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Warning! Gauges in a 91 coupe


Well-known member
Nov 22, 2008
Mystic CT
91 Dark Green Coupe
Today was an enlightening day for me.. Oil pressure seems a bit high water temp cool like a 160 stat.. oil temp ice cold so was getting ready to buy some sensors... and wrench next weeked.. My scantool came in this morning on ups.. Oil temp was 196 water 186 did not see oil pressure on the scan tool.. funniest part was the mileage was reading right previously well at least possible but today at least with the tool plugged in was getting around 87 mpg on the interstate at 70 mph... The Prius morons need to learn a thing or two from this..LOL.. It has to go in for emmission the reason for the scan and me not believing a car that runs so good would have such crazy readings.. It does tell me everything up to the ECM is just fine and between that and the instrument panel... How do you check that for errors? or is it a clean the connectors and hope it works ok.. Does anyone know how the data gets from the ECM to the dash?? Looking for a list of connectors to clean this weekend.. Guess what I am trying to say is take the dash readings with a grain of salt....;shrug
Seems to me there was a post on something like this recently.
Hopefully someone will chime in and know what that was.
You could try the search in the bar towards the top of your page.

Welcome to the site, you will get results here, best place on the net for corvette help.

I found several questions with similar problems though no solutions.. While waiting for the FSM to arrive I was wonder if anyone knows the data flow from the PCM to the dash. The dash reads wildly wrong and the scan tool has reasonable readings. I can not believe they would sell a new car for 37k I still have the window sticker for mine and that was the bottom line.. That has less accurate gauges than a pinto.. So my thoughts are either the data is getting corruted along the way by a poor connection or a module is scrambling the data. The problem I have with that mentally is this, the data I would assume is in digital format between the modules as it would make the most sense to me. Otherwise you would need a duplicate wire for a lot of different inputs. I don't remeber seeing oil pressure on the inputs from the PCM to the scan tool but it might be in the data but not checked by the tool. Oil temp and water temp is there though. My thoughts are that every input gets routed to the ccm and the ccm passes it out to the things that ned information. So I guess my quetsion is how do you monitor ccm data if my assumptions are true.

The other possibity I can come up with is the analog gauge is defective or the circut board or the driver chips for the gauge. Hopefully the FSM will have useful trouble shooting tips for the dash as I also assume getting a schematic for the dash a fantasy..

Dave, When my 1989 was posting odd MPG readings it was resolved by replacing the ECM. The defective ECM was not sending out serial data required by the cluster. If I remember correctly, I was seeing limited or no serial data at the ALDL port until I replaced the ECM. The oil pressure switch has both a switch and sensor. The sensor side provides resistance that is used only by the cluster.
I have a scan tool and it has all sensible data IE. engine temp 186 oil temp 197..etc buth the cluster has odd readings is my problem.. I don't see how the ECM has a problem or the sensors or the wiring to the ECM if the scan tool reads right. The oil pressure sensor might be bad as it is only used by the instrument cluster from what I have figured out so far..

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