted on the firewall - just to the rear of the strut tower on the drivers side. You will see that it has a couple of vacum hoses attached plus some wires. That's the best that I can describe it.

Looking at the wiring diagram (yours might be different), make sure that the brake lights are working. I suspect on mine that I may be missing the speedometer signal. We have had the instrument cluster out twice this summer for other reasons and I suspect that we may not have connected the yellow wire.

Didn't have time to look at it last night. Maybe tonight...

 

I finally had the time to take another look at this. I checked the engine codes again and found that the 3-1-1 code is back. This code is no speed sensor to the computer. Most likely the cause of my cruise control not working as well.

Both the ECU and the cruise control get the speed signal from the speedometer. I verified that both the yellow wire (cruise) and the blue/black stripe wire (ECU) are tied together in the instrument panel with the signal from the speedometer. Since both speedometer and odometer work fine, I know I am getting a signal from the differential. Therefore expect that the problem is in the speedometer.

Does anyone know how I can troubleshoot this further. I can get access to an oscilloscope if I need to look for waveform signals but I don't know what to look for.

 

It's my understanding the rear speed sensor signal goes first to the speedometer and from there to the ecu. Without the ecu receiving the sensor signal, the engine will not return to idle properly from higher rpm's. I'd double check the connections on the speedo and them check the ecu to see if in fact no signal is arriving

 

Thanks Swift, that's kinda what I thought as well. On a whim yesterday, I disconnected the yellow wire from the back of the instrument cluster. It is my understanding that this is the speed input to the cruise control. I then reset the codes on the ECU, drove it around 5 miles. So far it hasn't re-thrown the 3-1-1 code.

My thinking on this is that there may be damage to the cruise control circuit that is grounding out the speed signal.

Earlier this year, my son's took out the instrument cluster because we were having trouble with the cluster dropping out (all instruments would stop working). They cleaned the connections but when they put it back in, they connected the tachometer wire (white/red) to the same connector that the yellow wire connects to. Could it be that when they did this they somehow damaged the cruise control circuit?

By the way, where is the cruise control unit located? My Haynes manual doesn't say much about troubleshooting the cruise control other than "take it to the shop".

 

Still no codes thrown on the ECU. Can anyone out there tell me where the cruise control unit is located? I found the vacum pump under the hood near where the ECU diagnostic light is located but I have yet to find the CC unit. There is a square metal box located just above the steering column behind the instrument cluster. Is that the cruise control unit?

 

Yes according to the Bentley page 360-13. Make sure fuse number 12 is good. I say you are on the right track trying to isolate it to either the CC unit or the speedometer (not sending out the speed signal on the yellow wire).

 

Thanks Bubba. Yeah, I checked fuse 12 and it checks out good. Yesterday I drove the car to work and back and checked the codes again when I got home. The 3-1-1 code came back so I guess that rules out the CC unit (since I still have it disconnected).

I guess I am back to debugging the speedometer. I may have to disassemble it to determine the signal paths through it.

Thanks for your help

 

Act1292....did you ever find out what component in the cc circuitry was damaged on the cluster when they inadvertly plugged the tach wire to the cc terminal? I ask because it's happened to me too!!

 

Haven't pulled it apart yet. I'm not in a big hurry and my plan is to pull a speedometer from a car in the junkyard and pull it apart to learn the circuitry. That way if I break something, I have a backup. I am waiting for the junkyard near me to get a 240 as they currently have none.

I'll keep you posted when I get around to it.

 

For anyone interested, I finally tracked down the issue. What I found is that in the speedometer circuit board (not the instrument panel but the board on the side of the speedometer) there is a chip labeled UAF 2115. You can do a google search and get the pinout for this chip. Is is a speedometer controller and pin 3 of the chip feeds the output to the ECU and cruise control unit. Basically, connecting the tach input to the cruise control connector fried the output of this pin. It didn't fry anything else in the chip and that is why the speedometer and odometer still worked.

Anyway, I had pulled a speedo unit from the junkyard but found out later it was incompatible with my car because the donor car had ABS and I don't. The signaling from the transaxle used by the speedometer in later models also feeds the ABS controller. Apparently the ABS unit needs a higher resolution signal than provided by the older sending units making them incompatible.

So, instead I unsoldered the 2115 chip from the donor speedometer, installed it in my speedometer circuit board, put it back together and all is good. I have cruise control again and my 3-1-1 code is gone.

A quick test with an ohm meter should be able to determine if you have the same problem. Basically, test the resistance from the cruise control output pin to ground. If it shows anything but open, then you are probably experiencing the same problem. Mine read 57 ohms.

You can purchase new 2115 chips shipped from China for about $20 including shipping. My donor instrument cluster actually cost more than this but I got a spare odometer gear plus a few other parts from it. One kind person over in Brickboard actually offered to send me one.

 

I got 51 ohms! I Have a donor cluster I'll get the chip out of. Thanks again act!

 

Looks like you are on your way. Let me know how it turns out.