Guys,

I'm currently switching all my sensors over to the 383 from my 305tbi ('91). I have a few questions.

1. I'm switching to a carb, non-computer controlled. Do I need to keep the knock sensor? (I know I'll need one for a 350 if I do need it). Since all my new distributor needs is a 12v from the ignition, a tach wire and a ground, the computer can't retard the timiming if there's a knock anyways, correct? Or is it more complicated than that?

2. Coolant sensors. One at the front of the intake manifold doesn't fit the new intake (wrong size threads, new intake is 1/2 npt). Do I get an adapter? or do they make ones with the right size thread, but also uses the same electrical connection as the old one?... Basically same problem for the one on the head, which brings me to my next question...

3. I have a sensor on the pass and driver side head. I believe one is coolant and one is for the fan switch... which is which? The one on the passengers side was sheered off when i pulled it out, unsure how this happened, but this stops me from comparing it to a new one at the parts store.

Also, are any of these uneeded? and if i don't use them will i get a check engine light? I'm assuming I cant completely cut out the computer because it runs other things besides the engine, which means I'll always have a check engine light?

 

The sensor in the driver head is for the temp gauge in the cluster. The sensor in the passenger head is the fan switch, as far as the knock sensor and the coolant temp sensor I dnt believe you need either of those since your going carb with a vac advance distributor.

 

That's kinda what i was thinking. Also, I wasn't sure which one controlled the gauge and which was coolant temp for the computer, so that helps me out... I'm hoping some more people who have done a swap like this will chime in, but I would think what you said is correct.

 

If you are doing a carb swap all you require are the sender units related to the gauges and the fan switch ( if fitted );
everything else is redundant

 

Which brings me to two more questions,

1. Is the sensor above the oil filter the one for the pressure gauge?

and 2. I just shouldnt plug in anything else but the senders for the gauge? Im assuming ill have a constant check engine light?

 

Be sure to get a torque converter lock-up kit, on factory efi ( and carbed systems?) the computer controls the lock up in overdrive

 

Thanks but im using a t5 haha

 

Anyone else to chime in? Can i just unhook the computer completely? or are there other things besides the engine that is run by the computer?

 

I would say you can, but I'm not sure about the VSS and your speedometer. Idk if this runs through the computer for the speedo to work.

 

It does on the 90 - 92 TPI cars but not sure on the TBI's
Other than that there is no ECM related wiring that goes to the cluster
Pre '90 cars had the VSS input to the speedo 1st with a output from the speedo to the ECM

 

search for "carb swap "
A wealth of info on here

https://www.thirdgen.org/forums/ltx-...p-3rd-gen.html

 

[QUOTE=vetteoz;5639662]It does on the 90 - 92 TPI cars


So since his is an RS its possible the speedo may still work w/o the ECM?

 

It was orginally a tbi car

 

I can always swap to a mechanical speedo, but what about things like the fan switch? does that run off the computer?

 

As I think about it more, the VSS must go through the ECM. That's how it controls the speed limiter. I guess the real question is does the VSS go through the speedo first (then i can unplug the ECM) or does it go through the ECM first? In that case Im unsure of what to do about it.

 

https://www.thirdgen.org/forums/post...060-post6.html

As noted above
MAF TPI cars had VSS signal to the dash then ECM ; on MAP TPI car it went to the ECM 1st

 

Without a ECM you need a fan switch regardless



https://www.thirdgen.org/forums/cool...91-305tbi.html

 

No need for the knock sensor. Since that hole is actually the coolant drain, just put a brass ¼" pipe plug in it.

You definitely need the fan switch. (pass side head) If it doesn't fit your heads because the switch is 3/8" pipe but the hole is ½" pipe, you can use a bushing from the hardware store. Same for the temp gauge sending unit in the driver's side head.

Yes the sending unit above the oil filter is for the gauge, as well as for the fuel pump. You'll definitely need the gauge part kept working. Whether you need the fuel pump section depends on how, or even if, you're working the pump.

AFAIK the speedo is driven by the ECM, 90 and later. You'll need to either use a mech one, or a stand-alone electronic one.

 

Im going to be running the stock TBI fuel pump with a regulator... is that ECM run? or the relay is just run from the ignition switch? Also if I need to keep the ECM for the speedo, then I'll just have to deactivate the CEL somehow?

I got all the right bushings and the knock sensor is currently plugged

I know i'll need the fan switch, but does the fan switch need the ECM? If so Im assuming I'll need to wire up a manual switch

 

The way the fan switch works is when the head reaches that temp the switch grounds and completes the circuit and the fans come on. My TPI car has a fan setting thru the computer that controls the fan separate of the switch. You should be able to make the fan switch work w/o the computer.

 

So it doesn't need the computer then? Just a constant 12v and a ground? That''ll be easy to do. Luckily electrical and electronics is what i do for a living, so re-wiring anything doesn't really scare me haha They also sell stand alone fan switches which takes the temp reading right from the radiator, I might look into one of those. Only about $30 from jegs. Really the only problem left is the speedo, I guess we'll find out soon if it goes to the speedo or the computer first. I'll post on this thread to let you guys know for sure. Thanks for all the help.

 

I just came across this in a tech article.

"Factory Coolant Fan Circuit Operation
The fan is controlled mechanically through an in-cylinder head thermal switch and through the
mechanical A/C slider switch. The in-cylinder head thermal switch is designed to turn on the fan
when coolant temperatures reach 222°F (or higher) or when the A/C is turned on. There is no
ECM control for the fan on single fan applications."

So maybe the ECM doesn't control it? Its just only controlled by the relay?

 

The pump relay is run by the ECM but the pump switch mentioned by Sofa above supplies power direct to the pump anytime the engine OP is over 4 psi.

Delete the pump relay , just use the factory OP switch to run the pump.

As soon as you crank the engine the pump will run and it will stop when you turn the engine off.
No need for a separate pump switch you need to remember to operate as used by many in carb swaps

Take the bulb out

 

Well mine does work through the computer and it is a single fan. The reason I know this is because there is no wire plugged to the switch on the head (I can't find it) and the fan functions correctly. I've dug into the harness in several places and can't find a broken wire or anything, so I've just kinda quit worrying about it.

 

Sounds easy enough. So the fuel pump won't get primed when i turn the key then? It'll just kick on once i start cranking the engine?

Chad:

I don't think mine is computer controlled. My fan switch got busted a some point, which made my fan run constantly, so I'm assuming it was run directly from the switch.

I guess all of this will be verified in a couple weeks once i get all this buttoned up. I'll probably leave the ECM in until I can get time to change everything over like the fuel pump and possibly vss. That way I can at least get the engine broken in.

 

Good luck. Sounds like you can make everything work w/o the ECM, main thing in question is the speedo.

 

If it is wired through the ECM first, then I'm sure theres a way to just make it go straight to the speedo. I guess we'll find out. I just wanna get this on the road!

 

The engine should fire up on the fuel in carb bowls anyway

 

As I already said if you search on here you will find answers

https://www.thirdgen.org/forums/post...786-post7.html