'84 No Start - Digging Deeper

[Dale Soelter]

'84 cab has always started on the first tick of the key. Parked it last Friday, and tried to start it on Monday - cranks and cranks, but no go.

What I've done so far:

Swapped out DME/Fuel relay with my spare - no change
Relay in place is dated 2002, it replaced the original which is spare

Tested relays according to a post found here - both look OK and check out electrically

Pulled a plug and tested for spark - no spark

No smell of fuel, either from exhaust or at open cylinder while checking spark

Called John Walker, he mentioned flywheel sensors.

Tested sensors - Reference checks at 1020 between pins 1 & 2, and OL on others
Speed is OL on all pins, but should be the same as Ref on 1 & 2 ?

Can bad speed sensor cause a no start? Reading Bentley, while reference sensor will cause a no start, symptoms of a bad speed sensor are poor running, and until no start, car always ran perfectly.

Pulled wheel, looked, and decided that if they are bad, I didn't want to replace the sensors myself. Fat arms and tight access.

Before we tow to John's, any other things to test? Any way to jumper around the sensors to drive the car to shop?

Thanks DS

[ChkbookMechanic]

Have you checked the fuses? I had this same problem recently and it was the fuel pump fuse that blew. I found out that if you're checking a fuse, pull it out of the socket and examine it because it wasn't until I really looked at the fuse did I see where it was broken.

[Dale Soelter]

Yes, did check the fuses, first thing, but will give them a closer look.

[Tim L]

It looks like you found your problem; they’re not that hard to change if they’re not corroded into the holder.

[rick-l]

Originally Posted by dale soelter

speed is ol on all pins, but should be the same as ref on 1 & 2 ?

yes - OL as in open

Originally Posted by tim l

it looks like you found your problem; they’re not that hard to change if they’re not corroded into the holder.

+1

[Dale Soelter]

"It looks like you found your problem; they’re not that hard to change if they’re not corroded into the holder."

Maybe, but from what the Bentley manual seems to say, a bad reference sensor will cause a no start, but a bad speed sensor only causes poor running conditions. Unless the position of the connections in the engine compartment is reversed, I'm checking good on the reference sensor, and possibly bad on the speed sensor. I've owned the car for twenty five years, and know that the sensors haven't been moved from their factory position in that time. They still have the little DG and BG marking tapes attached and those appear correct for the position of the sensors.

Question that remains: can a bad speed sensor cause a no start condition? If so, yes, it appears we have found the problem. If not, would a test of the ECU be the next logical step? Anyone know of a shop in the Seattle area that could do this?

Still open to other possibilities...

Thanks!
DFS

[rick-l]

Not knowing exactly the software in the DME I can only speculate but.....
The ref sender (one pulse per rev) would tell the location of TDC and the speed sensor would tell you when to fire the plugs (number of teeth from TDC) and the speed of the engine. If it didn't have that I don't see how it would run.

I won't make things worse to change it if you meter readings are correct.

EDIT: Mark the wires before you unplug them unless you can figure what BG and DG (?) mean.

[KCTarga3.2]

Are you sure you have gas in it? Sounds stupid, but this happened to me last week. Sometimes the fuel level sender gets stuck and you don't notice you have been at half a tank for a couple weeks. Good luck.

[john walker's workshop]

the sensors are opposite of how the plugs are mounted. bottom plug=top sensor. leave the bracket in place.