I've been doing some data logging with Diacom on my '87 and have noticed the dash readout was consistently off from what Diacom reported my coolant temp to be. As an example, when my dash readout was 210* Diacom would show 190-195* and so on...........In fact, Diacom consistently reads 15 to 20 degrees cooler than the digi dash readout even while the car is warming up. This discrepancy has bugged me over the last few months that I've had the new motor in, so I finally had a buddy use another scan tool on the car and it also reported about the same differences!

They're all (scan tools & dash) getting their info from the same source, so I'd suspect the problem lies somewhere in how the dash interprets the informaton from the computer and displays that info (I don't know anything about how that works). I definately put my faith in the scan tools over the dash readout. This makes me wonder how many of us have chased heating gremlins needlessly? I'm a big believer in excessive coolant temp being one of the more dangerous factors in maintaining overall engine health............particularly in my case since I'm running a factory 400 block.

Soooo, based on my experience I'd say if your car runs around 210-220, you may not have a problem after all, and if you're one of those guys running 230 or more it may not be quite as bad as you think (although temps that high are an indication of something rotten in Denmark). Of course, YMMV...............

Conclusion? I think it would be prudent to use a scan tool of some sort or install an independent temp guage to see if your car also has this great a discrepancy...............now I'm starting to wonder about the other readouts: oil pressure, trans temp, amps, etc?

 

I've never been 100% convinced of the accuracy of digital displays, that said though, my 87 has a 180 thermo and a 200 temp switch.When the display reads 180-181 I can almost see the thermo opening because it starts cooling back down immediately-like wise on the fan switch-when the display reads 200-201 I can hear the fans come on. Just curious if you've checked the connections to your ECM-may not do anything but who knows.

 

The same inconsistency applies to my oil pressure read-outs.

Here's my tale:

Earlier last year I was trying to repair my digital dash problem and, with my ham-fisted self, broke the part of the digital gauge that shows battery and coolant temp.

My dash was not repairable (don't ask me to confess on how I know this to be the case, just take my word for it) so in order to be able to monitor BAT and Coolant Temp, I installed two AutoMeter mechanical gauges in the bread-box.

While at it I also installed a matching oil pressure gauge, so now the bread-box has three gauges. I ran the copper oil pressure line from the bread-box to a point just above the oil filter on the block.

Interestingly, my oil pressure is always 10 to 15 psi higher on the digital dash than on the mechanical gauge in the box. So, thinking my sender at the distributor, was the cause I bought a new sender and installed it. Same thing. The two gauges never agree, but since both are always in the ball park, I just live with it.

Jake

 

Where have you been. Long time no post.
Ed

 

First, don't lose sight of the fact that no two sensors are the same. Second, look at the location of the sensors. The ECM sensor is in the aluminum intake manifold, with plenty of surface to volume ratio to dissipate heat. That is what your scanner is reading from. The dash gauge sensor is screwed into the cylinder head, right where the heat is generated.

RACE ON!!!

 

There are 2 coolant temp sensors. One feeds the Guage and one feeds the ECM. One is in the front of the engine the other is in the rear - which is why there is a difference in coolant temperatures.

 

I drove up to the Chicago area the first week in July, then went east to visit my daughter, then my son at West Point. I didn't get back to Texas until December (ain't retirement wonderful, LOL). When I got back my internet had (somehow) died and I just received a replacement DSL modem today.

Anyway, like Paul Newman said in The Color of Money

"I'M BACK!"

Jake

 

That's exactly what I contribute my pressure differences to; different locations.

Jake

 

Exactly. I'm sure the TPI cars are slightly different, but the LT1's certainly have two sending units. One is on the water pump housing, the other is in the passenger side cylinder head. The cylinder head sending unit feeds directly to the analog gauge on the dash and nothing else. The water pump sending unit feeds to the ECM, from which the CCM gets its data for the digital gauge.

Of course you're just "guessing" on the analog gauge, but they should read close to the same thing. If the analog gauge jumps up and the digital doesn't it's a good indicator that you've got a problem with the water pump. This can be especially helpful if you have an electric water pump.