Quote:
Originally Posted by Felching
Have you actually done it like I have?
You'll notice most still have the same "diagnostic port" as the diesels, minus the output harness, like the 85 diesel models.
|
Well that pretty much says it there. If it works it works. This is good to know because a lot of 80s gassers have the diagnostic ports and pickups just like the diesels.
I’ve been meaning to use the diagnostic port on my 85 to run the tach on the dash. I held off because I thought I’d need a two pin tach. Now I just need a 2.5mm socket contact for pin 3 of the diagnostic port.
I already have an amp.
EDIT -
Hey @Felching I'm really confused now. I took your word for it that you have "done" this, but can you tell us exactly what you did and what you got working? You took the diagnostic signal off the firewall plug and ran it to an 85 tachometer gauge and you got a signal without replacing the gauge with the older two pin 82 type gauge?
Take a look at the image below. That is fresh from the three pin plug going straight to my gauge in my 1985 300D at idle. My signal comes from the bell housing sensor and presumably it reads the teeth on the ring gear like Diseasal says.
Here is my problem. On my 82 300SD at idle I get a 14Hz square wave as an input to the gauge. If you look below at my scope capture, I'm getting a 1.84KHz square wave. How the heck is an 85 gauge supposed to read the right idle rpm when it gets 14Hz from the diagnostic port than instead of an 1.84KHz wave coming from the bell housing.
Something doesn't click here. What is crazier is this (14Hz : 1.84KHz) is almost exactly a factor of 144 like Diseasal mentions is the number of gear teeth read out on the 1985.
I felt something was wrong when I know guys are bypassing the EGR computer with jumpers and getting their tach to work on 85s. That means the EGR computer is nothing but a pass through for the rpm signal. I guess the sensor is putting out a clean square wave from the get go like a hall sensor instead of the variable reluctance sensor on the balancer? There is literally no electronics between the sensor and the back of the tach in the 1985 car. No tach amp.
Please explain. I don't understand how your "done it" works. My take is the tach gauge in the 85 is a totally different circuit inside...unless it is so smart it can tell that there is a Khz signal coming in instead of a Hz signal and can adapt by itself. In 1985? No way.
