Need tachometer help on 85 300D

[Diesel Giant]

Ok got an 85 300 D with a non functioning tach. Here is what I have done so far:

1) I have tried another gauge with the same results.
2) Checked the EGR relay fuse and its good
3) Replaced the tach pick up in the bellhousing with a new one
4) The 85 Models DON'T have the tach amp on the fender, so that does not apply
5) Looked for anything obviously wrong with wiring, nothing.
6) All other gauges work just fine

On this year model if the tach does not work, then the klima relay won't engage the A/C compressor, which of course it does not.

What am I missing??

[lietuviai]

My A/C would work regardless if the tach was working or not. Check to see if your kick-down switch is working. I found that mine was part of my tach/tranny problem.

[InSaNeBoY]

Originally Posted by lietuviai

My A/C would work regardless if the tach was working or not. Check to see if your kick-down switch is working. I found that mine was part of my tach/tranny problem.

intersting... the 300D I test drove sunday had a problem with the tach, and it never kicked down either... hummm...

[Diesel Giant]

Ok I got the tach working. The EGR relay was bad. Still cant get power to the compressor feed. I think the Klima is bad.

Anyone know how to test it properly?

[AMH]

Was the a/c compressor operating prior to the tach failure? Before you start throwing parts at the problem, check the basics first. Do you have any freon in the system? Pressure readings, even with the compressor disengaged? Jumped the terminals at the drier pressure switch? Do you have voltage at the pressure switch when the correct button in depressed on the PB unit?

[Diesel Giant]

System is low on freon. I jumped the clutch with 12 volts and it works. Then I jumped the low pressure switch and checked for voltage at compressor feed wire circuit with climate control on and there is none.

Checked for voltage where the Klima is plugged in and I have 12v. Then I took apart the Klima and found the coil was not pulling the switch. Jumped the coil and I get power to the compressor feed wire. BUT, these Klimas are weird and EXPENSIVE birds, and there could be some other "feed" into the Klima that is not getting there. So before I get rid of it I need a procedure to test the Klima itself to verify its bad.

[Diesel Giant]

The main problem was the evaporator temp sensor. The Klima was fine.

[JimmyL]

Originally Posted by Diesel Giant

The main problem was the evaporator temp sensor. The Klima was fine.

How did you find that? Thanks for following up on your original post. Very helpful for the future.

[AMH]

Originally Posted by JimmyL

How did you find that? Thanks for following up on your original post. Very helpful for the future.

The evaporator temp sensor is mounted on the evaporator case. Two wires each with a spade connector, don't have the wiring diagram in front of me to look up the color codes. It is a square box, each side about 1", wrapped in felt. A long probe comes off the sensor, and into the evaporator fins. This senses evaporator temperature, and prevents ice up. If the sensor detects icing, no power is fed to the klima, compressor, etc. etc.