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Won't Rev Past 1,100 Unless Foot Is On Brake Pedal
Been gone a while, but I'm back with a '94 S420 that I'm bringing back to life. 142k miles.
The issue is the car won't let the engine go above 1,100 RPM unless my foot is touching the brake pedal. Sounds crazy, but it's true.
When I accelerate, the engine starts to fluctuate when it hits 1,100. The engine will go to 1,300 and then back down to 1,100 over and over again. But if I touch the brake pedal (even as lightly as I can) the engine will rev all the way up to redline. Basically the engine works great if I'm touching the brake pedal.
If I get the car up to 3,000 RPM, then remove my foot from the brake pedal, the engine goes back down to 1,100 and fluctuates to 1,300 again no matter how hard I'm pushing the gas pedal.
Any ideas?? I've replaced the vacuum line to the brake booster at the suggestion of someone from another board. I'm starting to think it's electrical, because of how lightly I can touch the brake pedal to allow the engine to work normally.
The engine wiring harness is a newer Delphi model, so insulation shouldn't be an issue. I don't know about the throttle body wiring. How can I find out?
Also, I've pulled fuse 23 (brake light, ASR) and the car wouldn't rev past 1300 with my foot on or off the brake pedal. Could this be an issue? What modules get their power from fuse 23 on down the line?
carefully peel a little tape off wiring in different places.Also on v8s there a wire that rubs on the block somewhere.Sound like brake light grounds the throttle body.Check engine ground too,they are never large enough,I always add extra ground strap to motors.
As from your description above, I would also believe it to be an electrical problem somewhere. You asked about the fuse # 23, different MY vehicle are wired differently! Example, 1998 MY, fuse # p23 feeds to Cruise Control switch, Stop Lamp switch, Auto Dimming inside Rear View mirror. and get its power from the ignition switch. according to diagrams from Mitchell. I would try removing the brake light switch and check its serviceability, if you can. (...its tricky on the two contacts!) Otherwise, just get a new one. Also, make sure that the connectors (2 off) at the ASR control module are electrically sound!
A good diagnostic scanner would come in very handy here, as in any or most of these situations. i.e. needed to erase some DTCs that can prevent engine from performing/run well. Beside of course telling us whats going on inside the many control modules. Just curious, does your model have a "base module" installed in the electronic compartment? (...coffin shaped box). This base module provides powers to a number of electronic control modules also inside the compartment. Good luck and hope this help!
I've replaced the brake light switch with a working one from my S500. No change in behavior. I'll look for the grounding straps and get a better ground. I wish I knew more about what wire usually rubs against the block so I could check it.
Also, yes my '94 has a base control module in the coffin box. All 4 fuses are good in it.
Is there any need to pull a throttle body from a junker car yet? Should I have it just in case?
mbs xk8, good to know that you got a base module installed and that the 4 fuses are good. Now can you remove fuse # 3 (...viewing from left to right) and with ignition sw. on position 2, check for battery voltage. Simply probe both legs! If nada, then your base module is "shorted" and needs attention! IF, the battery voltage is present, then we need to dig in a little deeper. For that matter, why not probe all the 4 fuses to ensure there is voltage there. Just ensure that the position of fuses, (...hmmm, i got only 3 fuses in there !) Maybe, one of the fuse is sitting on an empty slot!?
It is always good to have a spare working parts around. But I would not get one at this point. Obviously, no electrical shorts are tolerated anywhere in this vehicle. You mentioned that newer Delphi wiring harness was installed, then we can assumed that those wires to the EA and others on the throttle body assembly should be fine. Your vehicle is responding well as you've described, i.e. ...when brake pedal is "touched". Your removed brake switch is good after all since the change over does not help in your situation. You also mentioned that when you pulled fuse# 23 at F3 fuse box, engine does not revv up. This also is responding well, that this fuse 23 is doing its job! Now lets check out the base module!
All fuses (4 lol) have power. Using the coffin box as ground, there’s power at all 4.
Very interesting and you've done well so far! recognize that your base module has 4 outlets and has power to it, shows that is different circuitry from mine!!! Unfortunately my WIS expired and is not functioning. However, those circuit has the same idea to its operation. You'll have to substitute those pin numbers from what am going to mention below:
Now lets check to see if those voltages arrived at their respective control modules, M.E. and ASR. On mine, their are piggy back to each other by four bolts. Guess that would be the same arrangement here and you're familiar with this vehicle. IF, those voltages are present, then we need an even bigger "thinking cap" to put on! IF, nada, then that could simply be a wiring issue.
With box cover removed, disconnect connectors (Left side only or rear) of M.E and ASR control modules. Pin # 1 of ME and pin # 11 of ASR, lets call it "pair 1"." Pair 2" are pin # 3 of ME and pin # 4 of ASR. (...substitute where necessary) They receive voltages from the brake switch NO and NC respectively. With ignition switch at pos. 2, voltage should be present at pair 2, serve by NC switch. If the brake peddle is depressed, then pair 1 should have voltage!(...serve by NO switch) Now, the bigger thinking cap comes in when the above are "true"! If "false" , then its only wiring issue.
No ASR module, no problem. ASR is just a glorified ABS control module. Just trace or find the wire, carrying battery voltage from brake switch to the ABS module and M.E. (e.c.u.) or LH module.? Wiring diagram would be a tremendous help here I can see the connectors are all installed below. Just pull ALL the control modules out for easy access to the connector below. Some wish to disconnect battery Neg terminal before doing this. Know your radio codes. etc.
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Jumping ahead with this thought in mind. What IF, both battery voltages from brake light switch are found at the their respective modules? How do I determine which module, LH (ecu) or ABS module is the culprit? I would bet on the LH module! Unless, you have a strong hunch feeling that the ABS module is not functioning i.e. ABS warning light on dashboard etc. Basically, the car is in a 'limp home mode" But this is a more costly approach! Worst does LH needs variant coding before putting in a replacement item??? Mine does! (...damnit!) So, the bottom line here is confirming the battery line to its control modules would be more easy on the pocket and your heart! LOL!!! Also, in the process you get to know the vehicle better!!!
I was splitting my time between getting this W140 up and running, and diagnosing engine problems with my Range Rover P38. (I love those P38s and hate them at the same time. I wish the engines didn't have so many issues - lol)
So somehow I fixed the rev issue. There were several computer modules included when I bought the car. I was also told the 420 engine was replaced with a 500.
I replaced module 0175454432 (LH 4,2I) with 0175454532 (LH 5,0I) and I can now rev to redline without it cutting out at 1300. I can rev with my foot on or off the brake pedal.
My E-GAS module is 1405457132. Which I’m hoping is compatible with the new 500 module above. Someone wrote 420 on it.
Am I on the right path?
my idle is very low and I have to keep my foot on the gas to keep the engine running. Is there an idle control?
also, my tach, mpg, oil pressure gauge are not working now. They worked yesterday.
What a day! I think I found the source of my rough idle. I was looking at the rotors and followed 2 wires crack to the coils. One of the plugs on top is missing the metal part that goes down into the coil.
Take a look at the picture.
Now... where can I find a new wire (besides the dealer). There aren’t any older W140sin nearby junk yards. Would NAPA have this?
Good! now that the vehicle can run using the replaced LH module, go for a warm up drive, a looong warmup drive! Of course after fixing up the HT cable as mentioned above. Ensure U got clean engine oil and filter, air filter and ALL fluids are ...sound. Brakes are good! I would leave the "small trouble" aside for now. Even the E-gas module aka Tranny control module, leave it in there. Just ensure the vehicle is safe to drive around to "run-in" After many, many "trips" hope that the small troubles will go away! Sometimes, it fix itself! Congratulation! you made it!
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recognize that your base module has 4 outlets and has power to it, shows that is different circuitry from mine!!! Unfortunately my WIS expired and is not functioning. However, those circuit has the same idea to its operation. You'll have to substitute those pin numbers from what am going to mention below:
