Jump to content

Instrument Cluster Temp Gauge reading cold after obd2 swap


Dark Ride

Recommended Posts

I am officially stumped on this. Ever since I did the OBD2 swap on my 90 grand prix I can't get the temp gauge to read properly. Once the car is warmed up, the pcm sees 180f, but the gauge on the instrument panel on shows roughly 130-140.

 

I understand these work by resistance, with decreasing ohms as the sensor gets hotter. i confirmed there is almost no resistance from the sensor to the cluster and cluster to engine block. The sensor is giving roughly 340 ohms at 180, which seems in line but maybe I'm wrong. Please help, it's making me insane because I think the car is running cold.

Link to comment
Share on other sites

the circuit for the temp gauge sending unit on a 3-wire sensor uses the case as a ground. I wonder if you have a bunch of sealant or Teflon tape on it or something that is not allowing it to make a ground

Link to comment
Share on other sites

the circuit for the temp gauge sending unit on a 3-wire sensor uses the case as a ground. I wonder if you have a bunch of sealant or Teflon tape on it or something that is not allowing it to make a ground

That's possible, checking for resistance between the exposed brass on the sensor and the block/housing should confirm that correct?
Link to comment
Share on other sites

Yes, also resistance between ground and the single pin on the bottom row willtest the sensor. You can't rule out that the scaling may simply be wrong on that year of sensor for that year of gauge.

Link to comment
Share on other sites

I'll test the resistances tomorrow and let you know. I'm going to check the shop manuals on the 90 & 99 as well to compare the scaling like you mentioned. I feel like there were close when I initially did this, but maybe not close enough.

Link to comment
Share on other sites

Perhaps retest at 180 for the 340 ohms, but connect one of your leads to the block instead of the sensor body to rule out the sensor grounding issue. Sometimes testing for resistance can be inaccurate if the absolute value is small (ie engine block to sensor body) due to instrument error.

Edited by vipmiller803
Link to comment
Share on other sites

Confirmed only 0.1 ohms between sensor body and block. Jumping the wire feeding the gauge caused it to jump to 280 (as expected since there would be no resistance) also confirmed roughly 340 ohms at 180 again. I checked the electrical diagrams and the range between what the cluster expects and what the sender provides is slightly different. The cluster looks for 1400 ohms at 100f and 60 ohms at 280f. The sending unit provides 1365 ohms at 100f and 44 ohms at 280f. If anything it should read higher then right? Thoughts?

Link to comment
Share on other sites

You could try a potentiometer to dial in exactly what resistance the gauge wants to see at the right temp. The CTS is changing resistance but obviously 340 is on the cold side for the gauge. You could place the cts but those are unreturnable once installed.

Edited by vipmiller803
Link to comment
Share on other sites

I'm beginning to think it must have to be the "curve" of the resistance changes. That the new sensor is more gradual and the old one was steeper after say 130f, so that when it was 180f, resistance was only 150 or something. Either that or I have a lazy gauge...

Link to comment
Share on other sites

So I had some really old resistors laying around and i confirmed that I need roughly 150 ohms for the gauge to read 180. So... is there a way to reduce the resistance without messing up the lower end? I could wire a resistor in parallel but that would reduce the resistance too much when it's cold.

Link to comment
Share on other sites

After eliminating everything else, it seems I don't have a choice but to install the old sensor if i want this to read correctly, so that's what I'll end up doing. I'll let you know how it goes, thanks for the suggestions guys.

Link to comment
Share on other sites

Yeah, this is on an L36 so I'm thinking I'll drill and tap right above the stock one. I'm installing my turbo kit so I had it half pulled apart anyway so I popped the thermostat housing off and it looks like the thickness should be sufficient and it won't impend coolant flow or hit the thermostat.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...