'87 4.0L ballast resistor

I'm suffering from intermittent stalling/hard starting, and I think it may be the ballast resistor. Does anyone know the spec for the resistor? I have an ohm meter, but I have no idea what it should read.

relyt said:

I'm suffering from intermittent stalling/hard starting, and I think it may be the ballast resistor. Does anyone know the spec for the resistor? I have an ohm meter, but I have no idea what it should read.

Click to expand...

Check the terminal tabs - the resistance spec should be stamped into one of them near the ceramic body.

Just remove it. I did a year ago with no ill effects. One less headache

X2 - not needed. Remove resistor and splice wire.

Hallo. Mayby you have to renew the CPS, after removing the ballast resistor.
But there are plenty of other causes for a bad start. :wave1:

What is allowable tolerance? The resistor calls out 1.1Ω but measures a consistent 1.7Ω. That's 60% difference, but I would think that still wouldn't be enough to cause my problems. I guess I'll try bypassing it, but if that doesn't fix the problem, what would be the next logical course of action?

Would a bad CPS cause similar troubles? I was under the impression that a bad CPS would prevent it from firing at all as opposed to being an intermittent issue. I don't have the equipment to check fuel pressure, but I do know that there is fuel at the rail.

I've checked the TPS, but I only have access to a DMM. I know analog is the only real way to check them, but I'm currently pretty strapped for cash, and really only have money to buy parts and not test equipment.

Could it be the fuel pump itself, or would that cause other problems? Any advice you guys have to offer is very much appreciated.

After looking at the wiring diagram, I'm almost positive the ballast resistor isn't the issue. If I'm understanding it correctly, the ballast resistor is bypassed completely when the starter relay is active. Based on that, I would say that I wouldn't be having the hard start issues after it dies. Am I correct in this?

The ballast resistor is a current-limiting device added to the RUN fuel pump circuit (bypassed in START) to reduce NVH (because people were already starting to get pansified even then. I don't buy a truck because I want a quiet ride - if I wanted quiet, I'd buy a Caddy.) Thus, the specific resistance value of the thing isn't really important (as long as it passes enough current to allow for fuel pump function,) and it doesn't have any "opens" in it (the resistive element is wire wrapped around an insulating core, and the whole thing is cased in ceramic. The coil may be potted - closed - or not - open - depending on manufacturer and specification. However, unlike the typical electronic wirewound resistor, an automotive ballast resistor is not a precision device.)

When I had the ballast in my 87 go bad, I just made a 14AWG jumper and did away with the thing. Problem solved (mine developed an "open" due to heat and vibration. Since the coil wasn't potted, I was able to clearly see that the coil was broken when I removed the ballast. I chucked it in the bin shortly afterwards.)

But would a bad ballast in any way cause a hard start issue? I suppose if it killed the fuel pump long enough, it would have to rebuild pressure before starting, but I could be wrong.

Another thing to note (which I probably should have mentioned in the beginning) is that I've only experienced stalling when slowing to a stop. As long as the RPM are well above idle (not sure where exactly as I don't have a tach in mine) I've never had an issue. When it does die, sitting with the key in the "On" position for a while does seem to make it more willing to start, but even then it can be a chore sometimes.

It should not cause a hard start problem - it's more likely to cause a "Start, No Run" issue instead. You'd lose the engine a few seconds after you let the key back into RUN from START.

I'm surprised I hadn't thought of this before - check the ground strap from the firewall to the cylinder head. Make sure it's clean, make sure the contact patches are clean (particularly on the chassis,) and make sure it hasn't soaked up any crud - if it's original, it probably has. It's the primary ground for the chassis, and therefore for the engine control unit - and if it starts getting flaky on you, it's going to raise all kinds of Hell...

And, take a look at your positive battery cable (at the battery,) and make sure you don't have a corroded conductor on the smaller of the two cables exiting the clamp. Losing that connexion will kill power to everything, and can be goofy to figure out if you don't know what you're looking for.

5-90 said:

It should not cause a hard start problem - it's more likely to cause a "Start, No Run" issue instead. You'd lose the engine a few seconds after you let the key back into RUN from START.

I'm surprised I hadn't thought of this before - check the ground strap from the firewall to the cylinder head. Make sure it's clean, make sure the contact patches are clean (particularly on the chassis,) and make sure it hasn't soaked up any crud - if it's original, it probably has. It's the primary ground for the chassis, and therefore for the engine control unit - and if it starts getting flaky on you, it's going to raise all kinds of Hell...

And, take a look at your positive battery cable (at the battery,) and make sure you don't have a corroded conductor on the smaller of the two cables exiting the clamp. Losing that connexion will kill power to everything, and can be goofy to figure out if you don't know what you're looking for.

Click to expand...

Battery cables are good. The ground strap isn't pretty, but if that's the primary chassis ground, I would assume that if it dies, that would kill the radio and lights as well, which it has never done.

I do know that I still have a pretty severe blow-by issue. I changed the valve cover gasket and the whole CCV system, cleaned up the valve cover inside and out, including all of the baffles, and I still have oil blowing out the top of the valve cover, from under the grommet at the back with the smaller diameter vacuum line coming out of it. If this oil got on the the CPS, would that potentially cause the issues I'm experiencing?

I hate to say this, considering this thing has only 130k miles on it, but I'm seriously considering driving it a mile down the road to the junk yard and asking what they'd give me for it.

relyt said:

Battery cables are good. The ground strap isn't pretty, but if that's the primary chassis ground, I would assume that if it dies, that would kill the radio and lights as well, which it has never done.

I do know that I still have a pretty severe blow-by issue. I changed the valve cover gasket and the whole CCV system, cleaned up the valve cover inside and out, including all of the baffles, and I still have oil blowing out the top of the valve cover, from under the grommet at the back with the smaller diameter vacuum line coming out of it. If this oil got on the the CPS, would that potentially cause the issues I'm experiencing?

I hate to say this, considering this thing has only 130k miles on it, but I'm seriously considering driving it a mile down the road to the junk yard and asking what they'd give me for it.

Click to expand...

If it dies all the way, you'd lose everything.

But bear in mind that most sensor signals are either millivolts or milliamps (more often the latter,) and low-current stuff like that means that it needs a good ground to read properly. So, clean it up and recheck.

Would a bad CPS cause similar troubles? I was under the impression that a bad CPS would prevent it from firing at all as opposed to being an intermittent issue. I don't have the equipment to check fuel pressure, but I do know that there is fuel at the rail.

Hallo. At least you can do is to clean the connector.
And yes ,it can cause similar starting troubles.
But also all the connectors of the TPS or MAT sensors are suspect. As mentioned above : grounds.
Even the vacuum lines and a weak coil. :shhh:

Not that it matters much now with my '91, but I was always curious with my old '87 just how that ballast resistor worked. It was my understanding that it was wired in series with the fuel pump when in the run circuit. Meaning that if the resistor failed open, or you disconnected it, the fuel pump would shut off. I ran my '87 with the wires completely disconnected, but NOT jumpered for a week before I realized that I had forgotten to reconnect them after I removed them to clean them. Shouldn't that have caused a start but not run condition?

So you had a sticky ballast resistor bypass relay?

But if I look at the diagrams of a '88, I cannot find this relay. :laugh:


'92 XJ

What about your spark plugs? You say you have a lot of blowby, so they may be susceptible to fouling. Removing and cleaning with spray start can put a little more life into the plugs sometimes.

urbexx said:

Not that it matters much now with my '91, but I was always curious with my old '87 just how that ballast resistor worked. It was my understanding that it was wired in series with the fuel pump when in the run circuit. Meaning that if the resistor failed open, or you disconnected it, the fuel pump would shut off. I ran my '87 with the wires completely disconnected, but NOT jumpered for a week before I realized that I had forgotten to reconnect them after I removed them to clean them. Shouldn't that have caused a start but not run condition?

Click to expand...

Yes it should have. Someone may have jumpered it at a point away from the acual ballast resistor. That is the only way to have had a running engine.