W124 OM 606 electrical issues- gauges dead, won't start now - PeachParts Mercedes-Benz Forum

freesoul · #1 10-10-2009, 08:00 PM

Greets, did a quick search and came up empty.

Been experiencing intermittent disruption of tach and gauges when first starting, also idles real low. Usually I just give the pedal a little kick and the tach snaps right back up and idle is fine.

Today it did it and the gauges stayed dead the whole way home although it ran fine. There was a low pitched whine coming from the dash on the lower left side, and really odd- when I put the turn signal on it went away

Now it won't start, GP light and others come on normally, its just totally dead to crank. The battery is fine I had it on charger all day.

I got to thinking maybe this was related with my high beam issues and issue with a connector like was suggested in another thread.

Can anyone point me in the right direction?

thanks in advance

Junkman · #2 10-10-2009, 08:35 PM

Whenever a car won't crank, do the simple things first. Make sure connections are clean & tight ie use volt meter & look for voltage drop between terminal & battery cable. one end of the cable & another. Do you get full voltage to the large solenoid terminal? Are your grounds good. Use meter to test fuses. Check to see if you're getting full battery voltage to the small wire on the solenoid with the key to start. A wiring diagram is helpful after basics are tested. Have you tested your ignition switch & combo switch?

Jeremy5848 · #3 10-11-2009, 01:14 AM

You have a "transition" model with the W124 body and the OM606NA engine so I'm not sure what it has in the way of an over-voltage protection relay. It may have the same kind of OVP that the older 124s have or it may have a K40 relay like the W210 cars. Either way, it sounds like something has failed, taking out the tach and gauges and now the ability to start. We need to hear from someone who is more familiar with this specific year-model. In the meantime, follow Junkman's recommendations and check the basics.

TMAllison · #4 10-11-2009, 09:42 AM

The 95 would have the OVP; and also the flaky wiring harness too.

freesoul · #5 10-11-2009, 10:36 AM

wiring harness was replaced this year. I'll start doing some checks as suggested today. Problem is I don't know much about electrical systems but I have a voltmeter and will start slugging my way through it. Also the e-brake light doesn't come on now. Due to the sound being emitted from under the dash I'm leaning on looking in that direction. Where would this OVP be?

Jeremy5848 · #6 10-11-2009, 12:27 PM

The W124 has an "instrument compartment" behind the battery; the OVP should be there. Look for a tall slim relay with a fuse or two built into the top. If the fuse has blown, you need to find out why. You might get away with just replacing the fuse -- but it's important to wonder why the fuse blew.

If the OVP itself has failed, it must be replaced-- I don't think anyone has had much success rebuilding them. A failed OVP is hard to determine because there's nothing "bad" to see. You have to measure the voltage at the output; if 12 Volts is not present there then everything that gets its power from the OVP won't work. I'll try to get you a schematic from my 124 CD-ROM FSM.

Jeremy

-- edit --

The OVP in the '95 E300D powers the "ISC control module (N8)" and the "EGR/resonance intake manifold control (N37/3). Other things may draw their power from those two; I'm almost sure the tach gets its signal from the EGR module, for example. In any case, if these modules are not working, your car probably will not start. In the attached schematic is shown K1/1, the OVP relay. The external 10 Amp fuse is in the top of the OVP, possibly under a hinged cover. The two leads from the 87L terminal go to N8 and N37/3 as described above. If terminal 87 has no power then the fuse is blown or K1/1 is bad.

The starter is another problem. As the attached schematic shows, the only thing that might be related to your no-crank problem is the ATA module, N8. If it somehow is powered through the OVP, then the starter motor won't work if the OVP is bad. In this schematic, K38 is the starter lock-out module, S16/1 is the neutral safety switch/backup light switch, M1 is the starter, G1 is the battery, S2/2 is the starter switch and that's about all there is to it.

BTW, be sure to do a complete fuse check. Remove and inspect each fuse, don't just test them in place with a meter. Look for hairline cracks in the conductors. I assume the '95 has a fusebox near the brake booster like other 124s?

Jeremy

freesoul · #7 10-11-2009, 04:03 PM

Jeremy thanks for that info. I went to the fusebox and #5 was blown - stop lamp, instrument, front roof light, electric bulb control unit, cruise control, tachometer. Now it starts!

I've also been having intermittent issues with climate control lights working and then not working. I still need to trace back and find the problem. I'll admit those wiring diagrams are all greek to me.

I'll work on the high beam issue in the other thread.

Jeremy5848 · #8 10-11-2009, 04:31 PM

Quote:

Originally Posted by freesoul

. . . I've also been having intermittent issues with climate control lights working and then not working. . .

If it is only the climate control lamps that work intermittently, there may be a bad connection in the chain of wires that connects the illumination circuitry to the push-button unit (PBU). The PBU has two large plugs that carry all of the control signals including the illumination. A pin might be slightly corroded.

If the instrument cluster lamps work but the climate control and other console lamps do not work (check them all when it's dark) then you could be having a problem with a module that lives behind the instrument cluster. It allows the rheostat in the instrument cluster to control many lamps (lots of current) without burning out. The module contains a power transistor that takes the load off of the rheostat. The rheostat dims the instrument cluster lamps directly and everything else indirectly, through the module. To be sure, you need to make a list of which interior lamps work and which do not and then make sure all of the bulbs are good. This includes not only the PBU but all of the window switches, console switches, dash vent control wheels, etc.

Jeremy

freesoul · #9 10-11-2009, 05:28 PM

all interior lamps work except the climate control is intermittent. I'll tear into that soon

thanks!

djjonbrown · #10 01-05-2011, 09:08 AM

I just experienced the exact same symptom(as the original post) with my '95 e320 Cabriolet, but #5 is not blown. Was the ultimate solution to replace the OVP?

layback40 · #11 01-05-2011, 09:17 AM

Quote:

Originally Posted by djjonbrown

I just experienced the exact same symptom(as the original post) with my '95 e320 Cabriolet, but #5 is not blown. Was the ultimate solution to replace the OVP?

Probably but check the fuse on it first. It could simply be a bad connection.

djjonbrown · #12 01-05-2011, 10:42 AM

It appears to have been a bad connection. Thanks for the advice.

I'm going to replace all my fuses with the non-corroding kind now.

PhoenixRising · #13 02-28-2020, 06:19 PM

Wow thank you so much, this thread saved me a lot of needless work, I was having a similar problem with my 1995 E300d not turning over. It really seemed odd, we could not even jump it with another car, The battery voltage would drop when we turn the key, to 11v, but nothing from the starter. I also noticed the gauges were not working but did not think anything about it. Just about ready to remove the starter, then I read post 7 of this thread above and guess what my fuse # 5 was intermittently opening, changed it and problem solved.
Thanks again