'77 TBI Conversion, what year donor?

[Oilbrnr]

If you were to do this conversion, which year '87-'91 would you look for as the donor vehicle? Also, if you were not to use the factory harness, would you use the Turbo City or Painless harness?

As a side question, I noticed that Painless offers a VSS module (part 60116). Could that be used rather than modifying the speedo?

 

[Russell]

Do the real VSS, they are incredibly important to a properly running EFI engine, and also incredibly easy to install in these trucks. Just get an orange ringed speedometer speedometer with the green optical VSS module hanging off the back. You'll need to do some cutting on your cluster to make the bigger speedometer fit, but you'll have a working VSS then. You'll also probally want to switch out the gas gauge as well, so it matches colors Unfortunately, the orange ringed speedometers only go to 85 mph though, instead of 100 mph with the old ones, but thats is the only downside.

I'd get an 88 - 92 stock TBI harness and modify it. They are almost entirely stand-alone. You'd only need to cut the wire that runs back to the fuel pump, and any wiring that goes to the AC setup (if so equipped). Just cut the connector that runs a group of 5 - 6 wires off to the driver's side of the dash, you'll need to splice a few wires in there later.

Check out the ECM pinout / wiring harness diagram at www.chevythunder.com The TBI ECM part number ends with 7747.

I had the electrical finished on my 77 stepside over the course of a weekend, and did not have to make any modifications to my stock wiring less shorting the stock park / neutral switch wiring so the ECM could use the switch instead. I used a 85 - 86 TPI distributor instead of the stock small cap HEI. That meant I was able to use the stock engine wiring harness and did not have to modify the wiring for the remote coil.

Fuel was easy too. My truck was a shortbox, and I was literally able to run the stock TBI hose from the 89 I pulled my TI setup out of straight from my throttle body to the tank. For a pump, I used an 87 sending unit in my stock tank. I did have some problems with it stalling out while taking corners if below 1/4 tank of fuel, but otherwise it worked very nicely.

Bolting everything in was easy too, only thing I had to modify was the four center bolt holes on the TBI intake.

If you have any questions as you go, feel free to post up!

 

[dyeager535]

If you are going to do the speedometer, just put the entire newer cluster in. Any truck from about '82 or '83-89 will have the *provisions* for VSS on the speedometer, VSS started when the electronic cruise did, which I'm fairly certain was '83, not '82.

You could do a '90-91, BUT that means you can't use a speedometer cable, and would either need the 1991 205, or the 1990-91 NP241. The clusters are entirely different in the '90-91's, so you can't mix and match stuff as easily. '90-91 cluster is a huge improvement for a bunch of reasons, but having to swap transfercases for a cluster would be a pain. Although I did it, and don't regret it a bit. I should have, however, just started out with the '90-91 t-case.

 

[Oilbrnr]

The Painless VSS is a real VSS, it just goes inline with the speedo cable, wherever you decide to put it. I just need to know if it puts out the correct PPT that is needed. I've seen some threads here that claim '90 ECMs are looking for a different count than the earlier unit.

I'm trying to keep this unmolested '77 as unmolested as possible so that if someone wanted to go back to OEM in the future they can. It has a factory tach that I'd like to keep, and have the cluster all match.

FWIW, it does have factory A/C and since it will be my DD here in AZ, I'd need to incorporate that if the TBI system supports it in some fashion.

Reason I'm asking these questions now, is there is a guy selling a '90 TBI system complete, and I wanted to know if it was a good year for this application, or if I should find an '87.

I'll check out the chevythunder link for the pinout.

Thanks!

 

[dyeager535]

2000 PPM (pulses per mile) is what the ECM is looking for. '90-91 is SOMEWHAT different, the VSS on the transfer case is 40 pulses per revolution, and feeds that into the DRAC module, which splits that input into the various ones that the speedometer, ECM, cruise, and ABS need. The ECM still gets 2000PPM, it just gets there in a roundabout way.

If you might plan on selling the truck, you seriously need to consider what you are doing. If you do it right, nothing will end up being molested on the truck, you'd just have a pile of parts to put back on later. If you are going to add TBI, don't worry about it, put whatever on you want, none of it is irreversible.

As to the tach, you can't easily put it in the '90-91 cluster (I left mine out so far because of this) and it might be the same way from '87-89, since there was apparently no tach offered, so they changed the gauge housing a bit. Tach is a separate harness you can just unplug it and leave the harness in place if need be.

I've found I don't really need a tach. The ECM already has an RPM/MPH limiter, so I can't exceed those, and idle isn't adjustable either, so other than as eye candy, it's pretty worthless. You'll probably end up with winALDL or a scanner, those both have RPM gauges if you were doing diagnostics.

 

[Russell]

His 77 probably still has a mechanical oil pressure gauge, which would make it tough to install a newer cluster anyways. Thats why I suggested he just switch out his speedometer and fuel gauge. However, if you have a tach, and don't want to cut anything, then you'll just need to get one of the painless inline 2000ppm signal generators as you mentioned.

The only involvement that the ECM has with the A/C is that it detects when the compressor clutch has locked, and it bumps up the idle speed by a few hundred RPM to account for the load on the engine. It does not control the compressor at all, just reacts to it. I've never hooked up an ECM to an A/C setup in the past before, but I am fairly sure it just needs a 12 volt signal applied to pin B8, which should be a dark green wire on the 90.

If you want the swap to be totally reversible make sure that you use a single wire connector to connect to any parts of the truck's stock harness like the A/C stuff, that way the new owner can just unplug the connector and still have the original wiring working. That would also be another reason for switching to the big cap HEI EST distributor from either the early TPI cars, or the EST trucks so you don't molest the stock wiring on either the tach or the engine harness' accounts.

I would also highly suggest that you make the switch to a 105 amp CS150 alternator from the TBI trucks. If you can find yourself a 92ish Buick LeSabre with a 3800 they have a CS150 with the correct housing to literally bolt straight into your stock 77 alternator bracketry. You may need to rotate the bottom housing to put the wiring plug up in a place where you can get at it. Wiring them up is easy, just go get the proper CS130 adapter from your local parts house (with the built in resistance) and plug everything straight in. TBI is a power hungry setup, and you will likely run into problems with the old 63 amp SI alternator.

For the SES light, you could put the 90's (assuming burb, crew cab or K5) SES lense under either your tach or fuel gauge (whichever does not have any use idiot lights) and find yourself an NP241 4x4 light wiring harness to get the little light plug with the wires on it. Put one side to hot, the other to ground, and the ECM will turn the light on for you. This would again save you from making any modification to your stock cluster.

One thing I would highly suggest, if you don't already have one, is to hook up a charcoal canister. I didn't with my 77, and had a lot of trouble with gas fumes as the fuel pump built up pressure in the tank, and vented it out on my driver's side frame rail up front. Installing the canister is easy, just use the stock TBI port up front, the engine is already calibrated to be receiving that extra amount of vapor and does not require any canister solenoids.

 

[Oilbrnr]

Thanks for the info guys! It is a mechanical oil pressure gauge.

Can you explain the EST distributor a bit more? I'm not that familiar with the variants of the HEI as I probably should be. Further, I'm not that sure what is different about the OEM connection is for the tach.

What needs to happen as far a connecting the peddle cable to the throttle body? What did you do for the fuel pump? I know that in-tank is quieter, but I'm assuming that requires a swap of the entire tank.

Sorry if these questions are redundant, but I've done a lot of searching and still I'm not sure what the best route is to take, esp. donor year.

 

[Russell]

Some if the mid 80s trucks (I don't know the exact years) had the EST (electronic spark timing) system in them. Basically, there was a very crude ignition control module that changed the timing based on throttle position, vaccum levels etc. They didn't work that great, but were the fundamental beginnings of TBI and other fuel injection systems.

GM started off the electronically controlled distributors by modifying one of their regular old HEI distributors with a different ignition module, and by removing both the vaccum advance and the mechanical advance mechanisms. The distributor's timing was changed electronically in the ignition module instead of just firing every time the pickup coil was actuated.

GM did not change the signal that controls the ignition timing when they switched to the small cap HEI with the divorced coil when they introduced the NBS trucks in 88. The same distributor was used with the early TPI multiport systems up until 88.

The big cap HEI distributor looks identical to the old regular ones except they have a second pig tail for the ECM to control the timing, and don't have a vaccum canister.

The only modifications you'd have to make to the wiring would be to change the 4 wire connector from the small cap HEI version to the big cap HEI version on the TBI's wiring harness. So I didn't have to cut wires, I personally just picked up a couple extra connectors from the wreckers, de-pinned them, and soldered them together to make myself an adapter.

Worked very nicely If you put the big cap HEI in, it uses the same cap as the older HEI distributors, and as such uses the same power and tach wiring. You shouldn't have to modify anything to make your tach work properly with it.

As far as the throttle cable goes, just use the carb throttle cable. It should pop straight into the TBI bracketry. Might want to get someone to floor the pedal and play with the linkage up front to make sure that the throttle blades are being opened all the way by the cable. If it isn't, you'll need to replace your stretched cable.

Fuel pump was a stock in-tank TBI sending unit and pump from an 87 truck. I personally used a 91 crewcab sending unit, but it is the same as the 87's. A suburban or Blazer tank sender won't work for you though... They bolt straight in without any modifications to the tank.

However, if possible, get the whole EFI tank. They have special baffles in them that keeps the tank submerged even if you are going around corners, accelerating hard etc with less than 1/4 of a tank of fuel. Even if you do get the EFI tank, keep it above 1/2 a tank to prolong the length of the fuel pump's life.

I personally used an 89 NBS truck to get the wiring from, but it should be basically the same right from 87 up to 94ish or whenever they switched from the 700r4 / 4l60 to the 4l60e and began using a PCM instead of the earlier ECMs.