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fuel pump question-electrical

I've searched and trying to figure out:

What controls both fuel pumps and fails when the motor is hot?

Thermo switch?


I was stranded by the road today. It's not not the over boost relay, it's not the fp relays.

engine temp was about 190f and would start to sputter and not have any power. Both fuel pumps are running, but at some point they would both intermitently stop at the same time, the motor would stumble, and eventually the motor would shut down.

I saw some post by Jfairman that would indicate the WUR, but that was very recently rebuilt by Larry at CIS Flow tech and the fuel head was checked out.

What controls the fueul pump electricity.

The application is a 78 930 with SC cams and HF turbot with M&K exhaust. Otherwise stock.

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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-23-2012, 08:20 PM
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1978 wiring diagram shown. You are either losing power or ground. Power comes directly through the fuel pumps relays simultaneously so you could be dropping voltage at the relay coil (terminal 86 on both) or at the supply side of the relay (terminal 30). The other option is losing ground. The ground circuit can be opened by the boost relay, the air meter contact, or boost limit switch. I would bypass each one of those temporarily to see if any one of them solves the problem when bypassed.

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"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-24-2012, 02:59 AM
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Thanks Mark.

I didn't realize there was a air meter relay. I'll start with the easy stuff and move from there. I have been plagued with fuel probelms on this car. I almost want to run an independant fuel system (independent from the factory design) so it's easy to trace with off the shelf parts, but that's just the frustration talking.
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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-24-2012, 04:01 AM
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There's isn't supposed to be anything that shuts off the fuel pumps when hot but my first guess is the single overworked origonal fuse that supplies 12 volts to both fuel pump relays, fuel pumps, and the WUR and cold start aux air valve on the motor. That fuse gets too much current through it and it gets very hot and so does the old fuse holder it sits in. There are threads here to upgrade that fuse.

Next guess is the multi pin plug contacts to the speed relay under the driver seat. You can clean those if they become intermittant.

There's just alot of old relays and electrical contacts that have to be in good condition or there will be fuel pump problems. After 20 - 30 years they get funky.

As a back up power supply for when the pumps decide to shut off and you get stranded you can install a hidden double pole (to keep the two fuel pump relay and electrical circuits isolated) single throw toggle switch that bypasses all the fuel pump relays and supplies 12 volts directly from the battery to the fuel pump power leads.
Use a quality toggle switch rated for around 20 amps at 120 volts AC and that way it will safely handle more than 20 amps at 12 volts.

You can wire that fuel pump power supply bypass in behind the fuse panel at the two seperate power leads going to the pumps from the front and rear red fuel pump relays.
Old 03-24-2012, 07:34 AM
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I was stranded twice with my 87 930... the Yellow relays PCB solder connections were expanding and shutting down power to the pumps when it got hot. I replaced the relay and havent had an issue since.
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Old 03-24-2012, 11:58 AM
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Thank you guys for the input all good information.

On my first breakdown, did I mention there were two? Once in a parking lot, then about an hour later 5 miles down the side of the road. It appears to be heat related to some extent. All the relays were working, I changed out with known working relays. We are getting voltage to the rear pump (easiest to check), we bought a jack and I put a meter on it. All the fuses are good, did a visual inspection and took many out to phsically make sure they were intact. I think it is a switch of some sort that when it gets hot is interupts the current to the pumps.

Given the theory of heat being the catalyst for the event, we cooled down the motor with bags of ice.

After about 30 minutes it started and I drove it 5 more miles before the started to stumble and eventually fail again. At this point the car was not driveable adn I flatbedded it home.

I want to get it to repeat again and I want to try jumping out the overboost, then the air meter switch, then the overboost relay, and so on.

I do want to run an emergency bypass for the fuel pumps. If for no other reason had this thing quit on me while on "The Three Sisters" in Texas, it could have been dangerous.
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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-25-2012, 04:23 AM
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When I had this problem it was relay J16. It would overheat and open the contacts. It wasn't directly related to engine heat. The car would fire right back up, sometimes I was still coasting and could crank it up. Other times it would take a couple of minutes. The relay would be hot as a pistol to the touch. I think the control coil in the relay was shorted causing it to pull too much current and overheat causing a voltage drop to the point it couldn't hold the contacts closed. Replaced the relay and never had a problem again. Not saying that is your problem, just what I experienced.
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"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-25-2012, 04:35 AM
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Another possibility from my old VW Beetle days. Here in Florida on hot days I would have vapor lock from engine heat due to not having a full liquid seal in the fuel lines. I would have to let the engine cool before it would fire back up. Turns out the old mechanical fuel pump was failing so I installed an electric pump and solved the problem. Are you sure you have proper system and control fuel pressure as well as proper fuel volume? If not, maybe something to look at as well?????
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-25-2012, 04:40 AM
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equality72521,

I haven't had a chance to check fp's and vol since the event, but I have in the past and had it verifed by a third party when bench tested. I haven't done it on a "hot" motor.

I don't like that j16 relay back there, but there are tons of cars that run it. I was considering upgrading to a heavier duty more modern relay that may be more tolerant of the heat and vibration.

Any idea what the amp rating is on these relays?
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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-25-2012, 06:17 AM
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I haven't had a lot of time to work on this, but I have had a lot of time to think about it. I started jumping for bypassing switches. So far it looks like it's the boost limit switch. The car would not run before (albeit a 30 sec) and now it runs after bypassing the boost limit switch. I have ordered another switch to replace it with.
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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-28-2012, 03:15 AM
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I replaced the switch and it started and ran. I drove it until it started "popping" like crazy and then stalled. This is becoming a common site. At least I was only about a 1/2m from the house.



I am also learning to appreciate the dual angle flat beds.



Next is the air meter contact. Is this the blue switch on the back side that gets disconnected when checking fp?
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84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-29-2012, 05:36 PM
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Yes.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-29-2012, 06:51 PM
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Mark has been throwing some excellent advice your way. I don't know that I can add much, but it sounds like you've covered most of the standard "failure sites". It may be time to look under the drivers seat.

When my over-speed relay started acting up, it seemed always to be after running for awhile, shutting down for an hour or more, then trying to start back up. She would fire and run for a little less than a second then stall. Further waiting periods (cooling?) and eventually she would fire and keep running. Never stranded me on the road, just in the parking lot. Anyway, that's just another relay that can do the funky chicken on you.
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Old 03-29-2012, 07:12 PM
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I looked under the seat and didn't see a relay. I'll look again. Heat seems to be a consistent factor.
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84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-30-2012, 03:41 AM
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The relay under the seat isn't there on our model year RoW cars (1978-79)
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-30-2012, 04:48 AM
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Thanks. I thought I may be missing something.
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84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-30-2012, 06:01 AM
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How do you test the relay for the "delayed ignition cut-off"? I don't see it my version of the service manual.
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John- I blame and thank the longhood boys of Austin
84 911 Carrera w/ 930 t/p,efi'd,dizzy free,HFS turbo[/COLOR], 78 930 RoW no option sunroof coupe, 64 Corvette roadster, 71 911T, 73 911E, 67 Bronco survivor. 71 FJ40, 2001 Boxster
"Illegitimi non carborundum"
Old 03-30-2012, 07:22 AM
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Jump terminals "30" and "L".
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"
--clutch-monkey
Old 03-30-2012, 08:04 AM
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Thx
Old 03-30-2012, 08:12 AM
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I had your problem and it was the boost limit switch. If you have a multi meter you can check it.
Remove the switch, set meter to Ohms and hook to terminal and body. Should be a contact (if I remember circuit diagram right). This switch provides the earth for the pumps. When the little spring at 1.1 bar lifts it breaks the contact and shuts the pumps. Or, in my case if the terminal is loose in the body it can intermittently break contact. Your multi meter will show this when you try wiggling the terminal.
Alan

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Old 04-01-2012, 04:10 PM
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