820380SL still has running / idling issues - PeachParts Mercedes-Benz Forum

JustJim · #1 11-19-2015, 06:10 AM

Got the sl running enough to drive it finally, but it quit on the road. Found the fuel pump wasn't coming on. Jumped from the battery to the fuel pump and got it started and drove back home. Disconnected the jumper and the car kept running. Got under the dash and found the fuel pump/kickdown relay was very warm to the touch. Also, the car sputtered when I moved the relay. When I unplugged the relay the car quit. Plugged it back in and re-started the car with no problems. Shut off the car for the day. Came out next day and the relay was still very warm to the touch. Is the problem the relay, or something powering the relay, or both? Computer? Idle control module? The idle is ok when first started, but after about 10-15 minutes the idle goes up to 1700-2000 rpm.

Don't want to just throw parts at the problem. Any suggestions?

Thanks.

meltedpanda · #2 11-20-2015, 10:47 AM

original or after market fuel pump? MAny after market are not able to do what needs to be done on these ol gals. Fuel pump specs for pressure and GPS is critical.

Could also be a bad relay , swap a know good one

JustJim · #3 11-20-2015, 12:47 PM

Thanks for the suggestions. It is the original fuel pump. I already ordered the relay because I know it has a problem. I don't know if it's the only problem. Or if the relay can cause all of the issues I described. Also, the relay has to be drawing power when the car is turned off or it wouldn't be warm. I'm trying to figure out where the power is coming from, whether a defective relay can draw power on its own or if something else has a problem and is supplying power when it shouldn't. I'm not sure where to look for the source.

Any ideas?

Thanks again.

meltedpanda · #4 11-20-2015, 03:00 PM

a stuck relay can draw power, they are meant for most part to be open until energized.
Of course you could also have a short in the fuel pump circuitry

Using a tester , you can look for parasitic draw, pull one fuse at a time to see what circuit it is on, or you could also do that at the fuse panel , ground on the outside and checking with the hot lead for voltage

Good luck, electrical gremlins are not fun

JustJim · #5 11-20-2015, 06:03 PM

Thanks for the input. Makes sense. Any idea on where I can get a schematic of the relay circuitry so I can check it?

Thanks.

meltedpanda · #6 11-20-2015, 06:51 PM

PM Frank Reiner ( on this forum) or perhaps he will see this post, IIRC he knows this fuel system well

JustJim · #7 11-22-2015, 06:56 AM

Thanks for all of the help. I borrowed a fuel pump regulator from another 82 380sl and the car ran fine with this one installed. Also found that if I bumped the wires on the idle control module, the car sputtered. Took a closer look at the wiring and found one wire had been cut and spliced. When I touched the connector the car quit again. Removed the connector and found corrosion inside. Cut the wires and soldered them, problem solved. Back to the relay...The one I removed from my car is p/n 001 545 5305, and the one I borrowed is 001 545 3605. When I re-installed the 5305, the relay heated up again and the rpm increased to 1700-2000 rpm. Is there a difference between the two relays?

meltedpanda · #8 11-22-2015, 07:13 AM

Yes there is a difference the 5305 is wrong for your car. Has a higher RPM cut out I believe. For later models

JustJim · #9 12-15-2015, 05:55 AM

Ok. Thanks. Finally bought a new 001 545 3605 relay and the car runs much better. Still idles around 1600 rpm in park or neutral, but drops to 750 in drive with the brake set. When warms up is around 600 rpm in drive with the brake set, but jumps back to about 1600 rpm in neutral or park. Is this normal?

rowdie · #10 12-15-2015, 08:14 AM

Not at all normal. Check the over voltage protection relay for a blown fuse. There is an idle control unit. The solder joints can crack over time. Try flexing the unit to see if it changes.

JustJim · #11 12-16-2015, 11:48 AM

Over voltage protection relay fuse is good. Not sure about the idle control unit. Tried flexing it but don't know if I was able to flex it enough to do any good as I didn't notice any changes.

JustJim · #12 12-22-2015, 05:52 AM

Tried checking the wiring, and found that the RPMs dropped considerably when I bumped the computer. Unfortunately, I also had no power. Pushing down on the gas pedal caused sputtering. Turned the car off and re-started it. Still idled around 700 RPM but would sputter when gas pedal pushed. Turned the car off. Came back after 4 hours and started the car, and it was idled around 1500 RPM, but climbed back to 1700-2000 RPM when it warmed up. Will the computer cause this problem?

meltedpanda · #13 12-22-2015, 10:34 AM

yes sometimes the board connections become broken or less "connective" due to years of vibration. Or the connections themselves have become oxidized, clean them up and see if it helps

timm9 · #14 12-23-2015, 05:17 PM

Is it running rich? Is the exhaust staining the garage floor?

Similar situation some years back with an 82 107. Spent every free moment trying to resolve with replacement of oil temp sender, engine temp sender, analog brain, OVP, and more. After months finally found 7 huge 'cracks' in the Air Chamber allowing too much air into the system which was telling the computer to 'add' more fuel. $85.00 part resolved the high idle completely. It was only discovered after a 'smoke' test. This particular car only had 41,000 on the clock and was well cared for so I can only assume the material the chamber is made from was too weak for the heat.

nobby · #15 12-24-2015, 03:35 PM

Quote:

Originally Posted by timm9

Similar situation some years back with an 82 107. After months finally found 7 huge 'cracks' in the Air Chamber I can only assume the material the chamber is made from was too weak for the heat.

How about being 33 years old? Mileage and age have different affects on parts.

33 year old rubber is pretty much useless.