Coolant Temp Gauge (and others)

Greetings community. New member here. Avid DIYer but I just got a new to me 1999 GMC Suburban and since I’ve never had one before I figured I’d ask some people who’ve had one and would know.

99 Suburban K1500 5.7L

The instrument cluster was having issues so I had it rebuilt by a repair shop. Now I am uncertain if they did the job correctly. After the rebuild I put the cluster back in and I’m still having issues with the coolant gauge (including others).

To rule out the cluster I replaced both coolant temperature sensors and the thermostat. I also purged any air out of the system. After all this the gauge still only reaches the first quarter mark ~160F.

There’s nothing else it could be at this point right, they need to redo the cluster?

All the gauges should read 0 with the ignition key off correct? They shouldn’t display the last reading when the truck was running?

Thanks in advance.

Those gauges are run by stepper motors and the original stepper motors were notorious for their unreliability. There are replacement motors that are better. But here is the big issue with those repair shops, it involves soldering on a printed circuit board and it is very easy for an untrained person to do severe damage to the runs on the board.

Many of these repair shops only have a soldering iron and to do this job properly, it really needs a solder station. I used a shop that was in a back room of a used car lot, but they guy had the right equipment and had training in soldering on printed circuit boards.

Take out the instrument cluster and look at the runs on the circuit board. Either you will see that nothing was done, or only one stepper motor was changed, or you will see burn marks all over the board.

If you see burn marks, my suggestion is to go to YouTube and look for videos of guys repairing these boards. One of them, maybe more but at least one has a really good set up and he shows repairing a board done by either another shop or the owner whose DIY soldering skills were overrated by himself. This guy is a professional but he can repair the damage, but it will cost more than if the board was not damaged. He has to replace runs.

I replaced all my stepper motors with a soldering iron, not that bad, and all worked well afterwards. I am not sure if the people that did the repair have a way to test it afterwards, so it might need to go back. If you decide to order a new one it was $300 for reprogramming at the time I looked into it.

As far as I can see in the electrical diagrams, the 1999 Suburban still used good old fashioned analog gauges for temp, volts, fuel, and oil pressure. They switched to logic-controlled gauges (except for the oil pressure) for the new generation in 2000, though GM still referred to them as analog devices.

The older analog gauge use seems to be verified by the OP when it was said that both coolant temp sensors were replaced on the 5.7 engine. One is for the PCM, the other is strictly for gauge operation. The 5.3 engine used in the new generation only uses one temp sensor.

The new sensor (actually a sender) and/or gauge could be out of spec. The sender can be checked by checking resistance at various temperatures, if the correct values are known. The gauge can be checked by putting specific resistor values in place of the sender to see if the gauge reads correctly at both ends of its range. A resistance of 1305 ohms should read 100F on the gauge. A resistance of 49.5 ohms should read 260F on the gauge.

It’s also possible that the new gauge was not properly calibrated, or the newer 2000 gauge was installed instead, if that’s even possible (likely not since they are driven differently). The 1999 gauges just plug into sockets on the IP PCB. Coincidentally (or maybe not), the lowest reading on the new gauges is 160F.

If a scan tool is handy, it can be used to read the temp from the PCM to see if it correlates to the gauge reading.

It probably should be verified that the engine is operating at the proper temperature by other means. Was the new thermostat the correct 195F spec? It seems 160F thermostats are available.

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@Bugmenot, You may be right but I thought the Suburban was built on the same platform as the Silverado/Sierra which did use the stepper motors starting in 99.

The old “Classic C/K” and new “Silverado” generations of the pickups were both sold during the '99 model year. The 1999 Suburban was still based on the “classic” platform, while the new generation Suburban based on the new platform debuted as a 2000 model.

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OK then…

I disagree with you

On op’s generation of GM truck, the clusters were NOT notorious for problems

you’re thinking of the trucks that came right after

The 5.7 liter V8 is the key . . .

No full-size GM truck with that engine used a cluster with stepper motors

And I believe 1999 was one of those years in which both old body style and new body style were sold side by side


Assuming the thermostat installed has the correct rating . . . probably around 190 degrees . . . have you verified with a scan tool that the coolant temperature is in fact where it’s supposed to be. Should be around 200 degrees fahrenheit when everything is properly warmed up. Might take a few miles of driving to get there. If it’s correct, don’t worry about the cluster. That generation of clusters wasn’t all that accurate, as far as the coolant temperature gauge was concerned

OK guys, I stand corrected.

Didn’t that generation use a sender and a switch? The switch fed the ECM and the sender fed the gauge. If so, are you sure you changed the sender and not the switch? The actual temp can be verified as well as the sender output to see what is accurate and where the issue lies.

OP said both were changed. Both are NTC thermistor-type sensors/senders.

Thanks everyone for your replies.

To clear a couple things up, the cluster does use “air core” motors NOT the stepper motors. The reason I took it to a repair shop vs doing it myself was cost. The air core motors aren’t being made anymore and they run $80-$100 on eBay. The shop I took it to specializes in instrument repair so you’d think they know what they’re doing but who knows right? That’s why I prefer to DIY. I may end up having to do that with the cluster anyways. I’ve also been toying with the idea of CNCing my own mount and popping in some new autometer gauges and be done screwing around with it. This is my 3rd GM (I had a 2004 Tahoe and a 2006 Silverado) and they’ve all had issues with the cluster. But the others were easier to fix with the stepper motors.

Thanks for the suggestion of using the scanner to verify temp. I will definitely try that. Who knows maybe I got a bad replacement sensor or thermostat. Think it’s unlikely but it could happen, especially with the thermostat.

I can also verify that the thermostat was replaced with the correct 195F rating.

Anyways thanks for all your suggestions!

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If you would prefer to replace the whole cluster with aftermarket gauges, ready-made clusters are available, such as here. Rather pricey though.

EDIT: I changed the link above. The previous link actually showed a '92-'94 cluster, even though '99 was chosen in the search. The new link shows '95-'98, but should also be same as '99 “Classic” truck or Suburban.

Those “air core” motor gauges have resistors on the bottom between two of the pins. Perhaps the resistor on your temp gauge is iffy or the wrong value for proper calibration, or even missing. The gauges are also specific units for temp, oil, fuel, and volts. Make sure the right one(s) was(were) installed, and that they are in the correct positions.

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Wow thanks for the link! I had looked for something like that but couldn’t find it! That’s pretty much exactly what I was thinking to do. I was gonna trace the original mounting dimensions and make a CAD model and CNC it. The cost of the gauges plus the metal and machine time is almost the same as their product this just saves me the hassle. Man thanks again!