Hello, new here. Great boards, hopefully they'll help me out.
I just bought a 1991 Mustang Coupe, 2.3l, 5 speed. I was crusing down the hiway at about 110 kilometers/hr, and the check engine light came on.
Shortly after, my tach fell. Now going about 80k/hr, it was showing I was only at about 1-1.5 thousand rpm's, so I kick it down into third (I know, but I was new to the car, and according to the tach, it would be me right about nicely, I think), and the tach rose to 2.5, but the engine sounded MUCH more than that, so I pulled over, and took it easy and went by ear until home.
The car runs fine, but the tach is giving faulty readings...is this a sensor? If so, what could it be?
The ignition module has gone south. It's aboot $100 US to replace. It's the flat gray plastic deal attached to the lower intake manifold right behind the alternator.
DSS 331 stroker, AFR 185 heads, Performer rpm 2 intake, Accufab 75 mm race TB, 30lb injectors, 80 mm Pro-M mass air, Extreme energy cam 266.
Anderson ford power pipe the big one, Ford motor sport shorty headers, Bassani x pipe custom cat back with flows, World class T5, center force duel friction clutch.
It's a common problem with the DP engines. The check engine light will probably reveal codes 223 and 224 which points to the coils. Drove me nuts when I had the same problem. Checked coils, swapped coils, and replaced coils. Nothing changed. Read a couple of threads on other boards (Ranger and Mustang) and found folks with the same trouble. They all replaced the DIS module and all was well again. Soooo I replaced mine and all has been well since (2+yrs. ago).
Excellent, thank you. As it stands, the light only comes on after highway driving speeds. If I take it in when the light is off, there will be no codes, correct?
If it detects the codes you mentioned, Ill be sure to change the module...that is, when I can come up with the money.
Is it detremental to drive without this for a while?
Even if the check engine light is off at the time you pull the codes, You should still get that code under (cmdtc) continuous memory diagnostic codes. That means it is still in the pcm's memory as having a fault at one point in time unless the battery has been disconnected or died. Umm for the drivability part, it should be fine for a little while as long as you take it easy but keep an eye on it.
I think I drove mine for a couple of months that way. No noticeable change in performance or driveability. It was just irritating when the tach went loopy.
take it to an auto shop or a parts store that can test it. As I mentioned in other threads mentioned in my post above in this one - I went to autozone and had them test it over and over. about the 5th or 6th time it started failing. I bought the part and replaced it in the parking lot. it took about 15-20 minutes to change - no problems since.
If you take it to a parts store to have it checked, make sure the counter-jockey in question knows to run all four scans for EEC-IV with the typical parts-store code-reader (almost all of them use the same orange Actron PointOfSale code-reader) If they don't, you may not get all the codes. Most counter-jockeys are too ignorant or too lazy to run all four scans (KOEO Fast Codes, KOEO Slow Codes, KOER Fast Codes KOER Slow Codes).
The idiots I used to work with at the parts store were always amazed when I'd pull 1-3 more codes from a pre-1996 Ford than they would, even after I showed them the right way to do it. Some cars don't store codes in all four modes (not sure if the DIS 2.3s do or not) but it won't hurt to check all four.
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)
If you take it to a parts store to have it checked, make sure the counter-jockey in question knows to run all four scans for EEC-IV with the typical parts-store code-reader (almost all of them use the same orange Actron PointOfSale code-reader) If they don't, you may not get all the codes. Most counter-jockeys are too ignorant or too lazy to run all four scans (KOEO Fast Codes, KOEO Slow Codes, KOER Fast Codes KOER Slow Codes).
The idiots I used to work with at the parts store were always amazed when I'd pull 1-3 more codes from a pre-1996 Ford than they would, even after I showed them the right way to do it. Some cars don't store codes in all four modes (not sure if the DIS 2.3s do or not) but it won't hurt to check all four.
well that isn't exactly what I meant... The auto shop where I live won't even test a car older than '95 for trouble codes (because their "too updated" or something). I did the engine scans myself with a little tester that beeps at you for each number, but on the '93 you can actually just put a jumper on the OBD test lead on the car and the check eng. light will blink at you for the trouble code numbers. I can't remember how I did that once it was a long time ago... then I bought the tester that had a book with all the codes in it (but now you can find the codes online pretty easy - not so easy around 94-95...)
The way that I got my DIS tested was I took it off the engine at the store and they were able to plug it into a test module like where they test the alt's, batteries and starters. I ask several times that they could test it before I took it out in the parking lot. 10 minutes later it was being tested and failed, then about $100 and 20 minutes more and I was on the way. No more tach or Ck eng light issues to date.
74stang2togo is right though, if you can get the parts people to run the test for the trouble codes they probably won't do a very good job if they'll do it at all, unless you really know what your doing and tell them how to do it. But then you might as well do it yourself and know that it was right. (But to give them a little credit at the same time, I only have to work on my car and some of my families cars, they have to know a little about a ton of different cars, so they might know about more cars than I do, but usually I know about mine more than they do.)
(By the way, I think that I only ever had to run the key on eng off test - or what ever the first one is - the most basic one that just gets the codes stored in the memory- in order to get the 223 trouble codes that point to the DIS though). Untill I got that fixed I wasn't really able to get good readings on the other test with the engine running. That test is also the test that most auto parts stores will do for you if they "still own 'the-old-style' tester" (at least that's what they guy told me the when I asked him to double check the codes I was getting).
good luck with it all.
one final note: after I got this fixed I started getting a little better fuel economy again. so you may not want to put it off too long.
Help! Check engine light, tach & engine speed problems? 
$100 US to replace. It's the flat gray plastic deal attached to the lower intake manifold right behind the alternator. 
