bench testing, coolant temp drops continuously

[brendang2000]

MS2v3
I built this board back in 2015 and am just now getting back to this project. I have it connected to an engine on a stand as I’m still in the testing phase. I seem to remember back in 2015, that everything worked with the JimStim (JS) and when I had the relay board connected. Now I have a problem with the coolant temp, it drops continuously. I thought I had a bad sensor, I was using a new BMW E30, so I bought a new GM one, but it does the same thing. When first powered up, it read room temp (73*), but then after 5 or 6 minutes it was down to 34*. I didn’t do anything else, just waited and watched the temp fall. With the JS connected, I get a temp range of 51*-250*, but it takes a long time to get to 250* (minutes). It doesn’t drop below 51* with the JS. Maybe related or another issue, with the JS connected, the lowest I can get the MAT to read is 136*, at maximum it pegs to 250* then shoots over to 70*. I have also seen the temp jump sporadically, then return to normal. With the relay board attached, the temp is off by about 10* (used Ford 6 wire), and I have seen it “jump” here too.

What I have tried so far is, I’ve checked R7 & R8. They are colored correctly, and I get the right ohms at each, 2.49k and 2.2k. With key off, connector disconnected, I’m getting 2.9k ohm between connector signal wire and ground and almost zero between ground wire and ground. Key on, I’m getting 5v at the signal wire. I’ve looked at C6 & C7, they don’t look burnt, and C7 reads K5R 105K while C6 reads K1K. Looking at Digi-Key, I’m pretty sure these are the correct caps. Without de-soldering, not sure how else to check these. Solder joints all look good, nothing loose.

I believe everything else is working correctly. TPS, IAC (GM), and fuel pump (connected to a light) all seem to do what they’re supposed to. I have not turned this engine over yet, so VR signal hasn’t been tested either (RPMs work with JS). MSQ attached if needed. Any help troubleshooting here would be greatly appreciated. Thanks, Brendan

[Matt Cramer]

Try back probing the voltage on the sensor; what does it do?

[brendang2000]

Try back probing the voltage on the sensor; what does it do?

Thanks for the reply Matt. Not sure what you mean by back probing, but I did check the Vref signal as follows. I used the JimStim (JS) to have quicker changes in temperatures. I had my DMM grounded via the ground in the proto area and checked voltage at R4 & R7 on the Vref side. Voltage stayed at 4.98 regardless of which pot I increased/decreased. I then moved to the MAT/CLT side of R4 & R7 and got readings as follows:

MAT 3.85V 135*, 2.2mV 70* Any voltage below .722V (~345*) and the gauge jumps to 70*
CLT 3.80V 127* (continues to drop, but takes a long time), 1.7mV 170*

Turning the MAT pot while checking voltage on the CLT side has no effect and vice-versa.

Interesting thing happened while testing though. My DMM stopped getting a reading on the sensor side of R4&7, so I checked Vref at the proto area and the voltage was jumping all over the place. My meter isn’t the quickest, but I swear I saw 20+ volts, definitely 10+V. This has me thinking the Vref power is bad and maybe wiped out some other components? Other observation, the O2 sensor (via JS) only reads three values through the entire sweep of the pot, 12.1, 14.9 and 16.9. Guess I have some problems here as well. Again, any thoughts or direction is much appreciated. Thanks! Brendan

[Matt Cramer]

Back probing means to check the sensor in-circuit, often by sticking a pin or a needle-thin probe into the back of the connector.

Were the VREF jumps happening on the JimStim? If so, what exactly was powering the JimStim?

[brendang2000]

Hi Matt. I back probed the coolant sensor this morning, powered by “battery” not JS, and everything seemed to be working fine, grounded at proto and voltage on signal wire (sensor connected). I was getting 2.538V @ 80* and 1.798V @ 107*, steady reading, no jumping or rising/falling. I then connected the JS and still all seemed to be working. If I turned the coolant pot, gauge reacted quickly and steadily, no apparent rising/falling, no jumping. So, it appears I have the worst kind of problem, an intermittent one. Previous voltage jumps were happening on the JS. The JS is powered by a 12V wall-wart type p/s (the one that comes with the JS). I tested that with load/no load and it read 12.2V for both. Notes, my “battery” is a regulated 13.8V 30 amp p/s. I tried moving various components and the DB37 connector around while checking voltage, and had no fluctuations. I checked voltages again just now, both battery and JS, and all still appears fine. I’m at a loss. Until I can get this to act up again, not sure what else to do as I believe it’s working.

[brendang2000]

So I believe I've found my problem, or found a problem at least. For a while now, I'm been data-logging or just watching the gauges with this board powered through the rely board. All seemed well. Today, my coolant temp dropped to 39*, but the vref seemed normal. I started putting pressure on R4, R5, R7, R8 while watching the gauges, and I could get the gauges to jump around and give bad readings. So I'm going to resolder those, retest, and if I get the same type of bad readings I'll replace and retest.

I do have a question regarding AFR while engine not running. My dataview gauge shows 14.7-8 while powered up, but shouldn't this be reading 22.XXX since it's just air? It's an Innovated LC2 and I have the file set to single wideband. The gauge reads in the red while the sensor is warming up and jumps to the 14.xxx when finally reading, so I know it's getting something from the controller. Is this normal? Thanks!!!