w140 1998 need to disable immobililizer
 

I need to by pass this system.when temps get high in the interior car won't start.things done new ignition switch,new starter.Might be pse pump not wire correctly cause a can't find a damn photo of a 98 sedan wires.

I really don't think it's the PSE, and the 2 items you replaced probably not it. Well, since you already replaced them and it's not fixed, I guess we can assume they definitely weren't it. Would help to know what the car does or doesn't do when you have a failure.

The RFL module N54/3 is the heart of it, and that is connected via a CAN line to the engine control module, I would lean towards fixing the system, as anything that will get you around this would more than likely end up costing more and being less reliable than a properly fixed car.

Few suggestions, if you want to coninue to keep throwing parts at the problem (which I don't support but this seems to be the most popular course of action these days).
1. OK, even back in the day we did have problems with the antenna ring surrounding the ignition lock, if this is bad it might not pick up on the transponder in the key. I don't think the ring is terribly expensive or hard to replace. It is also commonly called the "coil", we usually called it the antenna ring.
2. Easy to try; if you have a second key to try out, the transponder in the key could be getting flaky. If the antanna doesn't fix it and you have no spare key, you could buy another key as a test (expensive key I realize). Added benefit is you would then have a spare key. I can't remember; did you get a new key with the ignition switch or did you just replace the electrical part (the switch itself)?
3. One "free" thing, is I know there was a bulletin on one of these DAS systems that warned about owners with a whole crapload (my terminology not MB's) of keys and do-dads on the keyring, because if the electrical part of the key is angled down the antenna ring also might not pick up on the transponder in the key.

Have you ever seen the odometer have the message "start error" in it?

Not really possible, built into the fuel computer that is married to the DAS computer.

I have no codes at all new key same lock
 

I checked codes,never in her life.She had something about can something,but I earsed it thinking it was the govenor code.
Using a snap code scanner hooked to comput acess under hood
I'm testing car today 89 degrees in hot sun with windows down she starts good,its when they go up,and get over 120 she won't start

Check the odometer screen when it fails to start for that message "start error", that's how you know it's a DAS problem.

no messages,but hey I will buy that anntena

the idea just came to mind what if neutral safety switch.Lets say the P.O. spilled drink on shifter. it didn't mess it up till car got hot and syrup turns into liquid it shorts?

Try starting in N instead of P as a test...

-J

no go to that one
However I can short starter out and start engine

it only happens about every two months in the summer

Well you already did the ignition switch, it sounds more and more like that's the problem.

Don't bother buying the antenna ring, that's just wasteful, if it was that you would get the "start error" message so it's not it.

Another idea would be (and i have seen various weird problems caused by this, I remember one on a 140 chassis too), where it could be something as simple as a loose screw on the bottom of the fusebox. Wires are joining up on those strips and through use and heat and what have you, they can loosen a bit and cause all sorts of weird things.
To do this, you would pull the fuse panel out of the fuse box to access the bottom of where the fuses are, you will see lots of screws holding wires to the metal strips. I'd disconnect the battery before messing in there though.

I will do as you asked
 

I guess do the rear also

Have you replaced the electrical portion of the ignition switch?
That can cause that issue & is a common failure.

Yes M.B I replaced the switch portion.What about if a coke was spilled,and sticky sugar only melts when hot

Yeah I had thought it sounds just like the switch. I had a similar case, where it sounded like it must be the ignition switch, (AND of course actually replaced it, although to my defense, I did some tests it and came up with the switch as the diagnosis), and it ended up being what I already mentioned, a loose screw on a bus bar under the fuse panel. Now it does seem to me that at some point they got rid of this system of attaching wires to the bus bars and went to more of a plug-in sort of connector, so if that is the case it blows my theory out of the water, or at least complicates it.
In the case I had it was malfunctioning turn signals. At least I didn't do what others would have which would have been to replace whatever module controls the signals, or the combination switch. I had a power problem right at the fusebox and looking at the diagram you would have sworn it had to the (common failure) ignition switch, but it wasn't, the problem was in the fusebox (or it could have been a wire, to test the wire I had to get under the fuse panel anyways, and went to pull the wire off the bottom of the fuse panel and found there was no resistance at all when I went to unscrew the screw). Again this was all after I had replaced the switch, so it was a hard lesson learned about jumping to conclusions and blaming the "failure du jour".