computer & timing - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Smokin86 · 08-12-2004, 11:27 PM

been workin on the truck a lot lately. it would crank all day, but never start. we got it finally figured out, but i got some questions.

this is what i did

the module and pickup were bad so I stabbed a rebuilt distributor in it. still wouldn't do anything. bypassed the computer under the dash by connecting a & c on the plug. it starts and idles now pretty good.

now the questions

what should i set my timing to? i adjusted the timing until it idled even enough by sound so I could drive it home from where it was dead at. Now i want to set it correctly with a timing light. everything is stock on the 305.

what does the computer actually do? my mechanic told me to bypass it. it worked, so i went with it. What are the consequences of bypassing it? what else is part of the system? any info about it would be appreciated.

Russell · 08-13-2004, 12:03 AM

The computer "listens" for detonation through the knock sensor on the side of the block, and retards the timing accordingly. You'll want to dissconnect the power to your computer for like 10 seconds or whatever it is to reset it and have it withdraw any error codes, then look for the bypass connection that is hooked up to the dizzy, and the other end on the wiring harness, connect the two together, start the truck up with the computer hooked up and on, then set your timing, and unplug the bypass. This is called setting your base timing, if you play with the timing without hooking up the bypass, it'll just retard / advance the timing until it is where it was before. Also make sure that the knock sensor is working properly, if it is sending a signal when there is no detonation, it'd cause the truck to run super retarded for no reason.

greg0380 · 08-13-2004, 08:07 AM

Smokin, I don't believe that your 86 has a computer, but more likely an ESC module (electronic spark control). It does use a knock sensor on the pass. side of the block down near the oilpan. If your distributor has a 4 wire connector hanging off it and a small module up under your glovebox, then that is your setup. 1987 was when the ECM computers started. I did the exact thing to my 85. Once you bypass the ESC by jumping out the plug you have to control spark yourself instead of the ESC. I put in a Crane adjustable vacuum advance kit on the distributor and it worked great and was easy to do. I was able to set mechanical timing at 10 deg and adjust the Crane kit to keep me out of the pinging range. Hope that helps.

**swervin ervin** · 08-13-2004, 08:10 AM

The ESC (electronic spark control) is not really a computer. All it does is retard timing if your engine spark knocks. Since you have bypassed it, it's doing nothing at all. The reason they are there in the first place is because 305's are spark knock prone engines mainly because of the head design. You want be able to run a lot of timing but I think several degrees past stock will be fine.

beaterC10 · 08-13-2004, 09:22 AM

Do you have to install a jumper on the ESC harness? My truck (84) has a 350 with a non ESC dizzy. The ESC connector is simply hanging on the firewall, with the power lead connected to the dizzy.

**swervin ervin** · 08-13-2004, 01:18 PM

No, all you need is the 12 volt power wire to the distributor. The rest of the ESC stuff can be removed entirely from the truck.

8t5Silverado · 08-13-2004, 01:44 PM

I had this same problem.

I tried to "jump" the ESC, but couldn't figure out how to do it correctly.

Luckily, I got an ESC from a guy on another board for shipping alone.

Mike, can I put a different set of heads (maybe off of a Camaro 350) on my 305 to help it breathe a little better?

'86_4.3 · 08-13-2004, 02:51 PM

The jumper is just on the distributer, not the harness(5 pin module on the ESC distributor) Those 350 heads will likely have larger combustion chambers which will kill the compression and hurt HP.

Smokin86 · 08-14-2004, 05:26 PM

thanks for the responses. i use the word computer very liberally. i know the "computer" was the ESC, i just called it a computer b/c ... well, i really don't know why. anyway, got a stupid question. please don't laugh. the timing tab has some teeth and then a circle. Is the circle zero degrees? For the line to be in the circle, i have to set my timing light to at least 34 degrees. I checked it last night with the vacuum advance plugged and it still did it. Wanted to make sure i didn't make an obvious mistake. if i check the timing on the other end of the timing tab, it shows 11. I guess the circle is not zero. I just want to be sure. This is the first time i have ever messed with timing, ignition, or anything of the sort.

SanitysBane · 08-14-2004, 06:35 PM

you don't have to jump it, all you have to do is unplug the nock sensor, and it won't do anything, i looked it up in my gm shopmanual.

08-12-2004, 11:27 PM	#1
Smokin86 Registered User Join Date: Oct 2003 Location: Wichita Falls, TX Posts: 71	computer & timing been workin on the truck a lot lately. it would crank all day, but never start. we got it finally figured out, but i got some questions. this is what i did the module and pickup were bad so I stabbed a rebuilt distributor in it. still wouldn't do anything. bypassed the computer under the dash by connecting a & c on the plug. it starts and idles now pretty good. now the questions what should i set my timing to? i adjusted the timing until it idled even enough by sound so I could drive it home from where it was dead at. Now i want to set it correctly with a timing light. everything is stock on the 305. what does the computer actually do? my mechanic told me to bypass it. it worked, so i went with it. What are the consequences of bypassing it? what else is part of the system? any info about it would be appreciated. __________________ Joseph 1986 Silverado with some things upgraded. Currently living in Arlington, TX going to school.

08-13-2004, 12:03 AM	#2
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	The computer "listens" for detonation through the knock sensor on the side of the block, and retards the timing accordingly. You'll want to dissconnect the power to your computer for like 10 seconds or whatever it is to reset it and have it withdraw any error codes, then look for the bypass connection that is hooked up to the dizzy, and the other end on the wiring harness, connect the two together, start the truck up with the computer hooked up and on, then set your timing, and unplug the bypass. This is called setting your base timing, if you play with the timing without hooking up the bypass, it'll just retard / advance the timing until it is where it was before. Also make sure that the knock sensor is working properly, if it is sending a signal when there is no detonation, it'd cause the truck to run super retarded for no reason. __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)

08-13-2004, 08:10 AM	#4
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	The ESC (electronic spark control) is not really a computer. All it does is retard timing if your engine spark knocks. Since you have bypassed it, it's doing nothing at all. The reason they are there in the first place is because 305's are spark knock prone engines mainly because of the head design. You want be able to run a lot of timing but I think several degrees past stock will be fine. __________________ Mike 1985 Chevy C-10

08-13-2004, 01:44 PM	#7
8t5Silverado Mudslides 'r my specialty Join Date: Dec 2003 Location: STL Posts: 1,106	I had this same problem. I tried to "jump" the ESC, but couldn't figure out how to do it correctly. Luckily, I got an ESC from a guy on another board for shipping alone. Mike, can I put a different set of heads (maybe off of a Camaro 350) on my 305 to help it breathe a little better? __________________ Always on the lookout for old cars and parts...(20s, 30s, 40s, 50s, 60s, etc.) http://www.cardomain.com/memberpage/500339

08-13-2004, 02:51 PM	#8
'86_4.3 Registered User Join Date: Jul 2004 Location: small town NE Posts: 5	The jumper is just on the distributer, not the harness(5 pin module on the ESC distributor) Those 350 heads will likely have larger combustion chambers which will kill the compression and hurt HP. __________________ My Website 86 chevy silverado 96 chevy tahoe 82 chevy camaro My dad's 67 chevy 4sp I6 daily driver 70 chevy short wide 307 th350--in lots of pieces 99 chevy tahoe

08-13-2004, 08:07 AM	#3
greg0380 Registered User Join Date: Dec 2003 Location: CT Posts: 454	Smokin, I don't believe that your 86 has a computer, but more likely an ESC module (electronic spark control). It does use a knock sensor on the pass. side of the block down near the oilpan. If your distributor has a 4 wire connector hanging off it and a small module up under your glovebox, then that is your setup. 1987 was when the ECM computers started. I did the exact thing to my 85. Once you bypass the ESC by jumping out the plug you have to control spark yourself instead of the ESC. I put in a Crane adjustable vacuum advance kit on the distributor and it worked great and was easy to do. I was able to set mechanical timing at 10 deg and adjust the Crane kit to keep me out of the pinging range. Hope that helps.

08-13-2004, 09:22 AM	#5
beaterC10 Senior Member Join Date: Jul 2003 Location: Indianapolis, IN Posts: 1,277	Do you have to install a jumper on the ESC harness? My truck (84) has a 350 with a non ESC dizzy. The ESC connector is simply hanging on the firewall, with the power lead connected to the dizzy.

08-13-2004, 01:18 PM	#6
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	No, all you need is the 12 volt power wire to the distributor. The rest of the ESC stuff can be removed entirely from the truck.

08-14-2004, 05:26 PM	#9
Smokin86 Registered User Join Date: Oct 2003 Location: Wichita Falls, TX Posts: 71	thanks for the responses. i use the word computer very liberally. i know the "computer" was the ESC, i just called it a computer b/c ... well, i really don't know why. anyway, got a stupid question. please don't laugh. the timing tab has some teeth and then a circle. Is the circle zero degrees? For the line to be in the circle, i have to set my timing light to at least 34 degrees. I checked it last night with the vacuum advance plugged and it still did it. Wanted to make sure i didn't make an obvious mistake. if i check the timing on the other end of the timing tab, it shows 11. I guess the circle is not zero. I just want to be sure. This is the first time i have ever messed with timing, ignition, or anything of the sort. __________________ Joseph 1986 Silverado with some things upgraded. Currently living in Arlington, TX going to school.

08-14-2004, 06:35 PM	#10
SanitysBane Registered User Join Date: May 2004 Location: Lubbock, Texas Posts: 2,121	you don't have to jump it, all you have to do is unplug the nock sensor, and it won't do anything, i looked it up in my gm shopmanual. __________________ '96 Nissan Pathfinder '02 Firebird Trans Am. '88 K5 Blazer



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