Mefi 4b help electrical issue

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Desertanddune

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Help please,
Fan shorted to ground rewired with relay, it wouldn't work off temp sensor so added switch, fan works now but now the car wont start It will turn over but not fire no signal to fuel pump if you ground relay fuel pump runs but car still wont fire power to computer and grounds ohm good
Mefi4b shortstar motor any help would be greatly appreciated
 
Do you have 12V to the Mefi? If you switch the ign on and the fuel pump doesn’t run… that indicates the ecu isn’t powering up
 
On the Case of the Mefi, grind off the Paint on one of the mounting bolts and run a ground wire from the case to the chassis
 
been a while since i touched a MEFI.

but something is way wrong, the fan relay should have NOTHING to do with the running of the engine.

a MEFI requires constant 12v and a switched 12v connection.

constant 12v is pin J2 pin 1
switched (key on) 12v is J2 pin 19

definitely check to see the ECU has a solid ground cnnection, there are THREE pins on the MEFI that should go to the engine block for ground, all on J1, pins 13 28 and 29.

the ground trigger for the fans should be from J1 pin 22.

my guess here wuld be the ecu is not grounded properly and/or been tied into the fan ground.
 
fuel pump control (ground) is J1 pin 6.

the fuel pump should turn on briefly when the key is turned on. if that isnt happening, then either you have lost the switched power in, or the relay is bad, or there is a problem with the crank/cam sensors and the ecu is not seeing RPM.
 
ChasingSand said:
Do you have spark?
Click to expand...
daveH said:
been a while since i touched a MEFI.

but something is way wrong, the fan relay should have NOTHING to do with the running of the engine.

a MEFI requires constant 12v and a switched 12v connection.

constant 12v is pin J2 pin 1
switched (key on) 12v is J2 pin 19

definitely check to see the ECU has a solid ground cnnection, there are THREE pins on the MEFI that should go to the engine block for ground, all on J1, pins 13 28 and 29.

the ground trigger for the fans should be from J1 pin 22.

my guess here wuld be the ecu is not grounded properly and/or been tied into the fan ground.
Click to expand...


Do you have spark? have power to coils but not sure on spark

im thinking ive lost ground somewhere , ohm on ground wire at engine shows no resistance
i didnt check constant power but i have switched power to computer{ Blue wire} so im going to double check pin 13 ,28,29 for ground
As suggested im going to run a ground from mounting bolt to chasis
 
daveH said:
fuel pump control (ground) is J1 pin 6.

the fuel pump should turn on briefly when the key is turned on. if that isnt happening, then either you have lost the switched power in, or the relay is bad, or there is a problem with the crank/cam sensors and the ecu is not seeing RPM.
Click to expand...
if i ground relay green wire fuel pump runs constant
 
Desertanddune said:
if i ground relay green wire fuel pump runs constant
Click to expand...
I dont know what color wiring your harness uses, but likely your green wire here is the trigger wire from the ECU, so grounding it would cause the pump to run.

grounding the ECU to the case should never be an issue.

a MEFI will do goofy things, if it doesnt have a constant and switched power input as stated above.

neither the fuel pump nor fan relay wiring should have ANY impact on the ECU running the engine.

I would manually turn on the fuel pump by shorting to ground your green wire, and crank it over. If it doesnt start, your issue has nothing to do with the relays.

again, if a crank or cam sensor was bad, this would do it.

another test would be to pull a plug and see if it actually has spark at crank. if it does not, then the ecu either isnt powering up due to a power or ground problem, or it isnt seeing RPM due to a faulty crank or cam sensor or their associated wiring.
 
Another common problem on this old mefi systems.
First…..your ignition key. Many used a toggle that was two position…..run and start. The switch will go bad and drop the voltage on the run leg during crank, starving the ecu for run power.

Second, many had exposed GM blade fuse blocks. Sand works its way in there and ruins the connections. The fuses will get loose and starve the system for power.
 
daveH said:
Another common problem on this old mefi systems.
First…..your ignition key. Many used a toggle that was two position…..run and start. The switch will go bad and drop the voltage on the run leg during crank, starving the ecu for run power.

Second, many had exposed GM blade fuse blocks. Sand works its way in there and ruins the connections. The fuses will get loose and starve the system for power.
Click to expand...
when i got the car 10 years ago it had the 2 position switch just left it .... 1st power/ fuel pump on, 2nd starter i probed from switch to computer there is power through to computer connection
has 4 blade fuse block, power to and through fuse block at every fuse and all fuses check good
 
daveH said:
I dont know what color wiring your harness uses, but likely your green wire here is the trigger wire from the ECU, so grounding it would cause the pump to run.

grounding the ECU to the case should never be an issue.

a MEFI will do goofy things, if it doesnt have a constant and switched power input as stated above.

neither the fuel pump nor fan relay wiring should have ANY impact on the ECU running the engine.

I would manually turn on the fuel pump by shorting to ground your green wire, and crank it over. If it doesnt start, your issue has nothing to do with the relays.

again, if a crank or cam sensor was bad, this would do it.

another test would be to pull a plug and see if it actually has spark at crank. if it does not, then the ecu either isnt powering up due to a power or ground problem, or it isnt seeing RPM due to a faulty crank or cam sensor or their associated wiring.
Click to expand...
i have ground the FP relay and cranked it, it didn't start
 
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