[b]Hey guys! I've been reading on this forum for some time and just signed up tonight. I purchased an 88 Cutlass Supreme Classic last fall (always loved those G bodies!) and I'm trying to get it going for the summer. I'm in the process of doing some body repairs (rear frame and lower quarters in front or rear wheels) with parts I cut off another car. It also has some drivabilty issues that I'm working out. I've made a few repairs, but I want to make sure all the basics are covered as well, which brings me to my question. In order to set the timing, the emissions label says to ground the C3 diagnostic lead. Where would I find that? I bought a Factory Service Manual at a swap meet only to find out, when I got it home, that it was for a FWD W body. I also got a Haynes repair manual and while it offers some good info, it doesn't address the C3 diag. connector. I worked at a Pontiac dealership in the eighties and seem to remember a lead coming out of the harness near the firewall. I don't think it was a G body (Grand Prix) though...likly a FWD...hard to remember (that was many beers ago!!) Anyway, hope someone can help me out. Thanks. al

The C3 system on the 307 is EXTREMELY sensitive to being adjusted correctly, so definitely get the correct Chassis Service Manual. I've found the Haynes/Chilton books to be pretty worthless.

Which diagnostic connector are you looking for, the one under the hood to check the dwell on the mixture control solenoid or the ALCL connector under the dash? The one under the hood should be a green single wire connector coming out the the harness directly in front of the carb. There will be nothing connected to it. Attach a normal dwell meter to this connector, set it to the six cylinder scale, and you can read the M/C solenoid dwell. Note that if the O2 sensor cools down, the computer goes back into open loop mode and the M/C dwell will be fixed. If the car has been idling, you may need to run it at 2500-3000 RPM for 2-3 minutes to heat up the O2 sensor and get the system back into closed loop mode.

I've been through the C3 system on two different 307s. They take some work to get everything working right, but after that they run very well. My 86 has been running great for two years now as a daily driver (well, until the 200-4R died...)

Hey thanks for the response. I'm not looking to set the dwell on the MC solenoid at this time, just to check the ignition timing and the emissions label says to ground that circuit. I seem to remember a tan and black wire that bypassed the EST for the purpose of checking the timing. I can't seem to find it under the hood. Like I said though, that was 20 years ago. Btw, what MPG does your well tuned 307 get. My 70 Charger with a 383 gets around 16 if I go easy on it. I'm thinking (hoping)the Olds should do better. Thanks again!!!

ps:sorry about the trans

You don't say but I will assume that you have a 5.0L VIN Y.
Each engine with a different VIN number has a different procedure. The spec I have is 20 degrees @ 1100 RPM's in park.

1)Engine must be at normal operating temperature with choke fully open. Turn A/C and all accessories off. Connect tachometer to engine. Connect timing light to No.1 spark plug.

2) With engine running, install a jumper wire between terminals "A" and "B" of diagnostic connector. Connector is located under left side of instument panel near steering column.

3) Set timing at specified engine RPM. Recheck timing after tightening distributor housing. With engine still running, remove jumper wire from diagnostic connector.

Hope this helps.