Tach signal wire

[vkh]

So I have an 81 c2500 with a 454/th400. It didn't have a factory tach but there still was a plug on the tach signal. When I got my tach cluster I just kinda unhooked it figuring it wasn't doing anything and made my own signal wire. Recently I traced the factory signal wire and found it connected to the relay plug thing attached to the drivers side valve cover. This has left me wondering what it was used for. Anyone know what it's purpose was? I noticed no difference when it was unhooked. Thanks all

 

[animal]

@RetroC10Sport

 

[chengny]

Speed switch - it (along with a solenoid) activated the TRCS (throttle return control system)?

 

[vkh]

Speed switch - it (along with a solenoid) activated the TRCS (throttle return control system)?

Would that be the little solenoid that bumps the idle up on the carb? If I remember correctly the original one had a vacuum line on it. I took it off at some point (I think when I was purging all my smog stuff and the mess of vacuum lines associated with it) and later added an electric one wired to up my idle when my ac was on. Now it's been quite awhile so the original may not have been vacuum run.

 

[chengny]

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

Would that be the little solenoid that bumps the idle up on the carb? If I remember correctly the original one had a vacuum line on it. I took it off at some point (I think when I was purging all my smog stuff and the mess of vacuum lines associated with it) and later added an electric one wired to up my idle when my ac was on. Now it's been quite awhile so the original may not have been vacuum run.

Proper operation of the system required 3 things:

1. a manifold vacuum source,

2. an RPM signal

3. and a 12 VDC supply.

Read the stuff below and you'll get why:

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[vkh]

Thanks, honestly I had no idea they had systems like that on trucks this old. Well, My idle is solid without it and all the vacuum components are long gone so I think I'll manage without it.