Continuity test doubt

[Ram__46]

Hi everyone. I have a doubt. If I have to test a ground pin of a 5V connector (suspect bad ground), to do a continuity test is it necessary to touch with the multimeter probe any point of exposed metal of the chassis? Or do I have to touch exactly and directly the negative pole of the battery?
Is it the same thing?
Thank you for help

 

[DPG]

What application is this?
Is the chassis definitely supposed to be connected to 0V?

 

[Ram__46]

Hi and thanks for your reply, it's a 5V (2 pin) connector of an exhaust gas temp sensor in a car. I suppose I have a bad ground wire, but I can't reach the ground collecting the wires of that sensor (and many other sensors) because it's located under the dashboard (hours of labor to reach it). I read 4.963V between the pins, but the ground pin is at 0.037V with respect to a metallic part in the engine.

Instead, from the other pin of the connector to the same metallic part of the engine I have 5.00V. So the ecu sends perfectly the 5V, but the ground it's not perfectly at zero.

The question is, is the ground wire a bad ground? I'm a novice in electrical repairs
For comparing, I checked another EGT sensor connector, and tension at the pins is 4.995V, quite perfectly 5V (I suppose also within the multimeter error sensitivity (it's a cheap multimeter)).

 

[DPG]

Yes, those very slight differences are to be expected. It may be worth you try to connect a temporary wire from ground to the pin which you know should be grounded.

 

[Ram__46]

That's what I wanted to do. Create a new ground wire and get it to another ground in the engine bay. If the sensor starts working properly again, I know the answer. But could a bad ground wire give that 0.037V value? I really don't get it thank you

The other sensor working properly had 4.995V at the connector. So I suppose that at the pins there must be quite exactly 5.00V and not 4.96 or so.

 

[DPG]

Rather than test voltage, what you need to do is a resistance measurement between the ground of the sensor connector, and a known ground. Set your meter to resistance/Ohms.

 

[Ram__46]

I did it but I don't remember the value. I remember I read around 3.6 kOhm (probably) but it seemed too low to me and strange, I'll repeat the measure. What range of values should I expect?

 

[DPG]

I did it but I don't remember the value. I remember I read around 3.6 kOhm (probably) but it seemed too low to me and strange, I'll repeat the measure. What range of values should I expect?

If the wire should definitely be grounded, then 3.6k is far too high. Depending on meter quality, lead resistance, length of cable etc I would expect around 1 Ohm. Again, don't worry about slight variations.

 

[Ram__46]

I'll let you know, thank you

 

[Ram__46]

Minutes ago I did the attempt. Cold engine. Diagnosis told 130 °C for that faulty sensor. I grounded it to another ground in the engine bay and I immediately got plausible results: 31°C while other sensors were at 29 °C. I think I found the problem. Now I have to detach wires from the connector but I don't know how to do it properly. Thanks again

 

[DPG]

Minutes ago I did the attempt. Cold engine. Diagnosis told 130 °C for that faulty sensor. I grounded it to another ground in the engine bay and I immediately got plausible results: 31°C while other sensors were at 29 °C. I think I found the problem. Now I have to detach wires from the connector but I don't know how to do it properly. Thanks again

Excellent! Post a photo of the connectors. Maybe we can see how they release.

 

[Ram__46]

Here are some pics. It seems that the white/yellow part (where wires get into) can be removed pressing that pin. I tried with a screwdriver but even with force it didn't move ?. I have a set of tools to remove wires from connectors, but I never did electrical repairs before

If you have ideas I'd be glad and grateful

 

[DPG]

I think you're right with pressing those side bits in. Maybe something more suitable than a screwdriver though.

 

[Ram__46]

Perfect. I'll try. But then, how to I detach the wires from that yellow connector?

 

[DPG]

Perfect. I'll try. But then, how to I detach the wires from that yellow connector?

Mmm, not sure. I think I'd be having a Google search if I were you. Bound to be something on the web.

 

[Ram__46]

It's a strange connector. Mercedes makes all things hard to repair diy. Engineered to go to a garage ?
Thanks again

 

[Ram__46]

Hi, I'm looking for an eyelet connector to put the new wire to ground. It's scarce in my city (not to be found), especially if I want a quality set. Do you think it's important to put certified connectors or it's enough using cheap chinese ones? I'm no expert about it and I'd like to avoid my car being on fire Thank you

 

[DPG]

Hi, I'm looking for an eyelet connector to put the new wire to ground. It's scarce in my city (not to be found), especially if I want a quality set. Do you think it's important to put certified connectors or it's enough using cheap chinese ones? I'm no expert about it and I'd like to avoid my car being on fire Thank you

To be honest, as long as it is solid and well crimped I would say you would be fine. Make sure you do a 'tug-test' to make sure the wire is firmly held.

Fire is not an issue in this case as the current is very low, but you don't want a bad connection giving you reliability problems.

 

[Ram__46]

Thank you a lot. I don't know what a tug-test is but I'll make sure is firmly crimped.

 

[DPG]

Thank you a lot. I don't know what a tug-test is but I'll make sure is firmly crimped.

Ah, yes that's a UK term sorry!

Try and pull the wire out of the crimp. It should be virtually impossible.

 

[static zap]

yes that's a UK term sorry!

We get in trouble with a Good yank , or an impulsive jerk !

(if you are building the skill with a crimp , allow for extra crimps
and spare cable and destruction test a few samples ..
( I completely distrust cheap crimp tools now )

 

[Ram__46]

Thank you. In fact today I tried crimping a few times with random wires just to try and the result was quite good. I only have a wire stripper that is also a crimper, so it won't be a professional result but who cares. I didn't find eyelet connectors of the perfect size but it doesn't matter. Tomorrow I'll do the repair

 

[DPG]

Thank you. In fact today I tried crimping a few times with random wires just to try and the result was quite good. I only have a wire stripper that is also a crimper, so it won't be a professional result but who cares. I didn't find eyelet connectors of the perfect size but it doesn't matter. Tomorrow I'll do the repair

Let us know how it goes.

 

[Ram__46]

Sure, I hope the CDI (it's a diesel) ecu will come back to working order. Thanks for help

 

[Ram__46]

Do you think it could be a good idea to sand the free ground point (at least I suppose it's a ground point) until it's bare metal? The already occupied one seems to have little room for a new connector. Thanks again

 

[DPG]

Do you think it could be a good idea to sand the free ground point (at least I suppose it's a ground point) until it's bare metal? The already occupied one seems to have little room for a new connector. Thanks again

Wouldn't do any harm yes. Cover it with a bit of grease when you've bolted the terminal up.

 

[Ram__46]

Wouldn't do any harm yes. Cover it with a bit of grease when you've bolted the terminal up.

Lithium spray grease? Like in car batteries connectors? ?

 

[DPG]

Lithium spray grease? Like in car batteries connectors? ?

To be honest any grease will be OK. Just to prevent any corrosion.

 

[Ram__46]

I just cut the ground connector's wire and the conductor is damaged. I don't know if it was the wire stripper or it was already like this. Do I have to cut in Another point or can I continue like this even with a reduced copper wire section? Thank you

 

[DPG]

I just cut the ground connector's wire and the conductor is damaged. I don't know if it was the wire stripper or it was already like this. Do I have to cut in Another point or can I continue like this even with a reduced copper wire section? Thank you

I'd try and cut back a bit further if there's enough spare. Not critical though.

 

[Ram__46]

Repair done. It seems to work fine. Now I have 4.997V at the connector, so it's a proper ground. And from diagnosis I read exactly the value of the sensor (translated from Ohm to Celsius obviously). Thank you for help guys. If anyone is interested I'll post some pics

 

[DPG]

Repair done. It seems to work fine. Now I have 4.997V at the connector, so it's a proper ground. And from diagnosis I read exactly the value of the sensor (translated from Ohm to Celsius obviously). Thank you for help guys. If anyone is interested I'll post some pics

Great stuff! All the best, and send a photo yes.

 

[Ram__46]

Here some pics. I jointed the wires making a braid and folding it on itself. Than I covered with shrinking tube and tape. If you carefully watch the engine bay you see on the left the new ground with the blue connector, and if you follow the wire you see the 1.5 meters path back to the connector. Hope it will last, seems solid.