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1995 E320 Cabriolet
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141 Posts
Discussion Starter · #1 ·
Can anyone direct me to a good thread for diagnosing an M104 that won't start? Looked in the 124 DIY links, and there's an M103 troubleshooting guide, but I didn't see one for the M104.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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18,079 Posts
Before anything else, put in a full set of brandnew copper core ceramic fuses such as those made by Flosser. Do not use the plastic Chinese dogsh!t autoparts stores sell as you risk melting down your fusebox.

Then determine if the starter engages. If it does, continue reading. If it doesn't, alarm module is probably acting up.

Have you put in a brandnew OVP (either OE MB or KAE)? Without a good OVP, a HFM M104 will not run.
 

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W163 and General M Gremlin
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7,666 Posts
Any CEL?
Use the appropriate scan tool for the w124s, to see what codes, if any.
 

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1992 CE 300-24
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214 Posts
Have you checked the 'StarTek' sticky
thread in the W140 section on this forum?

You should find diagnostic/test protocols
to suit your model of M104, there.
 

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Registered
1995 E320 Cabriolet
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141 Posts
Discussion Starter · #6 ·
Before anything else, put in a full set of brandnew copper core ceramic fuses such as those made by Flosser. Do not use the plastic Chinese dogsh!t autoparts stores sell as you risk melting down your fusebox.
Good idea, I'll check to see what's installed and probably replace all just for general purposes. Did that on my father's 126 420SEL and cleared up a couple of electrical gremlins that were plaguing that car.

Then determine if the starter engages. If it does, continue reading. If it doesn't, alarm module is probably acting up.
Starter engages. Earlier when it did this, I could get it to fire and run with a shot of starter fluid, so I replaced the fuel filter, and all was good for a month or two.

Have you put in a brandnew OVP (either OE MB or KAE)? Without a good OVP, a HFM M104 will not run.
OK, will check that. Have a spare in another non-running car, but not sure if that one is good. Is there a test other than replace with a known good unit?
 

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1995 E320 Cabriolet
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141 Posts
Discussion Starter · #7 ·
Any CEL?
Use the appropriate scan tool for the w124s, to see what codes, if any.
No CEL as the car doesn't start / run, and there wasn't one prior. I have cobbled together a flash code reader and did check and clear a bunch of codes before all this, when it would not start due to a clogged fuel filter. I'll check again and see what the new codes are.
 

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1995 E320 Cabriolet
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141 Posts
Discussion Starter · #8 ·
Check fuel pressure
That's a good thought. I have tried starter fluid, doesn't help, so I think this is spark related, not fuel related. Can you direct me to a section in the FSM for this test?
 

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1995 E320 Cabriolet
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141 Posts
Discussion Starter · #9 ·
Have you checked the 'StarTek' sticky
thread in the W140 section on this forum?

You should find diagnostic/test protocols
to suit your model of M104, there.
No! Excellent, I'll search that out. Thanks!
 

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1995 E320 Cabriolet
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141 Posts
Discussion Starter · #10 ·
I see that degrading wiring insulation is a common cause of issues, is a "no start" condition included? Where do I look to see if this is a problem? I DO NOT know my way around the M104, total newbie on this engine, so please be specific. What the heck is an ETA? Much to learn...
 

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12 odyssey (90k) 1995 E320 wagon (307k) 1983 500se (172k) 1995 Cabrio (123k)
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3,235 Posts
Eta = throttle body
Check behind splash shield near battery. Looking for label that says Delphi. Or look at wires on water pump. Insulation should be colors (blue or green) if it looks like speaker wire it's eco junk.
 

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95,94,93 e320/300e
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402 Posts
Check behind the battery right side for a silver tag on the main harness. If you don’t see one you should probably shop around for an updated main harness.

To check fuel pressure.. there’s a Schrader valve on the fuel rail. Get a pressure guage kit and tap into that.
 

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W163 and General M Gremlin
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7,666 Posts
Before any other troubleshooting.....


one example of many - check your tags/labels first.
You can also cut open carefully, the outer jacket or tesa tape and inspect the wiring.

Screen Shot 2021-07-22 at 6.37.10 PM.png
 

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Premium Member
2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,574 Posts
A few items:
No start on starting fluid, don't mess with the fuel system (yet).
OVP - On a '95 M104 bad OVP = no start, no questions asked.
Fuse issues, especially Fuse 7 = no start, even on starting fluid.
CPS - Bad Crank Position sensor = no start, even on starting fluid.
Ecco-junk wiring - a real problem for a '95, but a total no start on starting fluid would would unlikely, run like crap, yes, total no start, no. In the long run, this must be absolutely addressed though.
 

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12 odyssey (90k) 1995 E320 wagon (307k) 1983 500se (172k) 1995 Cabrio (123k)
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3,235 Posts
I'm with flooby, my experience on 94-95, it's the ovp.
 
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