Erratic Oil and Temp Sensor

[Jon_Winningham]

What would cause the temp sensor on a 1993 NAS D110 to all of a sudden read normal operating temperature and then cold? The engine is warmed up and the needle on the temp gauge wanders about from cold to normal operating temperature?

The oil pressure gauge is not reading any oil pressure when the engine is running, but before I start it (position II) that gauge reads the highest reading on the gauge. Once the engine is started the oil pressure gauge reads 0 and the oil pressure light is on. There is oil pressure because the engine is running fine. The battery light that sits next to the oil pressure light in the interment cluster is not on, but very dim.

I think that all these symptoms are related; any help is much appreciated.

[Monkeyboy]

Sounds alot like a bad ground behind the dash, Jon.

[Jon_Winningham]

What are the symtoms of a bad oil pressure sensor or bad temp sensor produce.

[Jon_Winningham]

I have replaced both the Temp sender and oil senders. I replaced the sender to the light and the gauge on the oil. I still have an erratic temp and no oil pressure showing on the gauge. The oil pressure has been tested and has the correct pressure. All grounds checked and are good. Could this be a problem the signal conditioner????

[ECR]

The signal conditioner could go bad (you shouldn't have the dam thing in there anyway), but I've never seen one jump around. That is usually a bad ground or loose wire.
I agree with Keith it sounds like a bad ground, especially if you have "dim" lights. Thats an easy bad ground indicator.
Check for voltage and good ground at the back of the oil gauge. Then ground the sending unit wire. The gauge should pin. If it doesn't you know it is the gauge. If it does it is likely the sender.

[Roadsiderob]

On a NAS 110 the ground circuit goes through the battery cut off switch. I have seen a bad cutoff switch cause problems such as this. To diagnose, pull the cables off of the switch, clean them and connect them together to remove the battery cutoff switch from the circuit.

[Jon_Winningham]

I opened up the dash and pulled the signal conditioner and one of the circuits was burnt!!!!!!!!!!!!!!!!!!!!!!

I bypassed the now removed signal conditioner, but the temp and oil gauge are both barley reading anything at all.

Thanks to everyone so far that has helped!!!!!!!!!!

Just to make sure... all black wires are ground right?

Any ideas on the gauges would be nice.

My next step it to replace the temp gauge with a VDO or LR. I think that the temp gauge may have been adversely affected when that circuit burned.

[Jon_Winningham]

I just put in a '94 D90 sending unit and a '94 temp gauge in my D110. I also removed the signal conditioner per ECR. Now the temp gauge reads in the red once the D110 is warmed up. I know it is not over heating, but why would the temp gauge read to high? Do I need to calibrate the sender or gauge?

[Hans]

Given that there was a wiring issue in the past, I'd personally run fresh, new wire for all the involved senders. All the way, gauge to sender, uninterrupted and unspliced if possible. And, if you can, try to use the same color wires and same connectors as the stock stuff, it WILL be helpful in the future. Burnt wiring also doesn't just happen, something caused that, and it may be part of your issue.

Also, make sure that you burped all the air out of the cooling system. If you have a big air pocket in there, it could cause these types of symptoms. And I wouldn't rule out overheating just yet either. Many of the original symptoms you described could also be attributed to cooling system issue, compounded by gauge problems. A bad clutch fan or sticky thermostat could cause odd fluctuations. But before we go down that road, lets make sure you are getting accurate information from your gauges.

Got an infrared thermometer? Or something else to accurately confirm the engine temp?

[Jon_Winningham]

OK, I ran new wire for the gauge, check temp by infrared thermometer, and it checked out fine.
The only thing that was burnt was a small circut on the signal conditioner, which has been removed.
I am still getting a temp reading in the red from the original D110 gauge, and the two different '94 D90 temp gauges that I have tried.
I just installed a new heater core, thermostate, and radiator.
What are the best ways to burp the coolent system? Per Hans...
Also, the fan does come on at the oddest times... It may switch on as I leave the house in the morning (when its cold)...
I know that online diagnoses is not the best mediam, but everyones suggestions have been most appreciated.
Anymore ideas are welcome...
Thanks, Jon

[Hans]

Fan comes on? Thats not a good sign, as the temp switch in that thing is pretty high.

To burp out the air, make sure to fill from the high-fill point that is above the valve cover, connected to the heater lines. also fill the overflow tank too. Then I like to grab the upper radiator hose and give it a few big squeezes with both hands. Really squash it down, pushing the bubbles into the radiator, and do it a bunch of times.

Just to make sure, you are using the '94 gauges with the 94 sender? IIRC the 110 sender has a different resistance range than the D90. And you're doing the sender that goes in from the front of the intake manifold, not the one that goes down from the top.