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1989 300SEL
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Discussion Starter #1 (Edited)
Car started immediately after 1wk vacation - 89 300SEL.
Checked all lights, etc as I was going to get it inspected.
Car started immediately next morning and I drove it only a mile or so.
Car failed to start that same afternoon [it knew it was going to be inspected?].
I have had problem before and after about 5 start attempts spaced 30 secs apart, it would start [falsely blamed fuel pump].
Tried 7 cranks - all powerful, but no firing.

Using service manual disc:
I started with the diagnostic for the fuel pump relay.
Jumper jacks 7 & 8 with ignition switch off - fuel pump runs - ok.
Voltage test jacks 9 & 11 with ignition switch on - no voltage - not ok.
Says to trace wiring for interruption.

Wiring diagram [page 108/2] says fuse to ignition system is number 7.
Fuse box diagram [page 102] says fuse 7 is horns, rear defog, turn sig/haz and warning indicators.
No fuse says 'ignition'

First would check to see if fuse is obviously blown.
Next fiddle with it and clean contacts, etc.
With fuse out, test voltage at ignition switch side of fuse with ignition on to see if ignition switch is ok.

I have seen where people jumper battery to coil primary or diagnostic socket jack 5 when ignition switch is the culprit to limp home - then have to remove jumper to turn off engine.
But what of over-voltage protection relay? Being bypassed not good?
How does the OVP relay work anyway? My battery voltage is 12.25.
 

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1989 300SEL
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27 Posts
Discussion Starter #2 (Edited)
More info from diagrams

In looking at diagrams 105/1 and 106 it appears clear that the #7 fuse shown in 108/2 is NOT used since the connection is on the power side of the fuse, not the protected side.

So there is no fuse to look for and the place to check in the RUN position is the hot side of this fuse - if no voltage to ground, then the ignition switch [or wiring to it] is the culprit.

Of course all the items protected by fuse #7 would not be working either. Hmmm.

Please confirm, thanks.
 

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hmm, sorry I can't help but here's what I have gathered about the OVP...The OVP provides consistent voltage the ECU and ABS systems to prevent their "computers" from being fried. It saved my ECU when I idiotically hooked the battery up backwards. From what I've seen symptoms of the fuse on the relay or the relay itself being fried are hard starts and very poor running. Bypassing it is definitely not a good idea, I would imagine you could easily fry the ECU if you tried to jump the car or something.
 

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1989 300SEL
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Discussion Starter #4
Yesterday I started with the fuel relay test procedure and ALL checks passed.
And of course, the car started and ran great as usual - only 123K miles!
So now it is a real mystery.

Does the 89 300SEL use a transponder key?
Could this be my problem?
Seems that it could keep the ignition from firing, but one would think the power in the RUN position should still be present.
 

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No transponder key... Was it cranking before when it wouldn't start? The ignition switch seems to be a relatively common failure point in these old things. Also did you try starting it in neutral?
 

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1989 300SEL
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Discussion Starter #6 (Edited)
Yes was cranking. No, only tried in park.

Next time it does it I will try wiggling the shifter in park and try neutral.
And I did not look for the fused items that should have been on.
Next time it does this I will check for them as none should be on if it is the ignition switch.

It has never quit while driving or idling.
Willl try to go get it inspected today.

Thanks for your comments.
 

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If it was cranking it probably wasn't the ignition switch or neutral safety switch. Maybe the ignition coil/rotor or distributor cap is going? Next time it happens check if you're getting spark. I really only know the basics so hopefully someone more knowledgeable will pitch in..
 

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1989 300SEL
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Discussion Starter #8
But certainly it could crank as that is independent of supplying power to the ignition.

What indicated that it was not getting power was that the fuel pump was not running but removing the fuel pump relay and jumping it, it ran.
But the power source on the jumpering is direct from battery, so it only proved the pump works.

The next relay test was power from the ignition switch, which read zero, thus the relay was never going to be able close.
Wires all looked okay, so it still seems to me it is in fact the ignition switch.

If it happens again, the proof will be that all circuits energized by the ignition switch will not have power which includes those on fuses 5, 6 & 7 when ignition switch is in the run or start positions.

Just started again - going to get inspected ......
 

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2012 CL550 4MATIC Coupe
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9,992 Posts
I would check the X26 connector. I had a bad pin there on Pin "1" which is the main power supply to the FPR and that caused my FPR not to work. I had to pull the socket, clean the corrosion and replace Pin 1's Molex connector.

Check Pins 1, 7 and 9. Those all feed the FPR and are a point of corrosion.
 

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1989 300SEL
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Discussion Starter #10
I hope you are on to something here. [thus NOT ignition switch!]

I completely missed the X26 as being in the path to the FPR on diagram 108/2.
How is the connector separated - I do not want to break anything.

The top section seems to wiggle relative to the bottom section.
I pulled a little but stopped - what is holding it?
Do you simply pry the two apart with flat blade screwdriver?

And, is that a cover at the top of the top section? Or just part of the section.

I will await your reply before attempting to take the connector apart.

Thanks.
 

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2012 CL550 4MATIC Coupe
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Do you simply pry the two apart with flat blade screwdriver?
Yes, insert laterally and gently pry apart. Also, once it's off, test the pins for continuity, opens and shorts to ground.
 

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1989 300SEL
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Discussion Starter #12
That was it - pin/socket corrosion

would not start again
checked for power at pin 1 on X26 with ignition-switch in run position - nothing
pushing on the underside with voltage probe - and ah there it was - power!
took connector apart - careful do not let pins/sockets slide out
cleaned pin surfaces lightly with emery board
cleaned sockets with gun barrel wire brush
added a small amout of Garden-Bender [GB] OX-GARG to top of inside of sockets.
pulled a part and reconnnected a few times to 'rub' OX-GARD in
so far so good

thanks for the help!
 

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2012 CL550 4MATIC Coupe
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My pleasure!
 
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