[jzanuttini]

Hey i got a 94 sierra and when i put into fifth gear my reverse lights come on.
I want to fix it but i don't have a clue where to start.

[303zuke]

There are 3, sometimes 4, switches threaded into the top of the gearbox. On the left side (but still on top), the reverse switch is towards the front while 5th is to the rear. I'd say somebody has taken your gearbox out or replaced a switch and reconnected the wrong ones into the loom. Swap 'em back.

The Reverse Switch is self explanatory, the other 3 are 4th gear, Neutral and 5th gear. 4th and 5th gear swithches are for EGR and Pollution control. Neutral Position Switch will only be fitted to vehicles with Air Con, it is for Idle Up circuit.

[jzanuttini]

Great well ill give that a go and see what happens.

[zukenutter]

Interesting, didn't know about the 4th and 5th switches for EGR etc.

[losfer]

zukenutter wrote:

Interesting, didn't know about the 4th and 5th switches for EGR etc.

they operate the vacuum switching solenoid on the front right hand side of the cylinder head.

[zukenutter]

would this cause the engine to cut out sporadically? We've worked through everything except for a carby rebuild so I'm running out of possibilities.
It runs fine otherwise and then when under load will just stop running.

Any other suggestions are welcome.

[zukenutter]

These are the two plugs coming from two gearbox switches. The large yellowed one is connected to the reverse switch and the smaller white one is connected to the neutral switch. I think these two switches are installed back-to-front. There are no others switches on this gearbox just blanked off holes. The cars wiring loom has no other plugs so I don't think it's the wrong gearbox.

I've disconnected the small white plug for now so the reverse lights don't come on when in fifth.

[want33s]

Sounds like someone has had the switches out and reinstalled them incorrectly.
From memory the reverse wires are red and yellow.

[zukenutter]

Yep I agree, we'll swap them tomorrow.

[Scrawny]

zukenutter wrote:

would this cause the engine to cut out sporadically? We've worked through everything except for a carby rebuild so I'm running out of possibilities.
It runs fine otherwise and then when under load will just stop running.

Any other suggestions are welcome.

Full cut out or just a stutter? I assume you've checked/changed the spark side? Maybe a can of carby clean down the throat while it's running as a start. Also check the secondary throat is opening properly

[zukenutter]

Yeah Scrawn ticks to all those. New pump, etc too. Only thing we haven't dealt with is a carby rebuild.
It cuts out straight away at idle but it can be kept running with revs. Runs rich too.

[Scrawny]

Does it have much blow-by? Oil in the intake? Does sound like its time for a carby rebuild but just trying to cover all the bases first. I assumed you'd have been over everything already

[zukenutter]

No blow by wasn't too bad. I think most other bases have been covered.
I haven't heard from him so I'm thinking that it hasn't cut out since we disconnected the plug. I'm pretty confident though that once we swap the switches it'll be sweet.

I'll keep you posted.

[zukenutter]

Swapped the switches this evening which has sorted the reverse lights. One step further, win.

Sadly it's still having idle cutout when reverse is selected but now it also cuts in reverse.

I've put my switch bypass (see pic) back in for now.

[zukenutter]

Oh yeah it's still dying randomly.
The pump seems to not always pump properly. Could the push rod be worn/too short?