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ECMlink Coolant Temp Signal Erratic

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obeeone420

Probationary Member
3
1
Aug 3, 2021
Salt Lake City, Utah
Hello guys,

I'm new to the DSM world and helping a friend put together a partial 1990 GSX build that he bought locally that has more issues than I care to discuss, but have one lingering issue I cannot resolve.

Build Background:
Old Holeset Turbo - Don't know the size
Denso 660cc injectors
2G MAF
AEM wideband gage with Bosch 4.2 LSU sensor (guessing cause it's really old) could be the 4.9
Running ECM Link for the computer.

The main issue I am trying to track down is the coolant temperature sensor has spikes throughout the graph and is never steady. It reads and increases in temperature, but it consistently spikes about 40 degrees every few seconds. For example if it's reading 140 for about 5 seconds it'll then spike a few times to 180 a couple time, but it's only momentarily. Has anybody experienced this while graphing a log. Unfortunately, the computer I am posting with does not have the ECM Link so I don't have the log to show, but could post one up if anyone has any input.

We have already replaced both the ECU temp sensor and the gage sensor as well.

Always appreciate any help. TIA.
 
The coolant temp sensor does not respond to temperature changes that quickly, so even if it was a malfunctioning thermostat, you wont be able to see a sudden 180F coolant pocket hitting the sensor at 140F. It would take a few good seconds to translate the analog temp signal to digital. You can test this with a boiling cup of water and dumping the sensor in it from room temp to boiling. It wont spike that quick.

The most likely scenario is you've got a break in the signal wires somewhere between the sensor and the ECU pinout. Hook an ohmmeter to the signal wire on the harness side plug for the CTS, then hook the other end to the harness side on the corresponding ECU pin. Measure the resistance, then start moving the cable and see if it changes values suddenly.

One other thing I can think of is the software itself. I'm not well versed on dsmlink (yet!) but at least in EVOscan, there are a few coolant temp options you can select for main readout and all of them read and respond differently. Some have big offsets which can mess with you.
 
I would also suspect some type of wiring issue that's causing a spike. That sensor is at a weird angle and those wires and connectors are often very old. Easy fix.
 
Thanks for all your replies. I am suspecting a wiring issue and I'll check for continuity of the wiring from the temp sensor the the ECU. I did already rewire the connection points at the temp sensor since the wiring was very brittle so we have a new plug on that side of it.

Looking more into this the previous owner spliced some type of GM O2 sensor plug and the wiring for that is spliced near the coolant temp sensor wiring so I'll tear that apart as well to check and make sure nothing was messed up there.

Also, noticed that the ECU grounding pins share quite a bit of other sensor grounds with the EGR temp sensor (removed), MAF, Coolant temp and TPS from pins 24 and 17 so I'll disconnect each one of those to see if anything might be giving me feedback by chance.

Appreciate the help.
 
Issue Solved!!

In short it was the daughter board on the ECU. We found this as one of the drivers for the ignition circuit went out and we lost spark on cylinder 2&3. After lots of testing of coil packs, powder transistor and verifying wiring, we ended up just throwing a known good ECU in and the car started and ran fine. At that point I scheduled the ECU to be repaired at ECM Tuning and had them look over a couple of the inputs which they found both corrosion and loose connections in a number of places. Fixed everything and the coolant temp signal is stable as it should be.

I'll try and get a log posted so there is reference in case anyone else runs into a similar issue.
 
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