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Old 08-02-2001, 11:50 AM
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Angry max RPM 3200, 450sl, '75, 110K

My car starts, but won't rev above 3200, and bogs down badly if I try to go above 3200.

Have been reading all the posts about OVP problems and see similarities to my problems.

When I got it back from shop that installed new valve cover gasket, it was running very rough. Exhaust is heavy and blue-white. Had some very black carbon exhaust, but that has cleared up.

I replaced points, plugs, plug wires (but not the ends at the plugs), coil, distributor cap, and rotor. It was due for renewing of all these parts anyway.

My new points have the black and white color code for the improved type that are not supposed to pit, so do not need a condenser. I clipped the wire on the distributor condenser, per instructions from a MB mechanic, and posts seen here. (Before I clipped the wire, I had all the same symptoms, and no change after clipping it.)

Have tried everything re: timing & dwell.

I have checked continuity and impedance for all wires in the ignition circuit. Also, no obvious high voltage leakage (no sparks from wires near grounding points).

I have not messed with the contact points that work the fuel injection system. No reason to believe something suddenly failed there.

Also, no reason to believe the timing chain has jumped a tooth or two, but I have not checked it yet.

Spark looks weak to me. Not a bright strong (white or blue) spark. Looks orange and weak.

Car had been jump started due to low battery (led me to believe OVP problem, from other posts). Bought new battery.

Have looked at the wiring diagrams in great detail and find no OVP. I think 1975 is pre-OVP.

I think I have a Cold Start Relay, if that has the same function. All fuses are fine. Cannot find a 10 amp fuse that would be in the ignition circuit.

Also, may have an Override Switch, according to my wiring diagram.

So here are my questions:

Q1: What is the 1 1/4" cubical aluminum component that sits just 1" forward of the ignition coil (driver side of engine)? It has one of the 6-prong connectors (1 1/4" X 2") attached. Number on side is: SR9852 0015422849.

Q2: Do these (part in Q1) fail often and does failing yield the described RPM problem?

Q3: Does a bad/failing alternator result in the limited RPM, as I am experiencing?

Q4: Is the Override Switch the same as Cold Start Relay?

Thank you,
Steve Guinter
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Old 08-02-2001, 04:41 PM
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Steve, although I don't have the answer as of yet (look under "124 Tranny" thread) - I have the same problem without the exhaust, etc. My 124 (88 300TE) will not go above 3200rpms either. At first I thought it was the trans - I am sure this is not the problem. I am in the process (this week) of having the cat. conv. looked at - however initial response from my tech was that I would be seeing power reductions all over the place if it was clogged. If you find out before I do, let me know. Other then the 3200rpms, mine runs and idles fine.
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Old 08-02-2001, 06:11 PM
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Join Date: Jul 2001
Location: Tampa, FL
Posts: 69
When's the last time you changed your fuel filter... sounds like it might be plugged. This would restrict your fuel flow and since your engine would be pulling more fuel the faster you rev it, it would starve if the filter was clogged. I'd try changing it.

Good luck!

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Old 08-03-2001, 09:02 AM
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I agree with Michael and for my 124 that is on my list besides the catalytic conv. Since Steve is experiencing almost the exact problem as mine though (3200rpms), it seems to me it may be something such as a vacum line, etc. I am puzzled by the magical 3200rpms, it runs fine below this (with A/C on or off), although maybe a little hot. According to local techs (I have been just talking with them - the haven't look at the car yet), if it was a filter or catalytic conv. problem, we would see power reductions at various other stages also - maybe. Anyway, anything helps and it seems we are getting closer.

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Old 08-03-2001, 10:01 AM
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Rick & Michael,
Thank you for your comments. I will use all suggestions for further diagnosis. I will post results, too.
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Old 08-03-2001, 11:59 AM
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There is no OVP on a 75 450. Check the distributor and see if you're getting any advance when you rev the engine. I had a 4.5 with similar syptoms caused by a frozen distributor years ago. Not only was it slow, but the exhaust manifolds would "glow" bright orange. You can also put a vacuum gauge on it to check for a plugged exhaust.
Randy D.
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