kombatrok

I bought an 86 turbo a while ago, got it cheap knowing it had some issues. It ran great though and the issues were electric. I sorted most of the issues out and in the process discovered that there had clearly been an electrical fire at some point under the fuse box.

Fast forward a month or two. I started having issues with the car starting in the mornings or after it had been sitting for a while after rain or it got wet. Found a poor repair in the battery tray, water intrusion etc... Fixed that. Continued to have morning starting issues and occasionally when the car did start if would idle perfectly but die as soon as I touched the throttle. Seemed like a no spark issue when it wouldn't start. Swapped DME/KLR for a working one from a member on here and still had issues.

It got worse and eventually would not start at all. So I put it in the garage and did this.

Replaced engine wiring harness with a "good" used one
New DME temp sensor
Speed and reference sensors
Cleaned/rebuilt injectors
New FPR
Known good DME relay

Now it still won't start. Has no spark and crazy tach bounce. If I swap the plugs for the S/R sensors it changes as seen below.





I'm stumped as to what would be causing that. Either the DME is getting a bad signal from the S/R sensors or both the DMEs I have are bad... or something else? Is there any wiring other than the engine harness that will affect the car starting and running?

Any advice for troubleshooting?

Thank you!

AScholtes

you didnt get the S/R sensors reversed did you?

kombatrok

The two videos are showing the S/R sensors both ways... After the first video I switched the plugs.

AScholtes

odd....

Clown

How did you replace the S and R sensors? Did you remove the whole bracket and then replace? Or did you just undo the screw on top of the sensor and pull out? These things are pretty touchy. They have to be gapped properly with a small margin for error. If you loosened the bracket which holds both sensors then they will need to be re-gapped.

kombatrok

All I did was remove the hold down bolt and slide the sensors out. Didn't move the bracket at all.

When adjusting the sensors what is a good thing to use to set the gap. I'm having trouble finding something the right thickness to put on the bottom of an old sensor to adjust it with?

Clown

Clarks garage gives the measurements. I just cant remember. I took and old box of washers out and started measuring them with micro meter until I found one. Then super glued it to bottom of sensor. It's a small space to work in with everything in the car.

YZ250

I've had a lot of crazy tach bounce like you show in the video. On my stock 86 951, the crazy tach bounce is a dead giveaway that the DME is getting low voltages and confusing it. The solution in my case was to clean the engine ground under the manifold and the ground at the kill switch (race car). To be safe, I cleaned every ground on the car.

If the battery gets low and into a certain voltage range, it can also confuse the DME and cause the crazy tach bounce. After charging the battery, if the crazy tach bounce continues, then disconnect the battery to reset the DME.

kombatrok

I'll clean the grounds again. I cleaned them when I had the intake off. but I will make sure and add a couple ground straps of the front of the engine as well.

Ronin-951

Sounds like multible problems. Have to ask, ... Factory Alarm? Touching the throttle takes the TPS off the Idle microswitch so you might want to go through that system too. Confirm that the Coil is getting 12 volts and a ground pulse when cranking by hooking up a 12 volt test light across the coil primary terminals, it should flash On and Off, - at least you will know which half of the ignition circuit to go after.

kombatrok

Sorry not to check back sooner. Been busy with work.

The factory alarm was bypassed correctly when I bought the car.

The coil has steady +12v with the ignition on. I will try to find a test light to check the ground pulse. To check it I would just put one lead to each side of the coil right? +&-?

I also just had a new set of ground cables made up and will put them on this week and add an additional ground strap from the block to the frame.

What I can't figure out is why I'm getting wild tach bounce like shown in the videos above. Seems to suggest a DME/sensor issue? Maybe ground?

kombatrok

Bringing back a thread I havent updated in a while... I just got the chance to do some more work on the car.

Replaced the battery ground with a duplicate of the ones Iceshark made and added a engine ground at the front of the block. Made sure the battery was charged fully and tried cranking it again. The crazy bouncing tach went away... now it bounces once, just slightly like it should then stops. Tried it with the S/R sensors plugged in both ways, no difference.

This is what it looks like when cranking now


Where can I get a test light or can I make one?

I will adjust the gap on the S/R sensors this weekend, other then that does anyone have an idea?

Thank you!

AScholtes

You should get an immediate bounce as soon as the key is turned. I think you are not getting a response from the reference sensor...

Bri Bro

If you want to look at the signals correctly, get a scope. The one below is $60 without case and the scope probe is $20. It will show you the amplitude and frequency of the sensors, coil, injectors etc.
http://www.dpcav.com/xcart/home.php?cat=254

I know the tach doesn't look right but what is the fuel pressure?

kombatrok

I get an immediate single bounce as soon as I turn the key to start. After that nothing...

I just bought the scope. Is there a good guide on how to use it to check the sensors? I've never used one before.

The fuel pressure should be good, It's a new FPR and I can hear fuel at the rail when I jumper power for the fuel pump at the DME relay. I will pick up a gauge just in case though.