1990 will not start - Chevy chassis

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tilliepeople

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I have a 1990 chieftain on a chevy P30 chassis 7.4L.
I hooked the battery up backwards and now it will not start.
Originally it was blowing the ecm/batt fuse but after some wiggle and jiggle every thing seems to power up except the crank sensor(no power at fuse box) and fuel pump (12 volts at fusebox)
It will even start with a shot of start fluid
Any ideas
I never did find the fuel pump relay and there is no check engine light to even check
 
Wow. There's no telling what you've fried. On the bright side, since the engine starts with starting fluid that means you have ignition and everything associated with that.

Look this web page over, it has quite a bit of electrical diagrams and sketches to at least give you a troubleshooting path.
 
John is right, you have the 3 thing required, air, fuel, ignition source and you know the engine will run but not stay running.
OUTDOORS, remove the fuel line from the injector body and crank the engine a few seconds while another person observes for the presence of fuel pressure from the open line. The results will narrow your search for the problem. The ECM tells the fuel pump to run when the key is on via that relay.
 
Finally found the fuel pump relay
That thing is so buried up under the dash and behind wire harnesses I'm going to do some serious swearing to get it out to even check it.
But at least I know where it's at and that's a big start.
 
There'll be a tag on the old ECM with the part number. Should be able to match them or find a cross-reference chart somewhere. In a '90, there's probably still a PROM (chip) in the computer that contains the calibrations for whatever drive train is in the coach. That PROM will have to be transferred from the old ECM to the new one or the coach won't run.
I've run across a couple of cases where the owner swapped out the ECM themselves and sent the old one back for the core charge without removing the PROM. They were not happy with what they had to pay for a new PROM.
 
Pedro Dog said:
Try bypassing the relay to the fuel pump with 12V. I'm not ready to declare a bad ECM due to reverse polarity at the battery if everything else is working.
Click to expand...
Your right. I am not nearly ready to call for a new ECM.I will start at the fuel pump relay since everything else seem to work and work my way from there. I know the relay energizes because that is how we eventually found it (listened to the clicking) but that does mean it works.
 
Also, those years GM had problems with the pumps just going dormant. Some people used to hit the gas tank (hard slap) and sometimes they would start. Those relays are standard parts and easy to get. May be easier to just buy one but if it clicks it's probably good.
Look at your electronics center and there is probably another relay matching and you can just swap them.
 
In their infinite wisdom, the GM engineers also put an oil pressure switch in the wiring for the fuel pump. Basically, the pump runs off the relay while you are cranking the engine. Once the engine starts and the oil pressure comes up, the pump is then running off the oil pressure sensor. They do it this way so the pump will shut off if the engine dies for some reason.
There should be 2 OP sensors on the engine. A bell shaped one with a single blade connector runs the gauge on the dash, and the cylindrical one with the weather-pak connector runs the pump.

Next thing I'd do is what Pedro suggested, however. Get the wife to crank the thing over while you lay underneath it and smack the heck out of the fuel tank with a rubber mallet.
 
Just an observation, automotive parts are designed to withstand a variety of environmental and operational extremes of temperature and fault conditions. For most electrical parts, overvoltage and polarity reversal are standard requirements. They usually aren't required to operate correctly beyond rated limits but will not be damaged. An ECM is a component that I would expect would readily survive a polarity reversal event. I would suspect some other component external to the ECM that might prevent it from operating correctly before I'd deem the ECM bad. Another observation is that during cranking the inputs to the ECM are limited - it's pretty much firing injectors at a preset level, spark and running the fuel pump. So if it's not starting there is something major not happening with fuel or fire and I'd prove those out before suspecting the ECM.

Mark B.
Albuquerque, NM
 
Mark_K5LXP said:
Just an observation, automotive parts are designed to withstand a variety of environmental and operational extremes of temperature and fault conditions. For most electrical parts, overvoltage and polarity reversal are standard requirements. They usually aren't required to operate correctly beyond rated limits but will not be damaged. An ECM is a component that I would expect would readily survive a polarity reversal event. I would suspect some other component external to the ECM that might prevent it from operating correctly before I'd deem the ECM bad. Another observation is that during cranking the inputs to the ECM are limited - it's pretty much firing injectors at a preset level, spark and running the fuel pump. So if it's not starting there is something major not happening with fuel or fire and I'd prove those out before suspecting the ECM.

Mark B.
Albuquerque, NM
Click to expand...
He says it'll fire if you spray starting fluid into the intake while cranking. This definitely makes it a fuel problem if that's so. Could be the fuel pump, the oil pressure switch, the fuel pump relay or the ECM not firing the injectors.
You can check for injector pulse by disconnecting the injector and putting a noid light or a test light across the terminals. Light should flash when engine is cranked. If that's good, then 8 times out of 10 it's a fuel pump issue. The other 2 times it's the relay or the oil pressure switch.
 
The news today is not too good.
I disconnected the fuel line at the carb and sure enough when i turned the key I did have fuel.
That means the injectors are not firing. I really don't know the procedure for testing them.
I do not have a Noid light and I tried the 9-volt battery trick with no results. That means either the injectors are bad, or the battery was to week to work.
I wrote down the numbers on the ECM, but they must be wrong because nothing comes up when I search for those numbers.
As far as I can tell there are a couple of switches left to check but I am not sure which way to turn.
ECM is around $100, and the injectors are at least $150 for a pair, and I really don't want to just throw parts at it.
Thank you for your patience with me
 
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