81 911SC starts then stops after a second

[hasaramat]

I'm working on an 81 911SC that will start and then immediately stops after about one second. If I jump the fuel pump relay with a wire it runs perfectly. I have tried two new RED relays and the same thing happens, (new red ones are made in china if you haven't noticed). Tried a used German one also, same results. Any thoughts? Could it be the rev limiter?

[timmy2]

You are bypassing the safety shut off by bridging the relay socket to force the pump to run.
Check the air flow switch wiring isn't grounded somehow.
You didn't happen to have the connectors off the CIS recently did you?
May have the plugs in the wrong locations?

[boyt911sc]

Originally Posted by hasaramat

I'm working on an 81 911SC that will start and then immediately stops after about one second. If I jump the fuel pump relay with a wire it runs perfectly. I have tried two new RED relays and the same thing happens, (new red ones are made in china if you haven't noticed). Tried a used German one also, same results. Any thoughts? Could it be the rev limiter?

Hasaramat,

Test the FP relay, really socket terminals, and other auxiliary devices connected to the relay socket like a rev limiter, engine immobilizer, etc. something is triggering to cut off the FP operation. Familiarize your self to the basic test for the FP relay. Search for DKLever48's thread about testing FP relay. I would focus on terminals #30 & #85. Lately people were having problems with new FP relays. Keep us posted.

Tony

[hasaramat]

The only thing that I have been working on is the CDI. I had to send it out to have it rebuilt. I will test the relays to make sure they are good.

[hasaramat]

Pin 30 is good and free to the fuel pump. Pin 87 is the culprit. I get temporary power while the starter is engaged and the engine fires then as soon as I disengaged the starter the power goes out of pin 87 at the relay. The relay is good. According to the diagram pin 87 comes from the ignition switch and I have continuity from pins 15 and 50 on the ignition switch to pin 87 at the FP relay. With the key in the "on" postilion shouldn't pin 87 have constant power?

Also where is the rev limiter switch in an 81SC?

[tirwin]

Rev limiter is behind the tach or maybe it's integrated -- I've never bothered to actually look at it.

I was just dealing with this the other day troubleshooting something else. Seems like I remember that 1 circuit is involved at start and then once the car is started the other circuit provides constant hot.

I just checked my car ('83). 87a has 12V with the key in ON position but not 87.

[boyt911sc]

Originally Posted by hasaramat

Pin 30 is good and free to the fuel pump. Pin 87 is the culprit. I get temporary power while the starter is engaged and the engine fires then as soon as I disengaged the starter the power goes out of pin 87 at the relay. The relay is good. According to the diagram pin 87 comes from the ignition switch and I have continuity from pins 15 and 50 on the ignition switch to pin 87 at the FP relay. With the key in the "on" postilion shouldn't pin 87 have constant power?

Also where is the rev limiter switch in an 81SC?

Hasaramat,

Power at terminal #87 is only present at the START position of the ignition switch. Once you turned the ignition switch back to ON/RUN position, the power at terminal 87 is cut off and at the same time AFS (air flow sensor) is lifted up by the vacuum created by the moving engine. At the same instant, the FP relay switches back from 87-30 to 87a-30 configuration that keeps the FP running.

Tony

[hasaramat]

The problem is solved!!!
It was the pin 85 ground at the fuel pump relay that was the problem. When the Air metering flap is raised the ground at pin 85 should be interrupted but pin 85 was still grounding. Pin 85 has a double ground! It has a ground at the AFS (air flow switch) on the back side of the air metering housing. This is a nasty little switch to test!
And it also has a ground at the ever allusive REV Limiter Switch. This is a UFO, Bigfoot, and Loch Ness monster switch that is hidden deep in the nether world of a 911SC. IT IS NOT BEHIND THE TACH. At least on an 81SC it's not. If you unplug this switch it will eliminate the second ground. After I did this the car fired right up and runs great. The rev limiter switch is bad. It is a box about 3 inches by 3 inches by 3/4 inch thick. It has a white plug going into it. the plug has four wires: brown, red, brown/red and white/red. It is held in by two Phillips head screws. The only access I can see is removing the fuel,oil gauge and MAYBE you can get the screws out. Putting it back in will be a whole different battle.


This is the location of the rev limiter switch but under the cowl along the inner fender wall


This pic is of the switch itself. Bad phone pic. The white plug in the middle of the pic is the plug removed from the rev limiter switch. The smaller black box to the left of the white plug is the flasher relay. The gauge that you see is the far left fuel, oil level gauge from the back.You can reach the switch and unplug it from the trunk side but I don't think you could ever remove it from the trunk side. I hope this will help some poor soul that cannot get there fuel pump to work properly. After reading a 6 page thread on this problem then at the end it left me hanging with a small mention of the rev limiter switch. This should make it complete.

[schoward]

"And it also has a ground at the ever allusive REV Limiter Switch. **This is a UFO, Bigfoot, and Loch Ness monster switch that is hidden deep in the nether world of a 911SC.*** IT IS NOT BEHIND THE TACH. At least on an 81SC it's not. If you unplug this switch it will eliminate the second ground. After I did this the car fired right up and runs great. The rev limiter switch is bad. It is a box about 3 inches by 3 inches by 3/4 inch thick. It has a white plug going into it. the plug has four wires: brown, red, brown/red and white/red. It is held in by two Phillips head screws. The only access I can see is removing the fuel,oil gauge and MAYBE you can get the screws out. Putting it back in will be a whole different battle. "

+1 -- The box in my '81 was also bad. A very obscure item that can directly affect your fuel pump as its job is to cut the fuel pump when revs of 7k reached.

I found mine unplugged. Plugged it in and it would allow the car to run but it was cutting the fuel pump on and off very quickly making the car run a little 'off'. Fuel pump sounded like it had gravel going through it from the intermittent power/ground. Unplugged it and all fine. Got a new used box, mounted it, plugged it in and all good.

You can run with it unplugged, you just wont have a rev limiter. So it is easy to test if this is your problem - just unplug it.

[tirwin]

Brings up an interesting question. For those that have a non-OEM CDI that also have a rev limiter function (e.g., MSD), should the original rev limiter be permanently disconnected? One less thing to troubleshoot?