External temp display help - PeachParts Mercedes-Benz Forum

bgkast · #1 03-06-2009, 01:57 AM

I am trying to retrofit an external temp display into my SD. I got the display and probe from a member here and pulled the wiring harness and display-compatible gauge housing from a junk yard car.

The problem I am having is that the unit only seems to power up ever 4 to 5 times I try it, and when it does power up it displays + 88.8*C most of the time with an occasional -27.0* or -37.0*. I think I have got it to display the correct temperature once. The back light seems to work fine. I have been testing it on the bench with a car battery as a power supply.

I have verified that my wiring harness does not have any breaks in it, and have tried re-soldering the unit with no success.

Is the unit dead or do I have another problem?

Thanks

pawoSD · #2 03-06-2009, 09:36 AM

Nobody has any idea how these work??

DeliveryValve · #3 03-06-2009, 11:46 AM

biodiesel300TD knows.... He installed one in his 240D. Slick setup.

I am fixin to do the same to my 300D. I got the display, but don't have the other stuff.

bgkast · #4 03-10-2009, 02:51 AM

Bump.

milner351 · #5 03-10-2009, 11:53 AM

this seems to be the one system on my car that works fine!

have you tried a few different types of flux while soldering? I found when rebuilding the wiring harness that sometimes one flux worked better than another for cleaning things and getting the solder to adhere properly.

bgkast · #6 03-10-2009, 03:49 PM

When I re-soldered the unit I only heated up each joint to repair any cracks that may have formed, I did not add any solder.

Can anybody that has retrofitted one of these try unplugging the sensor and report back what the unit displays?

bgkast · #7 03-12-2009, 09:32 PM

Bump!

Jeremy5848 · #8 03-12-2009, 10:37 PM

I tried that once . . . I simulated a sensor with a resistor substitution box in order to bench-test a display . . . as I recall (poorly), with a very high resistance (equivalent to a missing sensor) the display read minus a whole bunch, like -30F or -40F or something like that. As I decreased the resistance, the display read higher and higher until, at a few hundred ohms or so, it read 70F or something like that. So, if your display thinks that it's mighty cold, you could have a bad sensor.

Jeremy

pawoSD · #9 03-12-2009, 10:52 PM

There's no guarantee that the sensor survived removal from the car....it was 15F out and brittle.....and I had a hard time getting it out. I only pulled it for the heck of it. Perhaps its damaged....or was already not working right.

babymog · #10 03-12-2009, 10:55 PM

If you have power and ground, it should power up, read (IIRC) -40c or -40F (same temp in F or C, little trivia for ya) with an open-circuit sensor. The light and temp-sensor return are the other two pins.

I have a 126 display around here somewhere, I'll power it up tomorrow and verify for you. PM or email me if I forget.