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Old 12-17-2008, 11:21 PM   #1
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Default Neutral Safety Switch ??

Symptoms:
My 1994 automatic F150 XL sometimes does not start.
On these occasions, I move the shift lever through all the gears once or twice and then it starts. Recently, it does not start at all even with the movement of the shift lever. When not starting, I turn the key and hear a click coming from the dash that appears to be the the ignition switch circuit closing.

Here's what I know:
1.) Starter is brand new (I put a new one in thinking initially, that it was the cause.). Pos (e-motor), Pos (solenoid) and Neg leads excellent contact.

2.) I understand that there is a neutral safety switch located under the dash attached to the steering column (however, when I look under there I have no idea which wires go to what).

3.) The wiring diagram involves the Key Cylinder, Ignition Switch, Column-Shift-Actuated Neutral Safety Switch, Relay, Starter Unit. (see attachment). The diagram seems to imply that the NSS is at the Transmission housing.


My guess is that it's the NSS. Maybe corrosion or malfunction. I wouldn't mind bypassing it if that's the case. Otherwise it could be the relay. I don't know where that's located or how to check if it's burnt out.

Would someone please help me out here? Thanks in advance.

~Dan
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Old 12-17-2008, 11:50 PM   #2
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Technically your truck has a manual lever position sensor. It acts as a park/neutral safety switch but it also tells the computer what gear the transmission is in. If it is bad and you bypass it then the truck won't run correctly. The computer needs to know what gear the transmission is in to control the idle properly. It is on the transmission. You could bypass it temporarily just to see if that's the problem.
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Old 12-18-2008, 10:02 PM   #3
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Sean,
That sounds logical. Now I know what MLPS means from the diagram. I'll try bypassing the MLPS at the transmission tomorrow and see if it kicks over.

I'm already thinking that corrosion has set into that switch given its close proximity to the ground and given the fact that the guy who gave me this truck let it sit for 2yrs over grass & dirt. There is advanced corrosion on everything on the underside.

I'll get back to ya on results of MLPS investigation.
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Old 12-21-2008, 09:44 AM   #4
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I had the same problem with my truck and it ended up being a bad connection on the starter. You could hear the solenoid clicking but the engine wouldn't crank. A mechanic friend of mine told me that it is quite common on these older F-150s due to a clip-on connection on the starter. He told me to cut that connection off, attach a new ring connector and bolt it on like the rest of the connections. I did that and haven't had a problem since. I initially thought it was my neutral safety switch too because when I moved the gear shifter it would start but it wasn't that. Just thought I'd throw that out there. Good luck!
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Old 12-22-2008, 11:49 PM   #5
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Thanks,

I went to napa today and saw a photo of what the MLPS sensor / switch looks like. It's just a black box and a multi-prong plug. Going to check if theres good contact there. For $70, I'm not going to replace it. Maybe find used one on ebay or just put a one-way switch bypassing the sensor and run the wires back to the dash. Push and hold switch while starting, then let go once started. $3 fix.

We'll see.
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Old 12-26-2008, 08:35 PM   #6
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Admiller,
I understand what you're saying. However, unlike your truck, my truck has the solenoid attached in a package to the starter. All connections are good.

Sean,
I got underneath the truck and unplugged the MLPS to inspect for corrosion. Used a mirror to look at the male connection so I did not have to dismount the assembly. All connections looked good. There looked to be some type of yellowish grease in and around the prongs.... is this some type of anti-corrosoin / conductive grease??? It looked like it belonged there so I left it alone.

The starter relay would click but the engine would not start. But the engine would start when I jumped the two prongs on the relay. The ignition wire was intact and all other connections were intact. So I determined that the relay was getting power but not able to close and maintain the circuit. I swapped it out for a new one @ $13 and it works fine.

Thanks Everyone for all your help.
Enjoy a safe and merry new year!

~ginO!
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Old 12-26-2008, 11:35 PM   #7
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Quote:
Originally Posted by dfly9891 View Post
Admiller,
I understand what you're saying. However, unlike your truck, my truck has the solenoid attached in a package to the starter. All connections are good.
Sorry, I meant the relay was clicking, not the solenoid...
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Old 12-26-2008, 11:35 PM
 
 
 
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