6.9 - Turn the key then nothing

[MercedesMike]

Hi all. I'm an admitted novice when it comes to things mechanical so I thought I'd ask this community for your collective wisdom. I have a '78 6.9 with about 90K miles on the odometer. Runs great, works great, love the car. Except it's happened twice now. After a day driving about 30 miles to my office in northern Illinois yesterday, I called it a day around 3pm, settled into my seat, put the key in the ignition, turned it and...nothing. The car didn't turn over, didn't sputter, nothing. I turned the key back, tried again, nothing.

My first thought was uh oh, I left the headlights on. Nope. Lights are off. The buzzer sounds when I open the door with the key in the ignition. The interior dome light turns on. The gauges do their thing in anticipation of the engine turning over when I turn the key. Radio works. Windows go up and down. It just doesn't turn over.

Back when I was a teenager, I remembered this happening once with my old Honda Civic. I had left it in Drive when I parked the car and the weight of the car in gear was more than the starter could handle. So I tried shifting the Mercedes into neutral and turning the ignition. Nothing. Back to Park. Nothing. Time to pop open the hood.

Nothing seemed loose. No dangling wires. No connections amiss. I didn't see any ground wires hanging free. I wiggled and pushed all the wires I could see. Everything seemed as it should.

It's clear that the battery isn't dead. Perhaps its low on juice? Maybe my alternator is going and on the last run it didn't charge up the battery properly? Hmm.

I pulled out my phone, did a quick internet search for the nearest towing place and got ready to dial. One more try. I put the key in the ignition, turned it, and it started right away. What the...

Ran great all the way home. No problems.

So today, suspicious and wary, I used the car again to drive my many kids to an endless number of soccer games around the western Chicago suburbs. The weather was sunny, around 75 to 80 degrees F (24-27 degreees C). Same as yesterday. Started every time between games with no issues until after the last game. Same thing happened. Turned the key and nothing. I popped open the hood again, did my wiggle check of the various electrical connections and nothing seemed to work. Tried my gear shifting thing again. Nope. Pulled out my phone, looked up a local towing company on Google. One more time before dialing. Tried to start it again and vroom, started right away.

I now turn the floor over to all of you for your thoughts and diagnosis. Is it my alternator? Perhaps the coil? Are there any particular electrical connections that I should focus on? Any wires that I should carefully examine and replace? Is there a particular connection to the starter that I should be looking at?

I fear that if I bring it into the shop, they can't duplicate the issue. I'd appreciate your collective wise counsel.

Thanks in advance.

 

[TJ 450]

Do you hear a click from the starter solenoid? That would be the first thing to check afer ensuring that the battery is charged and the terminals are secure.

If you don't hear the solenoid, the starter lockout switch is probably to blame... it could be out of adjustment, faulty or the connector has come adrift.

Tim

 

[BenzBoy]

Let's assume that the gear lever detent switch is OK.
There is another possibility. Sometimes on older cars when the motor is hot and the wiring warm the wiring simply provides too great a resisyance to feed enough paower to the start motor. The solution os to have a sparkie wire direct to the battery with a solenoid switch in the loop. I have done this twice and it works well.
I say this necause the resistance is normal at col but high when the wire is hot and that seems to be when you have the problem.
Just a thought..
Regards,
Benz-Boy

 

[KenM]

For my money I think Benzboy is on the right track here, I used to have a '68 Chev Impala that did the exact same thing, I found that by pouring a bucket of cold water down the back of the engine and over the starter motor it would usually fix the problem, pretty much straight away. I have to admit that I didn't look too much closer into it as I sold the car but it has to have something to do with electrics, either an overheating starter or more likely poor connections.
Cheers,
Ken

 

[WGB]

I would start with the Starter lockout switch on the transmission and then suspect the contacts in the ignition switch.

Bill

 

[MercedesMike]

Thanks all for the replies. TJ 450, no I don't hear a click from the starter solenoid. Where might I find the starter lockout switch and what should I be looking for to determine if it is out of adjustment or faulty? I think I agree with many of you that if it is not the switch then it's likely a wiring problem. As a general question, what manual should I have on hand that has the relevant wiring diagrams?

 

[MercedesMike]

I found the wiring diagram in the service manual on W116.org. Now if someone would be so kind as to send me a picture of the starer lockout switch and associated wires I should be examining, that would be really helpful.

 

[Styria]

Hi MercedesMike, I'd probably second WGB's fault finding procedure, although neither of the two are particularly user friendly when it comes to fixing them. In my opinion, the inhibitor switch on the left side of your gearbox is the more likely source of your problems.

There are four wires that are part of a loom that connects to the switch. Taking a photo is just about impossible, even with the loom disconnected from the switch as it is all encased. The switch itself can be removed from the box as it is held in place by two 8mm bolts, but you will need to disconnect the gearchange linkage as well first. Unless you know what you're doing, you'd be best advised to consult the services of an Auto Electrician as he will be able to test the switch on the bench. Also, the wiring loom would by now be quite brittle and it is possible for any of the four bullet connectors to become partially detached from the wires.

It would also pay you to check your earth and live connections on your battery terminals. Other than that, I am just not sure which way you could turn - also, keep the ignition module in mind - it could also be faulty. See if you can obtain a spare unit and have it handy to connect up next time you have a problem - BTW, have not heard from you - did you get to the bottom of the problem ? Regards Styria

 

[BAR]

It sometimes happens when starter motors are old and hot, that they 'expand', that is the copper winding on the solenoid expands and then jams inside the case. I found once this happened to me, I could start the car from cold, but it needed to be off for at least 1 hour (and cool down) before it could restart.

The other aspect of 'lock out' from gear selector is also worth investigating.

 

[CraigS]

I would agree with all the suggestions given.

1. If it won't start in Park, try starting in Neutral. If it does, susupect the neutral switch
2. Check the terminal block UNDER the battery tray. You will need to remove the battery and 4 screws, and clean all wires on the terminal block
3. Check for breakages in the green wire from the ignition module to the distributor. Doesn't take much so check carefully.

 

[TJ 450]

Ah, the ignition control unit is completely unrelated to the starter circuit.

Tim

 

[MercedesMike]

Thank you all for your reply. I will try your suggestions and let you know what happens. Frankly I suspect the green wire from the ignition module to the distributor. It's one of the wires I "jiggled" on the two times this has happened. I found a replacement on Autohausaz.com ($90 USD!). I also ordered a new ignition coil just in case.

I'll also check my "earth" connections (we call them ground connections here in the US -- had to think about that one for a moment). I'll also pull the battery and check those connections as well.

I'm currently traveling the rest of this week so it'll have to be the weekend before I respond back.

The other suggestions regarding the electrical wiring to the gearbox sounds like a big challenge to tackle. I'll look at that as a Plan B.

 

[MercedesMike]

So I've replaced the ignition coil and have not had the problem recur. The unit I replaced sure looks 31 years old. Of course that means nothing since it could happen tomorrow. If it happens again I'll repost. Thanks to all again for your brainstorming and advice.

p.s. It's getting colder here in the midwest USA so only another month or two before the car goes into storage. Time to dig out the posts on storing cars for the winter. Anyone have good links that you could point me to?

 

[SEL_69L]

A guy from Stuttgart visited the the M-B NSW Club in Sydney a couple of years ago. He is a member of the 126 Club in Stuttgart and has a 500SEC. For winter, he uses his Audi only. He gives his 500 SEC a service, full detail, puts it on blocks under a cover in his garage, and there it hibernates until springtime. A bit like an automotive Yogi Bear!

 

[PSQ]

I had the intermittent won't go problem for a couple of years, kept thinking I'd fixed it.
Finally got it looked at (my wife got tired of sitting in the car waiting fr it to decide to go), and had an additional relay installed (don't ask me where in what circuit), but apparently it was the same thing he installed in Porsche 911s with a similar problem. Heat under the bonnet -> higher resistance -> sitting, waiting.
Never had the problem since, and I've even moved the battery to the boot since then with all the long wire that entails.

 

[Styria]

Electrical problems can be the most difficult to detect problems at times. Invariably, for an Auto Electrician to rectify a problem, it has to be present at the time of checking and examination.

It will indeed be interesting for Mercedes Mike to confirm (in several months and after Winter) whether, in fact, replacement of the Ignition Coil has rectified the problem in the long term. N. B. Hi PeterQ., nice to have you on board - believe it or not, I even looked at your website quite some time ago - e.g. Vauxhall Cresta etc etc. - quite interesting, but don't ask me today how I found it. Regards Styria

 

[SEL_69L]

Aha! Round pin relays are used throughout the 116. With these cars as old as they are now, the relays relevent to of any your intermittent electrical problems should be checked for operation out of the car. In most cases, they are easy to remove, and can be checked with a multimeter. Fortunately, they can be easily replaced for relatively small cost.

I had an intermittant problem with my 6.9. An auto sparkie could not find the problem, because the intermittant faulty relay controlling the fuel pump happened to be working properly at the time.

The engine on my car on a couple of occasions simply cut out while I was driving it. One of these events was on the motorway at 100km/hr. I can tell you, it can be frightening when this happens because when the engine stops at speed you loose your power steering assistance. You have to be a gorilla to wrestle with steering with a 6.9 without power assistance, with that extra heavy chunk of cast iron engine over the front wheels, and I am only a little guy, at 65 kilos. I would pull the car to a stop, start the car again, and drive away, considerably shaken, and wondering how I had avoided crashing into other cars at speed.

Often, but not always, I would try to start the car, but it simply would not start. On the day of finally getting to the reason why the car would not start, I had an NRMA roadside assistance mechanic visit my car at home. The standard approach to getting a car going again is to check if the engine is getting spark (which it was), and fuel, (which it was'nt). On this occasion the car refused to start. The first thing to suspect was a faulty fuel pump. That proved to be working; it had only been newly replaced. The next thing to be suspect was the electrical supply to the fuel pump.

This proved to be the cause of the problem. The relay controlling the fuel pump was removed from the car and tested, and found to be intermittently faulty. This was replaced and I have not had a problem since.

The lesson I have learned from this saga is to always carry a variety of spare relays in the glovebox, and to always make sure that all relay modules are pushed into their connectors properly. Blade relay pins, I think, make more positive connection to their mountings.

 

[Lukas]

What happens when the motor stops while travelling at speed - does the auto engage neutral / freewheel?

I imagine in a manual car that either the driven wheels lock up and/or all sorts of bad things happen to the engine and driveline.

 

[John S]

In a manual car you smoothly come to a halt, no fuss or drama - in an automatic it's even smoother due to the action of the torque converter.

 

[SEL_69L]

No problem with the transmission at all, just that goddamnawful heavy steering, which although you may expect it, is almost impossible with that cast iron 6.9 engine over the front wheels, especially with a little guy like me. John S is right about the transmission.