So I got in my 190E to head to class today as I had a quiz and it cranked over then died immediately as soon as the key went back to the on position from the start position. Tried a few more times and nothing. I also noticed that my tachometer was not reading at all. I checked the fuses and rolled them but nothing changed. I checked the diagnostic light to see if there were any fault codes but had no response from the engine computer. I had someone pick me up to take me to get some furniture we had bought for my house and my friend who is a mechanic stopped by with my spare engine computer and changed it, but that changed nothing. I am currently suspecting the OVP is bad but it could possibly be a malfunctioning ignition switch or bad crank position sensor. The engine will run as long as the ignition is in the start position.

Any ideas?

Starter motor?

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1990 190E 3.0L

If it starts how can it be the battery?

You need a wiring diagram , and stop randomly throwing parts at the car.

What is the history of this car?

The wiring diagram will show the ignition switch internal connections. While you are cranking apparently the computer is fed through one set of contacts and when int he run position another.

A wiring diagram will also show if this car has a ballast resistor in the ignition circuit ( given it is electronic ignition that is doubtful )

I don't know how the OVP interacts on your car but a wiring diagram will show that.

Put a test light on the positive side of the ignition coil and see if you lose power in the run position. If so I'd be tempted to use a fused jumper to apply power there.

It could be the ignition switch. The switch has a momentary contact position for starting. This supplies current to all the running circuits, plus the starter. When you release the key, the switch then moves to the "on" contact that only supplies the running circuits. Are your fuel pump relay and ECU getting power when in the "on" position?

Yes, the car runs, but only when the key is held in the start position. I can hold the key at the start position and the engine will run until the key goes back to the the on/run position and it dies. I just had my friend change the engine computer cause I had a known good spare and it takes 5 minutes to change. I have a good OVP so I'll try that next then check the crank position sensor to make sure the resistance is still in spec. I can also check to see if the ECU is getting voltage while the ignition is at on/run

Idea #1:
There are two signals that will close the fuel pump relay; a) cranking signal from the starter motor, b) run signal (TD) from the ignition. Since the pump, and the engine, run when cranking, that signal is good. The relay may be defective, and not responding to the TD signal when key is released,

OR

Idea #2:

No TD signal is coming from the ignition module, or wiring in the TD circuit is faulty, which would explain no tach reading.

The OVP certainly was not the issue, changing it with a known good one changed nothing. The crank position sensor, while probably not a possible cause, tested fine.

The engine is not really wanting to run at all at this point. It'll start but very weakly, then the exhaust smells really gassy.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

Adjust the fuel mixture. Turn it counter clockwise a quarter turn

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1990 190E 3.0L

If it been running before dont move a thing...You will be in more trouble then. You need to check out the vacuum lines for leaks .And all fuses ,give them a turn in the holders Earth points are the next to clean up and grease. You can bridge the fuel relay socket to see if it will run without stopping On my car its number 7 and 8 .If it runs the the fuel relay is shot.

You pins to join are 30 and 87 on the relay .You do have a throttle micro switch on the bowden cable. Check to see if they are working ,you will hear it click if you press it .

This is what you need to do to do to see if its the fuel relay at fault.
If you do this , then dont let it run with out the engine running .If you stop the engine remove the relay as it will keep pumping fuel in to the engine.

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Define " engine runs while the key is in the start position".

If the engine is barely running while the starter is spinning ( starter helping the engine along ) , you have an entirely different problem than if the engine runs fine in the crank position and suddenly shuts off when in the run position.

I pulled the MAS relay out then reinstalled it, figuring it could be sitting crooked in its plug or something. I don't think that was related, maybe so, but I tried starting it back up, and was on the start position for about 20 seconds. I noticed the tach started working so I slowly eased the key back to the run position and it died but not immediately. Repeated that again and the engine stayed running.

It ran very badly and idled at 550 in park (850 was what I was getting before) and would bog down whenever I tried to give it gas. It smelled like it was running rich. It started running better and better as it warmed up and at one point the idle jumped back up to 850 and stayed there and I could rev the engine like normal. Shut it off and restarted it and it ran correctly. I am going to let the engine cool off until tonight and try starting it again. If it has more issues when it is cold and gets better as it warms up, my guess will be that something related to the engine temperature is bad. The CIS coolant sensor, air flow potentiometer, and idle air control valve are dependent on engine temperature.

I am still not getting signal from the computer to the diagnostic light so there's definitely still an issue somewhere.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

I determined it is temperature related. Once the engine warms up enough, the ECU can be checked for codes through the diagnostic port.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.