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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3927
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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I have a 1991 Mercedes 500sl engine, will not start. Key fob

Customer Question

I have a 1991 Mercedes 500sl engine, will not start. Key fob does not function. Car has a Radio Alarm Anti-theft system.
Submitted: 10 months ago.
Category: Mercedes
Customer: replied 10 months ago.
I just replaced the engine with a used motor.
Expert:  Lou P. master tech replied 10 months ago.

Hello, did the engine start and run before you swapped the engines? why did you exchange engines?

Customer: replied 10 months ago.
The engine did not run. It was locked up. The starter was removed, It looked like it was overheated. It also had a dead battery too. So I did nothing.
Expert:  Lou P. master tech replied 10 months ago.

OK so did you replace the engine, and starter as well?

Customer: replied 10 months ago.
No, This way gives me time to think about what you write...
Expert:  Lou P. master tech replied 10 months ago.

Ok Robert so does the starter crank the engine over ?

Customer: replied 10 months ago.
it does.
Expert:  Lou P. master tech replied 10 months ago.

Ok good. Next step does the engine run if you spray starter fluid ?

Customer: replied 10 months ago.
no, there is no voltage to the coil with the key on or when the engine is spinning over.
Expert:  Lou P. master tech replied 10 months ago.

Ok. Good. Good to know. Is the crank sensor plugged in ? It's the sensor at the rear of the engine on the drivers side on the bell housing.

Customer: replied 10 months ago.
I'm sure it is, I will have to go look.....
Expert:  Lou P. master tech replied 10 months ago.

Also inspect all the wiring , those years had a issue with the wiring harnesses cracking and falling apart. Make sure the wiring does not look brittle.

Customer: replied 10 months ago.
The sensor is plugged in. On the drivers side fender, right. I have looked at the wiring harness and the only bad wires I see are one to the a/c and to the temp sending units. They must be under the motor mount brackets.
Expert:  Lou P. master tech replied 10 months ago.

Ok so far so good. Give me a second.

Customer: replied 10 months ago.
while I'm waiting, l will say this, with the ignition switch on. The windows do not operate, the horn does not blow the door locks do not operate. The key remote has new batteries, but only shows a red light when pressing the button. I think there should be a green light come on when the doors are unlocked. The first time I tried to start the engine the alarm sounded, I had just connected the battery and when I opened the door, the alarm sounded off. I think at that time I inserted the key into the ignition and the alarm quit. I think, not real sure.
Expert:  Lou P. master tech replied 10 months ago.

Yes please tell me as much as possible. That tells me the theft protection may be active.

Customer: replied 10 months ago.
Well, that is what I think, if so, how do I deactivate it. Sorry, most times when I use this site, so many techs want to ask all the questions and make the statements. I think this is most of what I can help you with.
Customer: replied 10 months ago.
Oh, another thing. With a test light is connected to the plus side of the battery. It lights up when touching either post on the coil, the light will come on....
Expert:  Lou P. master tech replied 10 months ago.

Oh no brother this is a relationship. We both need input to make it work. I need as much info to help you. That way we don't waste time going over things you may have already done.

Customer: replied 10 months ago.
OK, thanks,
Expert:  Lou P. master tech replied 10 months ago.

Currently I am looking over the diagrams fyi

Customer: replied 10 months ago.
ok
Expert:  Lou P. master tech replied 10 months ago.

Ok check fuse 5 in the fuse box for power. That fuse supplies power when the key is on.

Customer: replied 10 months ago.
Ok, let me be sure, the fuse box is under the drivers side hood hindge.
Expert:  Lou P. master tech replied 10 months ago.

Correct

Customer: replied 10 months ago.
OK, back to the shop. I'll be a few minutes.........
Customer: replied 10 months ago.
no voltage at; numbers 1-2-3-5-6-7-8-13-14-15-16--B
voltage at; numbers 4--A-C-D-E-F-G-H
Customer: replied 10 months ago.
Ignition switch is on while testing....
Expert:  Lou P. master tech replied 10 months ago.

Ok good man. Give me a second

Expert:  Lou P. master tech replied 10 months ago.

When you turn the key in the ignition does it move smooth with good clicks at 2 positions Before you crank ?

Expert:  Lou P. master tech replied 10 months ago.

See if you can press your test light into fuse 5 and wiggle the ignition key on and off and see if it lights up. I think we may have a ignition switch issue. Cause those fuses are key on power. And they come from the ignition switch. So far from what I see. The old diagrams are not as friendly

Customer: replied 10 months ago.
It turns very difficult. I almost could not get the key inserted. I sprayed WD-40, which helped alot, but still does not turn smoothly.
Expert:  Lou P. master tech replied 10 months ago.

Ok. Can you access the bottom of the ignition switch? Or back of it ? I have to see what style you have

Expert:  Lou P. master tech replied 10 months ago.

But try the wiggle test I am betting the switch is faulty and the internals came apart

Customer: replied 10 months ago.
I will, I will have to wait til Monday to do that. Actually I concidered that the switch may be the problem. I will get the info for you. Until then thanks. A tip will be coming your way.
Expert:  Lou P. master tech replied 10 months ago.

Appreciate it. On a side note. Fuse 6 is a battery power fuse. So it should have voltage. Is the fuse ok and. It blown ?

Expert:  Lou P. master tech replied 10 months ago.

Wait. Hold on that

Customer: replied 10 months ago.
OK on #6, I wasn't sure if I was on the hot side or the other side of the fuses
Expert:  Lou P. master tech replied 10 months ago.

Ok I was looking at fuse 6 of the rear fuse box. So you are saying fuse 6 now has power ?

Expert:  Lou P. master tech replied 10 months ago.

So if you are testing the fuses with the key on test both too sides of each fuse. All or most should have power. On both sides of each fuse. If not the fuse is blown. I don't know how much you know about cars so forgive me. I don't mean to offend.

Customer: replied 10 months ago.
OK, now you are saying there is a second fuse box behind the one that has a easy to remove cover. The back one then has screws holding the cover on.
Expert:  Lou P. master tech replied 10 months ago.

But the statement of "hot side of other side of fuses " does not jive. If the circuit is alive and the fuse is not blown both sides of the fuse will show power.

Customer: replied 10 months ago.
correct, I only checked one side of each fuse
Expert:  Lou P. master tech replied 10 months ago.

Ok. Check both side of each fuse.

Customer: replied 10 months ago.
I did not see any blown fuses. Anyhow I will double check the fuse box
Expert:  Lou P. master tech replied 10 months ago.

Ok so far all the fuses that you say do not have power all get power from the ignition switch. And it's hard wired from the switch to the bottom of the fuse box. So if you have no power at the fuses then the ignition switch (or the power to the ignition switch which I am looking for ) is bad.

Expert:  Lou P. master tech replied 10 months ago.

Ok. Check the battery in the trunk. If should have a big red cable and a few small red cables on the positive connection. Make sure they are all tight as I think there are two nuts if I remember correctly.

Expert:  Lou P. master tech replied 10 months ago.

Also the head lights should come on with key or no key

Customer: replied 10 months ago.
OK, I will check this out. The battery plus side does have two nuts. Not sure about how many wires. I will check the ignition switch too.
Expert:  Lou P. master tech replied 10 months ago.

Ok let me know. That may be the issue. Either the battery or the ignition switch. Hope this information helps. Please take a second and click the box and rate my service GOOD OR EXCELLENT SERVICE if you feel I have been helpful and please request me in the future. If you feel I have not been helpful please DO NOT RATE BAD OR POOR AND WASTE YOUR MONEY , I will opt out to see if another expert can provide more or better assistance so you're not wasting your money!!. Thanks for choosing just answer. Lou. also , if you feel i deserve one , BONUSES are GREATLY APPRECIATED ! 😉 Keep in mind then when you rate it does not end our conversation. you can still return here to me for follow up information if needed, Lou.

Customer: replied 10 months ago.
I was not offended. I have seen fuses corroded at one end before. Anyhow all that checked out. The battery has two red 10 gage wires along with a main heavy wire.
I disconnected the wire harness plug from the ingition switch. While looking up at the plug with 7 pins on the right side with #3 from top is hot as it the #7. On the left side has one hot wire #6. And #6 fuse in the fuse box has no voltage.
The bottom of the ignition switch makes a 90 degree turn into the steering column.
But, beyond that I read that ignition switches in this car can be troublesome. Can you give me any tips?
Expert:  Lou P. master tech replied 10 months ago.

Ok a later later I can access the diagrams and see what you can check

Customer: replied 10 months ago.
how to remove the ignition cylinder lock?
Expert:  Lou P. master tech replied 10 months ago.

I will get you the info tomorrow. If you don't hear from me by 10 am shoot me a reminder. Mornings are busy but I don't want to keep you waiting.

Customer: replied 10 months ago.
Just a reminder
Expert:  Lou P. master tech replied 10 months ago.

Ah sorry about that I have easier access to Mercedes info during the day at night my 2 month old and 2 year old make it harder for me to get to the computer lol. I'll check back in the am. So sorry for the delay.

Customer: replied 10 months ago.
ok
Expert:  Lou P. master tech replied 10 months ago.

I will have to opt out. My sincere apologies. Hopefully Marty or Pete and assist. This is not typical of me so I apologize for keeping you tied up. Hopefully some one else can get you going. Please do not respond or it will lock your question back to me.

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