Reverse light switch missing... What??

[OneShot]

Hi all,

Over the winter I did a TH400 to SM465 swap, plus lots of little tune ups and improvements. Went from 11mpg to 17.5mpg on my bi-weekly trek from Indy to Lafayette. Only problem is that when I did the swap I didn't hook up the reverse lights until now. I interrupted the wire going to the reverse lights with the switch on the side of the 465 but I can't find the switch on the column from the AT to save my life! I've found the neutral safety switch down by the firewall, but there are only two pins coming out of it, so no reverse switch there... I feel like I've looked everywhere without dropping the column.

Any help appreciated!
'79 C20 long bed standard cab SM465 350 Q-Jet

 

[OneShot]

Nobody knows where I can find that switch? I don't like having no reverse lights.

 

[CSFJ]

I didn't think there was a switch on the column. Always thought the switch on the trans handled all the output to the lights. Are you sure the switch in the new trans is good? Not some other wiring gremlin between it and the lights?

 

[OneShot]

Well after I got the 465 in and before I interrupted the wire to the lights, when I would rotate the shift collar on the column (I kept the auto column) the reverse lights would come on. So it has to be in there somewhere, or at least that would make sense. Still can't find it.

 

[OneShot]

And yeah I'm sure the switch in the 465 is good cause when I rotate the shift collar to the reverse position on the column and then put the 465 in reverse, the lights come on.

 

[CSFJ]

Is the neutral safety switch still hooked up?

 

[CSFJ]

This may be a question for one of the other members who are more familiar with the wiring on these trucks.

 

[CSFJ]

@chengny

 

[OneShot]

Yes the neutral safety is still hooked up. I've considered jumping the neutral safety and just running the truck in "reverse" on the column...

 

[CSFJ]

That's kinda what I was thinking, but really the neutral safety should be replaced with the a clutch safety switch. Otherwise that would be one solution to get your back-up lights back.

 

[OneShot]

True, when I was doing the swap I broke the clutch switch though. I've just been diligent about not starting it in gear. I should fix that at some point.

 

[CSFJ]

Don't feel bad. I managed to break the safety switch when I pulled the manual pedals from the donor truck for mine too. I just haven't gotten far enough to worry about it yet though.

 

[GTME94]

I'm not sure that the switch on the column is a Neutral safety switch, I think the safety is in the column and you can't actually turn the key with the auto trans shift lever in gear. The switch on the column works the reverse light because I had to adjust my switch to get the reverse lights to work properly when the auto shifter was in the reverse position. I think you'd need to bypass this switch for a manual swap since you won't be using the column shifter.

I don't know anything about how it works on a manual trans.

 

[OneShot]

I'm not sure that the switch on the column is a Neutral safety switch, I think the safety is in the column and you can't actually turn the key with the auto trans shift lever in gear. The switch on the column works the reverse light because I had to adjust my switch to get the reverse lights to work properly when the auto shifter was in the reverse position. I think you'd need to bypass this switch for a manual swap since you won't be using the column shifter.

I don't know anything about how it works on a manual trans.

It has to be the neutral switch. In addition to having jumped it already and tested what it does, what if the truck was in drive? You couldn't turn the key off, because of the mechanical mechanism, but turning the key to start wouldn't do anything, cause the truck is in drive. So there has to be an electrical switch somewhere that prevents the starter from firing when it's not in neutral or Park.

Thanks for the input - still haven't found the switch yet - but that's a neutral switch.

As always, more perspectives would be great.

 

[chengny]

In 1979 these trucks used an NSS with an automatic transmission (or a clutch safety switch if equipped with a manual transmission). The switch was a dual function type with two sets of contacts. One set prohibited the starter from cranking unless the gearshift was in either P/N, the other set was for the backup lights (when the gearshift was in R).

Around 1985 the electrical interlock that prevented the engine from being cranked (the NSS) was replaced with a mechanical interlock in the steering column.

After that, the wiring that formerly passed through the NSS now went straight from the ignition switch to the solenoid. The wiring and contacts for the backup lights remained as earlier model years.

The lead from the ignition switch to the NSS was/is yellow. The lead from the NSS to the "S" terminal on the starter solenoid was/is purple.

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I am quite sure that the yellow and purple wires are connected to the NSS located at the base of the steering column. If not - and as you say the solenoid gets no power unless the gearshift is in P/N - someone modified something.

Bottom line: get under the dash and located these two wires.

The purple wire comes through the firewall and the yellow comes out from the column. When you have located them, trace them to were they meet and you will have found the switching device that only allows cranking in P/N.