P0120 Check Engine Light On When Cruise On - PeachParts Mercedes-Benz Forum

samu · #1 06-17-2009, 12:21 PM

My 1996 E300D Check Engine Light with P0120 code came on after using the cruise for a couple of minutes. Anybody knows what is wrong?

Here is what happened. I was back on a long trip. The cruise was on. When I was closed to home, the Check Engine Light came on. At home, I use my Actron scanner to erase the code, drove for another week without using the cruise. No Check Engine Light. Then, I tried and put on the cruise again, LOL, the CEL came back on. Please help.

TMAllison · #2 06-17-2009, 12:55 PM

That is a circut code for teh TPS.

Talk to Jeremy.

samu · #3 06-17-2009, 03:05 PM

Quote:

Originally Posted by TMAllison

That is a circut code for teh TPS.

Talk to Jeremy.

Thanks Terry. per your recommendation, I've sent a pm to Jeremy, waiting for his response. THX Sam

Jeremy5848 · #4 06-17-2009, 06:21 PM

Here is what I said to Sam:

P0120 has to do with the TPS, the throttle position sensor. However, a number of things can cause that code. Questions: Does the cruise control stop working when the Check Engine light comes on? Does anything else stop working correctly? Engine runs correctly? Can you still change speed with the cruise control, turn it on and off and all that?

I would suggest unplugging, cleaning, and plugging back in the cable at the TPS and the cruise actuator. I don't remember where the actuator is, I have never messed with it. You may have to pull the intake manifold to get at it or you may be able to access it from underneath after removing the belly pans.

It is also possibly the K40 relay which is in the relay box on the right side, where you would expect to see the battery. The K40 relay has a number of connectors and fuses on top. Make sure all of the connectors and fuses are clean and good.

It is unlikely to be the TPS itself as it has no parts that would wear out, only two Hall Effect sensors (special semiconductors) that rotate in a magnetic field. Plus it is a $500 part that you don't want to have to buy.

The car is probably safe to continue driving while we figure out what this is.

Jeremy

samu · #5 06-18-2009, 08:57 AM

Thanks Jeremy,

As soon as the CEL came up, I hit the brake and continued to drive. Beside not using the cruise, everything was normal.

I'll remove the intake manifold and clean the cable. I remembered seeing one cable attached to the manifold.

As far as the K40. I just replaced it last summer. I'll take it out and clean it again and see what happen.

Driving is no problem even the CEL came out. Just a pain to see it.

Jeremy5848 · #6 06-18-2009, 11:21 AM

So the cruise continues to work even with CEL on? Are all of your tires the same size and age (miles)?

The wires attached to the manifold are for the "tuning" flaps, one at each end. Probably not related but make sure the IM and the flaps are clean and there is no binding. If you wish, you can block the EGR at either its inlet from the exhaust manifold or the outlet to the intake manifold (or both!) and the IM will stay cleaner.

samu · #7 06-18-2009, 11:27 AM

Once the CEL on, I hit the brake and cruise control.
All tires are the same sizes. I'm going to just take the IM off and clean the cables and the flap and see what happen.

TMAllison · #8 06-18-2009, 11:44 AM

Being a circuit code I'd rule out the simple stuff first Jeremy initially pointed out. Check/clean the harness connection to the TPS, confirm fuses on the K40, etc. Circuit codes usually point to a loss or interruption in communication. Can be as simple as a voltage issue, loose connection, or a failed/failing part.

Jeremy5848 · #9 06-18-2009, 04:43 PM

Just called my indy, he has never seen or heard of such a problem, unfortunately. Let's hope that this is just a loose or dirty connection, as Terry says, rather than an impending expensive failure in one of the electronic subsystems.

samu · #10 06-18-2009, 08:46 PM

Thanks guys.

I'll work on cleaning all connections: K40 fuses, tps connections sometimes next week.

My other 2003 jetta tdi is having a rear main seal leak too. Two problems at the same time.

Not my month.