Instrument cluster electrical question...

Hey all,


Been going through electrical issues lately and have made several posts, and I know someone asked me not to make several threads pertaining to the same issue because that makes my progress hard to follow, but I think this question is different enough to warrant a new thread.

I am curious about my instrument cluster situation. I have a 1991 f250 7.3 idi with e4od transmission. I am having transmission issues due to the failure of my tachometer, but it isn't just the tachometer that isnt working. All of the electric gauges on my truck have stopped working. The only gauge that does work is the speedometer as it is a mechanical gauge. What confuses me is that my instrument panel illumination lights work, my o/d light worked before this problem happened, my check engine light works, the 'brake' light on the dash works, as do my blinkers and hazards. My 4 wheel drive and low range lights are not working either.

Can anyone tell me what this means? There is obviously some electrical failure of the cluster because none of the electronic gauges work, but I still have most of my lights. OR is there some electrical component that supplies power to all of my gauges that I need to check?

I know the older (bullnose) trucks had some kind of voltage regulator on them that was prone to failure. Anyone on here know if the bricknose clusters are the same?

That does ring a few bells form when I first started digging into this truck. From what I have ready there is some sort of Instrument Voltage Regulator (IVR) on my truck, but the reason I quit looking into that is because I couldn't seem to find a separate voltage regulator from the one on the passenger side near the fender, which I think controls other electric systems like alternator, etc. So that begs the question, is there a separate voltage regulator for the instrument cluster, and if so where is it located? If there isn't a separate one, does the voltage regulator on the pass side near the fender control my cluster?

That ICVR is what I was talking about that I KNOW the bullnose trucks had but I don't know if the bricks do. I believe it was mounted directly on the circuit board in the cluster.

Here's some more information. http://www.garysgaragemahal.com/icvr.html

Here is a picture of an 89 cluster that I have out of a parts truck, mine is a 91 but both clusters look pretty well the same. They both have that circuit board mounted on the bottom right corner in the picture, do you think that could be an icvr in the bricknose's?

87 and up clusters do not use an ivr, all the gauges are fed from fuse 18, the circuit board just turns on the check engine light if the oil pressure gets too low or the engine temp goes to high, you can remove it and all you will loose is the check engine light.



A truck with an E4OD has the tach sensor going to the TECA then it feed the tach signal to the tachometer, if your tachometer is dead and the trans is in limp mode there’s a problem with either the sensor, wires to the computer or the computer itself.

Well turns out I was on the right track throughout this whole process but I didnt fully understand the wiring to the dash - I do now tho. None of my gauges were working because fuse 18 was blowing. I thought I had fixed that (i disconnected the main engine wiring plug and all my gauges, other than the tach, came back on.) that plug was melted and crumbling, so I assumed the problem was a short circuit in that plug. I fixed the short circuit at that plug (which wasn't really there) and never checked fuse 18 again, which had blown because its my fuel/water separator switch that has shorted out. I unplugged that, and now my gauges are back (Tach excluded). Therefore... Either my tach sensor or my TECA is a gonner. so, i borrowed a different tach sensor off a parts truck and tried it, no change, however the tach sensor out of the parts truck could be equally as bad. I tested them both for AC volts with the sensor unplugged by touching the leads of a multimeter to the two prongs in the tach sensor plug. One tach sensor gave me ~.6 volts and the other gave me ~.4, and if I remember correctly they should be giving me ~2.5. I read that testing procedure ona forum somewhere. I currently have texts out to a couple of relatives who also have 7.3 idi's to try an use one of their tach sensors to see if my tach will start up. If not, it must be the TECA. Needless to say, I have made a whole lot of progress from the original assumption that its was the throttle position sensor.

SO that leads me to the question... is there any way to definitively test if the TECA is working?

I don't think there is a specific voltage to look for on the tach sensor unless you are using the same rotunda meter Ford used. Multimeters are designed to measure at 60 hz. I would supect they would not read accurately at a different hz. I think any increasing with rpm voltage would be an indicator the sensor is performing.

Sorry I'm no help with your tranny. Have you pulled codes on it? That's where I would start.

Pulled codes a long time ago, there weren't very conclusive, just confirmed some sort of limp mode. Haven't pulled codes in a while, it the new tach sensor doesn't pan out I will probably pull them again to see if I can isolate a different problem

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Even if there isnt a specific voltage for me too be looking for, are the two voltages from the two sensors different enough to tell me that at least one of them isnt working?

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jonleroy2506 said:

Even if there isnt a specific voltage for me too be looking for, are the two voltages from the two sensors different enough to tell me that at least one of them isnt working?

Click to expand...

Not likely, there probably is a threshold voltage at which the signal is strong enough to make it to the computer but the difference between your two is probably inconclusive. That sensor works b/c the teeth of the timing gear are passing through a magnetic field set up at the tip of the sensor. As a tooth moves through the field an electrical current is induced into the wiring. Your voltmeter is reading the rise and fall of the voltage that is generated as the tooth enters and then passes the field creating a sinewave of Alternating Current. The difference between your two sensors is because of the strength of the magnetic field produced by the sensor. Another contributing factor is how close the teeth of the gear is to the sensor. The closer it is, the more “disturbance” in the field as the tooth passes and the greater the measurable voltage. If you have a scope, you can attach that to the sensor a see the sinewave generated and make sure it is regular and not full of spikes or missing humps. It’s also possible that your meter can measure frequency, if so could possibly get a frequency number from your meter and do some rough calculations to see if it’s close. Idle speed is approximately 650 and the cam is half and so is the pump gear. Then you need to know how many teeth are on the gear. Then you could see how many teeth are passing and divide it out to get a rough rpm.

That's some pretty complicated stuff for a guy like me, I'm gonna try a working tach sensor off my uncle's truck tomorrow, and if that doesn't work I'm gonna have to look into troubleshooting the teca, although I think I've already ruled out a teca failure. I'm really just banking on a new tach sensor fixing the problem at this point. For a 17 year old kid still in high school from 7 to 4 every day who just got laid off work that's about all I can afford right now lol

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Everything I described is learned in high school physics and pre-cal. At least I hope it still is. I didn’t catch on to that till much later myself but it was there for me to sleep through or wonder how it would apply to the rest of my life but now my circle is complete I guess. Learn from the experiences offered on this board and from what you have to suffer through every day 7 to 4. School these days seems more about teaching things and facts and not about making those things relevant to everyday life which is the real goal of learning something, to be able to use what you’ve learned to do something useful. Like fixing your truck without wasting money on parts that aren’t bad.

And unless you’re studying for a test go to bed!!! Good luck =)

Thanks, yeah school sucks sometimes but I sure do learn a lot so I try not to complain overly much. I said that mostly to emphasize the fact that I'm trying to keep this as cheap as I can and not too time consuming, as time and money aren't things I have a whole lot of at the moment

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