1988 F-150 starter, constant voltage, neutral safety switch
 

1988 F-150, automatic, 5.0

Sometimes when I attempt to start the truck and then release the key the starter keeps turning the engine over - engine does not start - and welds the contacts on the starter solenoid. removing key does nothing - have to pull a battery cable.

Doing a few checks I found :
- with the key turned to the "on" position I have 7 volts at the red/Lt Bl signal wire to the solenoid - or "S" terminal on the relay.
- 7 volts in and out of the neutral safety switch
- disconnect neutral safety switch and I have 12.2 or full voltage coming into the switch.
- two sets of contacts on the switch - measuring resistance across the contacts for the start signal I read 33 ohms - the switch operates correctly as I move through the gears - closed on park or neutral - open otherwise

- disconnect the main harness at the engine compartment firewall - drivers side - I have full voltage coming from the ignition switch - replaced the switch - no change - key cylinder switch is aligned correctly for all switch positions.

According to what I have read the red/lt bl starter wire should come from the ignition switch - go through the NSS - the straight to the solenoid

All tests done with the red/lt bl "S" wire removed from the solenoid.

Thoughts?

Have you replaced the starter solenoid? I had this problem on my '87 and the new solenoid fixed it. When this happens, does the starter keep running if you pull the small wire off the solenoid?

Very high probability it's the solenoid ("internal plunger hang" if you will, keeping the circuit connected even after the ignition switch tells it otherwise) -- I recently cut several of these open and compared the windings, plungers, internals of the Adv Auto, NAPA, Auto Zone, etc. parts --- noticeable difference in construction quality........NAPA part was superior by a country mile..........

Thanks for the reply - I have tried every brand of solenoid in existence - the floating 7 volts is not enough to close the solenoid - but it is enough to hold it. I tested this with a new solenoid not connected to the starter. The starter signal wire (red/Lt Bl) goes to full battery voltage when you turn the key to "start" - when you release the key to the "on" position the signal wire voltage goes to 7 volts and the solenoid stays closed.

I know they say the signal wire to the solenoid starts at the ignition switch - then through the neutral safety switch(auto transmission) - and then straight to the "S" terminal on the solenoid. I have discovered this is not the case - there is a branch/loop circuit that joins with the signal wire - it involves two other wires - gray/yellow & purple/yellow - these two join under the dash at a connector that is used for a manual transmission(clutch switch or something) - since it is not used, there is a terminator/jumper cap plugged into it. At this point it jumps the gray/yellow & purple/yellow wires together - the purple/yellow then joins the red/lt bl signal wire at a factory splice before going through the firewall.

Once through the firewall, the signal wire goes to a large, rectangular connector and then to the neutral safety switch(NSS) - it comes back from the NSS to the same connector - out of the connector it splits - one goes to the "S" terminal at the solenoid - the other goes to a large, black connector(round on the inside and square shaped on the outside).

Lot of info I know - few other things to note
- if I break the loop anywhere the signal wire goes to 0 volts as it should
- if I disconnect the large, black connector the signal wire goes to 0 volts
- if I remove the jumper cap for the optional clutch switch the signal goes to 0 volts.
- the large, black connector seems to power the fuel pumps - they do not engage until you plug it back in(key at "on" position
- this truck has dual fuel tanks - neither sending unit work right - flaky readings - and the rear tank will not always supply fuel.

SN - I had a friend come across a drygrass field in an
88 or 89 f 250.
Shut the engine off, took the key out and handed it to dad.

Smoke from under the hood led us to the external solenoid
on the internal fender.

Very easily could have started the whole town on fire.
New solenoid fixed it but like ...not sure I'll ever trust to park it inside of anything and feel I should disconnect the battery every time.

Wish I had better usable info for you but it was a couple years ago.
oh, starter shaft was blue from heat but still usable.

Shouldn't have any voltage on the Red/Light Blue wire unless the ignition key is in the Start position.

At the ignition switch is a Yellow wire. One end is attached to the Battery side of the starter solenoid lug. This provides 12 volts to the ignition switch. When it's in the Start position 12 volts passes thru the ignition switch on a Red/Light Blue wire and to the neutral safety switch then to the starter solenoid primary relay coil. The 12 volts also powers the TFI module when the key is in the Start position.

I'm wondering if a (bad) EEC-IV is feeding a spurious 7V signal into your neutral/safety sw (it's "close by" on the circuit) ??

I have check about everything I can - starting to think like you that it's a bad EEC-IV - any method to check the module VS replacing it?

I also disconnected every connector under the hood just to see if the 7 volts was coming back through something going bad - the only thing that affected it was unplugging the TFI ignition module on the distributor - the voltage went from 7 volts to 12 volts. Regardless - still have a high signal on the solenoid "S" terminal with the key in the on position.

Appears some of your wire colors don't match the Ford documentation.

Should be a Gray/Yellow wire called Neutral Drive Switch that comes from EEC IV pin 30.

It goes to C175 a Black 10 pin connector on the driver side wheel well apron.
From there it goes to C190 a Gray 2 pin connector called the Clutch Look Out Switch and is connected to the Red/Light Blue wire. Poke the Gray/Yellow wire out of C175 and see if that stops the 7 volts from being placed on the Red/Light Blue wire.

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Any resolution on the 7 volts at the red/ltblu?? I have just come across this identical issue on an 88' F150 5.0 Auto.

Not as of yet - back working through it again though. The post by F150xlt has great detail - I am making sure I covered the checks he mentions.

I did a KOEO scan and got -

Code 67 A/C on during KOEO or neutral drive switch open or manual lever position sensor out of range or neutral pressure switch closed or Park / Neutral switch open

With a 67 they tell you to measure the voltage on the Pin 30 on the EEC 60 pin connector a Brown/White wire called NDS Neutral Drive switch.

67 says to go to chart FA7
If voltage on pin 30 is less than 1 volt go to FA9
If voltage is greater than 1 volt go to FA8

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Thanks for the great detail and info. I do remember my NDS reading 33 ohms - looks like it will need to be replaced.
I assume i can use a wire penetrating probe to test vs getting a breakout box?

:-X10:Or a sewing needle or strip some insulation back.

Must be a misprint in the manual. The schematic posted says pin 30 of the EEC 60 pin connector is Gray/Yellow. In the other page listing the pin outs and wire colors it says pin 30 is Brown/White. Use the pin number on the connector and disregard the actual wire color.