k jet cold start problem on M110

[CraigB]

Having bad cold start problems and also warm start problems on the 1976 280SE. I pulled the cold start injector and turned the car over but nothing came out. Connected power to the injector and it worked fine. I checked the resistance indicated in the factory manual on the thermo time switch thing in the head at sub 15 degrees C and it seems ok. So I swapped one in off a 380 motor, still no luck. And before i started I checked the voltage at the wire to the injector and it was about 9v. At some point I thought that could be a problem - should it be 12v - but one side of one of the ignition resistors is about that voltage, so I used that to feed the injector and that worked fine - so i don't think that's the problem. It could be that the 380 thermo switch is also stuffed.

I have searched but though i would ask here for any experience. The main question is to check if there is any other cold start relay in the system or its just the power to thermo switch then wires off to the injector and back to the thermo switch (that's what the factory workshop manual wiring diagram suggests.

And then on to the warm start issue - I have my suspicions about the accumulator. I'm not sure how similar the systems are, but on the 1991 420sel that is parked next to it, i loosened the pip to its cold start injector and there was still fuel under pressure to it, despite it sitting not started for many many months. But I don't think it is my cold start valve issue because under cranking with the injector open with a power source and it is spraying plenty of fuel.

[TheMadRacoon]

Hi Craig,

Could it be the coil not giving you enough spark (since it seems you have enough fuel)...? Or something on the ignition side? Are you still having wet spark plugs (or was that on a different car?)

If it's not ignition related night it be fuel flow related, eg blocked fuel filter or yank strainer?

[CraigB]

yes different car with wet plugs - that has solved itself without a trace. But it has been a bit drier. My money is on the suggestion that it is an earthing issue somewhere - somebody had the same issue being hard to start when wet and ended up being an earth strap somewhere needed cleaning up.

I used this car on Sunday in a motorsport event where it was pretty much consistently redlined, full power, hitting speeds of up to about 100kmh on dirt, really crisp crack from straight through exhaust all the way..... I just don't think this is an ignition or fuel problem.

I put this on facebook too and getting similar responses so just clarifying that the cold start injector is not working - I can see it, not a drop of fuel coming out of it. If i connect some power up to it direct it works perfectly. So I reckon Mr. Bosch put that system in for a reason and called it the cold start injector to make it easier to start cold.... which is my problem! I don't reckon I have made that point clear enough.

But thanks - no responses at all on how the cold start system triggers, so I guess nobody has had the problem. Will have to dig deeper with Mr. google! Just finding it hard to get clear wiring diagrams that include the whole system. I have access through W116.org to factory manuals, but they split it all into smaller systems for wiring diagrams and doesn't look like anything complicated. Maybe its a broken wire and I can jump straight across to injector, but I have power at the injector, but perhaps the wire that returns to the thermo time switcth. Or maybe I should pull a time switch out of one of my working cars. Will have to check if v8 and 6cyl k jet have same switch.... lots of V8's here but only the one m110 6.

[VanCalledHelga]

no responses at all on how the cold start system triggers, so I guess nobody has had the problem. Will have to dig deeper with Mr. google!

If you'd asked me 25 years ago I would have been able to answer. There was a Bosch fuel injection book at my local library. It explained all the workings of the various Jetronic systems. But I imagine it has long been pulped as they ran out of space and started selling books a long time ago.

If K-Jet its anything like the mechanical Kugelfischer injection in my old BMW you should be able to rig up a momentary switch to activate the cold start injector. That'll bypass the cold start timer which should help you track down the issue.

While you're getting things going again with the switch, rig up a light and or meter to the timer circuit to check that it's activating properly.

[CraigB]

Thanks, that's a point - I can just wire it direct.... just not sure how long it stays on for but it does narrow down if that is the issue and even if it helps with hot start - maybe it puts a bit in when hot too in order to kick it off? Not a long term solution - I think if i can just get that injector to go click, I should be good.

[Bartman4800]

Craig, looking at this schematic:

http://howcarpartswork.blogspot.com/201 ... ystem.html

I would short circuit the thermo-time switch and see if the cold start injector gets activated.
If it does, the switch is the culprit.

If not, maybe a wire break?

Bart

[CraigB]

Thanks Bart. I worked out this morning pretty much that as the next step - pull the plug off the switch, check voltage to one terminal and then earth the other one. I found enough of a diagram to show that it gets power from terminal 50 and I gather that is from starter voltage - maybe fuel pump relay.

Thanks for that link. I had been looking for diagrams on accumulators. Do you know why / how it works with the two outlet accumulator? I will have to get dirty (hate playing with fuel if I can help it) and the standard test seems to be just to put the vacuum pump and gauge on it and see if it holds. I should be able to work out what two outlets do.

[Bartman4800]

I think the line from the bottom of the warm up regulator is a vacuum line to the intake manifold?

kjet2.PNG

kjet1.PNG

To be honest Craig, I have looked at lots of Kjets but never worked on one. So I am just giving you an educated guess.

I found this else where:

The warm-up regulator, also called the control pressure regulator, lowers the control pressure of the K-Jetronic when the engine is cold. This further deflects the control plunger in the fuel distributor and correspondingly supplies additional fuel. The warm-up regulator is installed on the engine to absorb its heat.

This website is actually gold:

http://www.bosch-classic.com/en/interne ... ts%20heat.

Bart

[TheMadRacoon]

https://youtu.be/a4fJAfXYxWk

I came across this a few years ago but haven't watched it again since... so apologies if it only tackles running but not starting. Still, a good animation iirc.

[CraigB]

Finally got back to this today and all sorted. I studied the wiring diagram and the pink and blue wires provide power both to the thermo time switch and the cold start injector, and I could measure voltage going to both when motor turned over. Then the brown/blue wire goes from the other terminal on the cold start injector back to the thermo time switch - earthing through the switch when cold or not when warmed up. So I pulled the plug off the thermo time switch and earthed the brown blue wire..... started instantly. So the switch i swapped into it was also stuffed. Raided another k jet car and fitted.... all working. I haven't got it hot and tried to start but I assume I still have an accumulator issue.

[Nabstud]

Excellent, a nice simple fix for once!!!

[Bartman4800]

Very rewarding! Glad to hear you fixed it.