PerryB

Mach 5,

The body of the DME does not need to be grounded by mounting. All the grounds are through the pins.

I also recently replaced my harness with the on from 944online. Are the female plug ends on the DME harness secure in the clip? Do the male pins on the DME push them back into the clip? Were you able to re-install the "combs"?

Did you also replace the sensors themselves?

Perry

Mach 5

That's good to hear regarding the grounding. Well, good in that I may not have to pay for a rebuilt DME. Bad in that I can probably rule that out as a problem. With respect to the pins, yes the combs are both installed and the female ends do not appear to be backing out at all. The sensors were replaced about two years ago. I checked them with an Ohn meter and they checked out OK. Also, the car was running fine (except for the offending wire) before the harness install, so I am inclined to believe the sensors themselves are OK.

Thanks for the suggestions.

The path that I'm going down now is to check the sensor gap. I'm wondering if the replacement harness somehow presents greater signal resistance (or poorer conductivity...I'm not an electrical engineer!) and now the gaps are too wide to send a strong enough signal to the DME. I know an oscilloscope would probably prove this out, but I don't have one. So I started down the agonizingly difficult task of getting to the bracket adjustment bolts for the sensors so that I can set the gap.

Again, if anyone has any thoughts on this idea, or any others, feel free to chime in.

Thanks.
Roger

Mach 5

After spending several hours trying to get to the sensor adjusting bolts, I'm starting to think getting an oscilloscope may be a less painful path.

Question: Does anyone have any experience with the Hantek 1008C PC based scope? Amazon has it for around $100. Most of the positive reviews seem to be from automotive users.

Thanks.
Roger

PerryB

Mach 5,

If you have a digital volt/ohm meter you can do a cranking AC voltage test on both sensors through the harness at the DME connector. Specialized ECU Repair has a YouTube video of how to do this test. I would do this before spending money on an oscilloscope.

Perry

joes

An easier way to test is use a 1.5 volt LED plug into either the DME or directly into the sensor. crank it over if it lites you are good. make sure the you have the correct polarity as the LED has a positive and negative.

Mach 5

I do have a voltmeter. Don't think I have an LED.

It is weird that I'm not getting tach bounce when I'm trying to start it, yet the fuel pump does run creating pressure at the rail. I thought these were both tied to a reference sensor signal?

These are the times that I miss carburetors and points

Roger

Mach 5

Update: Tested the sensors with a voltmeter. I am only getting .3v from the speed sensor, versus 1+v. I swapped sensors, even swapped plugs, same result. Then I hooked up the old wires and checked voltage, .8v. Still lower than the 1+v that it should be showing, but significantly higher than the new wires. I can't imagine how/why the new wires would be less conductive than the old wires. However, based on this, I'm a bit concerned that even if I go through the "royal b**ch" process of adjusting the gap, I still may not receive a strong enough signal at the DME with these new wires. I've been corresponding with the shop that made the wires and he said he has never heard of this before. I don't doubt that, but it still leaves me f'd for the moment.

Any suggestions?

Roger

JacRyann

Getting variable voltage outputs shows it's not a problem with sensor-gap. Especially since they were not changed between tests.

I had problematic connections on BOTH male connectors on the SENSOR side. I had erratic no starts that were resolved by wiggling the connectors. I narrowed it down by wiggling only one of four connectors. Turned out the harness female side was fine (wiggling those didn't resolve no-start).

I replaced both sensor-side male connectors and had no more issues. Since we can and should assume that your new harness & connectors are functional, test your sensor-side connectors. Do the ohm and voltage tests while someone wiggles the male sensor-side connectors. An oscilloscope trace would clearly show the change in before & after signals.

Mach 5

"Getting variable voltage outputs shows it's not a problem with sensor-gap."

The fact that the voltage is less than 1v with both sets of wires could imply a sensor gap problem. These readings should be over 1v for the speed sensor. The wider the gap, the lower the voltage.

With respect to the connectors, they are new, since they came with the new wiring harness, so that isn't the issue in my case.

I'll adjust the gap on Monday and keep my fingers crossed!

Roger

odonnell

When you redid the wiring, how did the instructions discuss the shielding material around the 2 wires per sensor (the shielding terminates to form the 3rd wire)? I wired mine recently (not from a kit) and it was the only thing keeping it from being straightforward. I can't remember which pins were to which wire but it makes sense that the signal is a voltage between the two internal wires - maybe the shielding is compromised and/or is crossed with one of the internal wires?

Mach 5

Are you referring to the shielding on the ground wires, pins 5 and/or 23? When you hook up the volt meter, you don't use the grounds (just 8/27 or 25/26), so I don't think this would have any impact on why the old wires would produce .8v versus .3v for the new wires.

I appreciate the suggestion.

Roger

Mach 5

I reset the sensor gap today. Still no start. However, speed (DG) voltage readings have increased. I'm now at .5v with the new wires (trying it through both DG and BG wires produces the same result) and over 1v with the old wires (using BG wires to test, since DG is cracked/frayed). I think it's safe to say there is something wrong with the new wires. I guess I'll have them send me another one. What a pain.

Roger

odonnell

Have you measured the resistance of the wires/harness?

txhokie4life

Was in the process of ordering new CPS for my 84 race car.

Rock Auto had some reasonably priced -- but when I went
to check out -- it said it was for european versions only?

Is there such a thing?

Mike

Mach 5

Not yet. I'll do it tonight.

It is strange. I get the lower voltage readings using either sensor, running through either new wiring harness. But either sensor running through one of the old harnesses produces an acceptable voltage. This would imply that the sensors are not at fault, nor is my multi-meter. This isn't really that complicated. The sensor, and some wire. It's got to be the wires. The vendor is sending me another set. I test them before I install them through the firewall.

Roger