Black51

So last fall I installed VEMS. Since then the car hasn't started. I had the engine out prior to that for some unrelated exhaust/turbo work. Virtually every sensor under the hood was replaced, minus the TPS. The TPS has been tested and is working perfectly though.

So I'm at a loss why the engine won't start. I get no tach bounce whatsoever when cranking. I've re-gapped the reference sensors probably 10 times now, and still no change. I've tested both the sensors' resistance and voltage output with an oscilloscope. So where I'm at with the readings are the sensors's resistance are right in spec; ~900-1000 ohms for both. Both are I think 3 years old and in good visual shape. The voltage output on the other hand, is not so good; ~.8V and .5V. The measurements are supposed to be >2.5V for the speed sensor, and >2.0V for the crank reference sensor. So my output readings are way low.

This is where I'm at a loss. I don't know why they're so low. Is there something in the VEMS software I need to do to make the sensors work properly, or perhaps something else I'm not think of? The reference sensors seem pretty straightforward otherwise. They're just hall sensors putting out a voltage to the ECU. I've visually confirmed both grounds on the bellhousing are bolted down and clean.

gpr8er

didn't someone here just go through this problem? Solution was to use Bosch sensors. What are you using?

Black51

I'm fairly sure they are bosch sensors, but can't confirm. The sensors where changed when i got a new clutch a few years back, but the mechanic didn't say what brand they were. Just the price. They were expensive. So I can only assume they were bosch. If the resistsnce checks out for both of them, and the car has ran with those same sensors in, albeit with a different engine management, why would they be considered bad?

Humboldtgrin

Bad ground? Maybe incorrect set screw height on the flywheel? Maybe broken wire in a 30+ year old wire harness? Sounds like a lot got touched and changed.

DrZ1

I had a similar issue with a stand-alone and the crank signal was recieved interference. Once I used the updated bracket with the sheathing for one of the sensors it fired up.

Hopefully something simple. I chased for quite some time and even went to a custom engine harness before finding.

Porvair

Have you tried swapping back to the stock Motronic/KLR to see if you can get the car to run or at least a tach bounce? That would at least help narrow this down to a VEMS vs another related issue (i.e., speed or reference sensor, other wiring or coil).

I had a fair bit of frustration with VEMS last year (in my case it ran ok with VEMS but would not rev over 4700) and ended up swapping back to the stock units,and also replacing the speed/reference sensor harness as the original was a bit crumbly. That worked fine for a while but I again lost spark a couple of months later at a track event.

After a fair bit of added frustration and testing the signals from the speed and reference sensors with a $99 Amazon oscilloscope I determined that there was virtually no signal from the reference sensor. In my case, it turned out that the flywheel pin that provides the reference sensor signal was damaged, so poor signal and no spark. Replacement flywheel and the equivalent of a clutch replacement in labor later and it is back and running strong. But I have not yet attempted the VEMS reinstallation as I'm not sure if it is worth the effort.

There are a number of threads out there that may be of use, but most are based on the stock ignition and not VEMS. Here is one particularly extensive one.
https://rennlist.com/forums/944-turb...troyed-19.html

Black51

Thanks Tom. The oscilloscope I am using is a very basic scope. It's pretty much just a multi meter with a scope function. So not sure what you're asking about regarding the gains.

I haven't tested the sensors without being connected, so the next time I have a helper I'll test that out.

For anyone with experience with VEMS, is there anything that needs to be done in the triggering out of the box?

Black51

It does have a scope that works pretty well for diagnosing stuff... like reference sensors.

So I just tested the unplugged sensors with a helper. One read as high as ~.7V momentarily, then settled at ~.5V. The other was ~.1V. Way low. Thanks for the suggestion. I never thought to isolate just the sensors to see if they were bad. And no, I was getting 0 tach bounce. None. So it sounds like I'll be ordering up some new sensors. At least I'm in practice in putting them in and taking them out.

Anyways, I'll get some news sensors put in and report back.

Black51

I see the waveform as well as the number on the screen. It shows both. The voltage spikes to ~.7V for just a moment as the starter energizes, and then I get a steady ~.5V as the engine is turning over.

I'm not an expert on electrical stuff. Should I be testing in AC voltage, or DC voltage on my meter? I tried testing DC voltage and didn't really get anything. I did get the described readings in AC voltage setting on my meter. Also, which pins exactly should I be attaching the clips to? I tried the 3 combinations for each 3 pin connectors.

Tom M'Guinn

Without knowing your meter, it's hard to say what settings to use. Typical voltage readings on a multimeter won't work to test these sensors' output. Not sure what you mean by "steady .5v" since you are looking for a sine wave on the speed sensor and EKG-like pulses on the ref sensor? The output from the sensors comes from the middle pin and one of the outter pins. If you find the two pins that have 800 to 1000 ohms of resistance between them, those are the ones.

gruhsy

Check that the pins on your engine harness for the reference sensors are seated properly in the connector. I had that issue on a new harness. The locking tab had not engaged. I managed to push them back into place and lock them, then the car started right up.

Black51

The meter is a Banggood MUSTOOL MT8206. It does has both regular multi meter functions as well as an oscilloscope function. I guess I'm not describing what I was seeing right... Yes, I see the sine wave for the speed sensor, and an EKG-like pulse for the ref sensor. But the voltage reading is too low. From peak-to-peak, it's only .5V. By saying "steady .5V", I just meant that the voltage wasn't fluctuating all over the place. And that was even after hooking directly up to the sensor connectors.

I'll double check to make sure the pins are seated properly.

ealoken

have you changed the way the fuelpump works? Or do VEMS handle that?

Tom M'Guinn

I see. I'm just not sure I'd trust a $50 multimeter/scope thing. For them both to actually be that low, the gap would have to be pretty far out and/or they'd both have to be suffering from the same fault. Seems more likely that it's the meter, but hard to know for sure without testing them on a real scope.

Humboldtgrin

Maybe racerboy could chime in with some help? If not too busy and is willing. Not sure who all knows VEMS very well (I’m sure quite a few) but I know racerboy has delt with it a few times. And that’s what I call an understatement.