Background Info
- working on a 1989 GMC Jimmy (full size)
- engine is a 5.7L (i.e. 350 cc) V8 that is a crate engine from "Blue Print" engines (
blueprintengines.com/products/blueprint-...ron-heads-roller-cam
)
- resistance/ohm reading of the coolant sending unit for the gauge as it sat on the bench was about 2500 ohms
- the "Blue Print" engine had the coolant sending unit for gauge (which is physically different and in a different location then the coolant sending unit for the ECU) from the previous engine that was in the truck
The Problem
- KOEO - the temp gauge's needle will peg all the way to the right (i.e. way beyond 260 degrees F)
- KOER - the temp gauge's needle stays pegged all the way to the right
- with KOER, the voltage in the coolant sending unit circuit fluctuates between 8 - 10 V as the needle stays pegged beyond the 260 degree F mark
The diagnostic portion of this job so far:
- we have ordered 4 temperature gauge coolant sending units. one lead of the volt meter is placed on the blade of the sending unit while the second lead is place on the threading of the sending unit. This gives the following results:
1) the first sending unit has a 2500 ohms resistance reading as it sits on the bench;
2) the second sending unit for the gauge has a 2900 ohms resistance reading as it sits on the bench
3 & 4) the 3rd and 4th sending units simply reads "OL" when checking ohms
- there is good power and ground at the gauge as measure at the cluster where the coolant temp gauge is mounted
- the sending unit wire has no voltage drops and the old school test of using an incandescent bulb yields a nice bright bulb when using the coolant sending unit wire as a ground source
- KOEO, battery voltage is 12V and the power source to the water temperature gauge (i.e. power in the cluster as measure at pin that provides power to the coolant temp gauge) is 11.2 V
Here are my questions based on the above
- I am assuming that the sending unit that I would be screwing into the block needs to have an ohm reading on the bench vs the two sending units that simply read "OL". I guess these "OL" sending units where a Monday production... i.e. fit for the pit?
- as noted above, I have a 0.8 V drop in the instrument cluster vs the battery voltage. Did GM engineers want this lower voltage in the cluster, or am I indeed dealing with a voltage drop issue?
- if I can't get this gauge to work, should I just be reducing the voltage in the coolant gauge's sending unit wire by trying different resistors until I get the desired reading from the coolant temperature gauge?
As always - thoughts appreciate and thanks in advance.
