89 635 csi no start

[bunty635]

Hello all,

need your help with no- start

my 89 635csi one fine day decided not to start,

occasionally i would fire up for a sec and then died.

First i suspected fuel as i couldn't hear the fuel pumps.
so i jumped the pins on the fuel pump relay ... fuels pumps are fine , pressure seems ok.
replaced fuel pump relay and main relays still no start.


what should be my next step..

[hornhospital]

Without the relay(s) being jumped, does it have fuel pressure at the rail? The crank sensor (either one) could be bad. The ECU won't fire up the fuel pump if it doesn't "see" the crank turning.

[bunty635]

Great direction thanks ill check it.

[bunty635]

crank sensor was covered in oil smudge,

bench tested it, came out 562 Ohms

replace it ?

[bunty635]

Replaced crank sensor from the yard

Replaced good used ecu

Replaced all spark plug wires

New coolant temp sensor.


replaced main and fuel relay with used ones from e30.


What else am i missing here

[hornhospital]

Test the crank sensor. I've seen too many of them from the wrecking yard that were DOA.

The main and fuel pump relays from an E30 may not be compatible with the E24.

[bunty635]

Thanks hornhospital !

Will do that

[bunty635]

Car fired up when i jumped both main and fuel relay.

What would that point out ?

[baders]

What happens when you jump the main relay by itself ?
What happens when you jump the fuel relay by itself ?
Is the E30 relay the correct type ?
Do you have a year specific Electrical Troubleshooting Manual ?
Do you have a multimeter ?

[bunty635]

What happens when you jump the main relay by itself ?
What happens when you jump the fuel relay by itself ?
Is the E30 relay the correct type ?
Do you have a year specific Electrical Troubleshooting Manual ?
Do you have a multimeter ?

new finding car runs when jumping relay by itself.
fuel relay seems fine.

i have put my multimeter to use, there's voltage at pin 86 on the main relay.

[Radical Racing]

I had what seems like the same problem with my '88 635 Csi - starts for a second and then dies. It turned out to be microscopic cracks in some solder joints inside the ECU. There is tech article that describes how to remove and open up the ECU and points out the three solder joints attached to a aluminum heat sink inside. I touched each joint for just 1 - 2 seconds with a fine tip Hakko soldering iron. Reinstalled the ECU and after months of frustration it started immediately.