• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

740 Tacho needle bouncing around like crazy, car dies

beepee

beepee

how hard can it be?
Joined
Sep 25, 2008
Location
Southern MD
Ok, so it seems like no matter how much love I give LH2.4 it hates me in return. :grrr:

Car is a 1990 740 Turbo w/15g and Mike's chips. I have been tweaking the fuel tables but this shouldn't be related as it happens on factory firmware as well.

When driving suddenly the ignition will cut out, tacho will drop to 0. Sometimes it will only do it briefly and the car will continue running, sometimes it will "fritz out" for several seconds and the needle will go bonkers.

Sometimes I can put it in neutral and restart, sometimes I end up coasting to the shoulder and playing with it to get it to start again. Sometimes it will start right back up, sometimes it will start rough, backfire, and generally misbehave until it dies.

So far this has only happened when the car is warm.

Throttle input seems to make no difference. My TPS is set correctly because I can see the overrun fuel cut happen on the WB02 when I coast for a few seconds.

I would normally suspect the usual ignition components (cap, rotor, coil, etc) except for the fact that at least two or three times after cranking has stopped the needle has continued to bounce around on its own, sometimes even pegging out at 7k briefly. :wtf:

I have swapped EZK and LH computers with no difference. The only thing that makes sense to me is the crank sensor. Is this correct or am I barking up the wrong tree?
 
gsellstr said:
Crank sensor is toast.
Click to expand...

So, according to the maintenance logs that the old guy who sold it to me the crank sensor has already been replaced once. I didn't think that they were a wear item. Venture a guess as to why they would they keep failing?

This couldn't be related to my transmission (auto) which has recently started to slip... could it? The trans wasn't misbehaving at the time of these incidents or I would have mentioned it...
 
Crank sensor, most often this behavior due to a worn cable harness and torn ripped insulation, sensor itself is probably OK. Look at the cable, if it has a yellow band on it, get a new one. At one time the earlier run of sensors had a yellow band on the cable and had poor insulation, these were recalled and replaced with a white banded cable which last a long time (except aftermarket ones).
 
vvpete said:
Crank sensor, most often this behavior due to a worn cable harness and torn ripped insulation, sensor itself is probably OK. Look at the cable, if it has a yellow band on it, get a new one. At one time the earlier run of sensors had a yellow band on the cable and had poor insulation, these were recalled and replaced with a white banded cable which last a long time (except aftermarket ones).
Click to expand...

Thanks for this. That hike to CT was going to be a pain in the ass :lol:

I'll take a look when I get home from work. It would be great if it was just the cable. :nod:
 
Bosch P/N 0 986 280 401, Crank sensor: Device used to make driver cranky.


Ok, update. Bought a new crank sensor, installed, car was fine for 1.5 days.

Get up this morning, start car to warm up in garage (about mid 30s outside), start drive to work and about 15 minutes in to my trip it just loses rpm flat out. No warning, nothing, just dies.

Luckily I was near a gas station so I sat in the parking lot for a bit wiggling wires, resetting computers, etc. because at this point I am convinced that it is not the crank sensor.

I tried:
- disconnecting battery
- disconnect/reconnect crank sensor from harness
- disconnect/reconnect spade terminals on coil
- disconnect/reconnect EZK
- swap EZK with non-chipped working one (my spare)
- re-load fuel ECU with factory firmware (it was working fine the whole time)

Typically while cranking the tach needle would bounce a little every 2-3 seconds.

After letting the car sit for approx 10 minutes, it randomly started and idled perfectly, only to quit about 45 seconds later.

After checking everything again (about 15 minutes later) I swapped out the new crank sensor for the old one I kept in my trunk. Car started immediately, drove the remaining 10 minutes to work without incident.

I think that it's worth mentioning that when I pulled the new sensor today it looked just fine and wasn't unusually hot or anything. No metal filings stuck to the tip or anything like that either.

So I am nearly at my wits end with this ****. :help:
 
Next guess would be the power stage or the coil. Either could have a temperature-based failure, either will affect how the tach reacts. Try either one first, both are easy enough to swap out. Ohm out the coil hot as well, as that may clue you in if there's an open in it.
 
gsellstr said:
Next guess would be the power stage or the coil. Either could have a temperature-based failure, either will affect how the tach reacts. Try either one first, both are easy enough to swap out. Ohm out the coil hot as well, as that may clue you in if there's an open in it.
Click to expand...

Thanks for the info.

That reminds me, I forgot to ask... is the tachometer wired directly to the EZK? Or does it get it's signal from somewhere in the power stage?

It would make a lot more sense if the got it's signal from the igniter. Might be time to hack my spare BIP373 into this mess... or make a Crazy Ray's run...
 
I know my 2.2 car get's its signal from the EZK box, I had the wiring on my hall effect sensor start going bad and it did just what you're describing, though it was during the bad wiring years so ymmv
 
WindowsBreakerG4 said:
I know my 2.2 car get's its signal from the EZK box, I had the wiring on my hall effect sensor start going bad and it did just what you're describing, though it was during the bad wiring years so ymmv
Click to expand...

Actually just had a talk with our office janitor who "used to be a mechanic" and he actually had a pretty good idea: could the flywheel be warping when it heats up? I haven't really checked it, but has it happened to anyone else?

I'm still going to try the power stage first; will be much easier and seems much more likely to me...
 
The signal for the tach comes from the negative side of the coil. Try the power stage first. It should be mounted to the inner fender with heat sink compound, to help dissipate heat. You can get heat sink compound at Radio Shack or an electronics parts supplier.
 
reseat the connection on the powerstage and start going over your grounds. My 940 has a somewhat similar issue that I'm pretty sure is ground related.
 
Slightly more accurate...the tach gets a piggybacked inductive spike from the coil by way of the ezk and power stage. With no coil or something like an MSD you'll have no tach signal, even though the ezk and power stage are still 'firing' the coil. If you get an open or short in the coil you'll no longer get the needed inductive spike to fire the coil, and there's the obvious lack of spark at the same time. Along those same lines, if the power stage fails to fire the coil, you'll also have no spark and no tach.

I wouldn't really consider the flex plate to be an issue. Warpage is VERY seldom an issue, and if it's warping that bad, you've got bigger problems and probably some associated noise with it hitting other parts.
 
You must log in or register to reply here.
Back
Top