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Old 07-18-2003, 09:26 PM
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Join Date: May 2003
Location: Atlanta
Posts: 398
Question Cold Temp, R16 resistor, High RPM – HELP!

Check out this madness. I have a 1990 300E with the following symptoms:

1) Temp gauge reads slightly over 40…all the time (in traffic with AC in Georgia!). Well actually not all the time. Every now and again it will actually read the correct temp.

2) Intermittently, upon start up of a *warm* engine, the RPM will zoom up and idle at 1800-2000. After maybe 40 seconds, it drops down to the normal 750 or so.

3) When I remove the R16 resistor and start up after the engine is warm, it will rise to 1800-2000 and STAY there. I have to turn off car, reinstall resistor, and restart car at which time it will do what I described above.

Here is a totally make up theory of mine. As I understand it, when the car is cold it raises the RPM to help with the start up. So now, because of a bad temp sensor somewhere, the car perpetually thinks it's running colder than it is. Thus the low gauge reading and high RPM.

I have no idea what the relationship is between the RPM problem and the R16 resistor…if there even is a relationship. Unless I am totally confused, the R16 resistor advances the engine so by removing it I am retarding it back to 0 degrees.

Does this make any sense or am I on drugs?

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Old 07-19-2003, 11:31 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Must be the drugs (bg).

The R16 resistor will work exactly opposite as you describe. Removing the resistor makes the timing advance (6deg I think).

I think your inside temp propblem is unrelated to the idle problem as there are different sensors involved with temp sensing on those two systems on that car.

to evaluate the idle problem one might wish to view the control. An unplugged idle valve will not run the motor fast on that system as it will the early v8s. To get that idle speed requires either such a command from the controller or a significant varaible vacuum leak. The way to watch control is with the same ammeter arrangement used for EHA current measurements. Proper control usually ranges from 600-700ma at idle to reading of 800-900 while driving as the valve is openned to be prepared for a sudden stop. It should immediately drop to the 6-700ma range at idle condition.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-19-2003, 12:49 PM
haasman's Avatar
Registered User
Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,097
I would also verify the coolant temperature. The 91 300E had a low temperature problem, never seemed to get above 80C. I went through the car including new Tstat etc. It turned out the temperature sending unit was bad.

If would wiggle the connector and the gauge would jump to normal. Like we have all heard, check the basics firts.

My 2 cents,


'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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